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The combined effect of ultrasound and ozone on bacteria in water Amna Mohammed Al-hashimi, Timothy J. Mason, and Eadaoin Maria Joyce Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5045437 • Publication Date (Web): 16 May 2015 Downloaded from http://pubs.acs.org on May 18, 2015

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Environmental Science & Technology

The combined effect of ultrasound and ozone on bacteria in water

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Amna M. Al-Hashimi, Timothy J. Mason and Eadaoin M. Joyce*

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The Sonochemistry Centre, Coventry University, Faculty of Health and Life Sciences, Priory Street, Coventry, UK

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Tel: +44 2476888075; Fax: +44 2476 888173; *Corresponding author email:[email protected]

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Keyword: Ozone,ultrasound,water treatment, flow cytometry, Escherichia coli

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*Corresponding author e-mail:[email protected]

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Abstract ........................................................................................................................................... 2

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Introduction ..................................................................................................................................... 3

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Material and methods ...................................................................................................................... 5

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Preparation and viability assessment of bacterial suspensions ................................................... 5

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Ultrasound and ozone system (USO3) ........................................................................................ 6

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Transmission electron microscopy ............................................................................................. 7

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Results ........................................................................................................................................... 10

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Measurement of dissolved ozone in water ................................................................................ 10

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Effect of ultrasound on the viability of Escherichia coliusing theUSO3 system. ..................... 11

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Effect of ozone on 75L Escherichia coli treated with the USO3 system for 16 minutes ......... 13

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Combined effect of ultrasound and ozone on the viability of Escherichia coli treated using the USO3 system ............................................................................................................................. 14

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Discussion ..................................................................................................................................... 16

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Acknowledgment .......................................................................................................................... 21

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References ..................................................................................................................................... 21

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Abstract

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the biological disinfection of water on a large-scale application using viable plate counts and

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flow cytometry.

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Methods and results: Escherichia coli B bacteria in saline suspension was treated using a

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commercially available combined ultrasound and ozone system (USO3 - USS, Ultrasonic

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Systems Gmbh, Germany) for 16 minutes. Two analytical methods were used to assess the

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results in terms of live and dead cells in the bulk liquid: standard viable plate counting recorded

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in terms of Colony Forming Units (CFU) per ml and flow cytometry (FCM). In the latter case 1

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ml bacterial suspension was stained simultaneously with the fluorescent stains SYTO9 and

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Propidium Iodide (PI). Transmission electron microscopy was used to generate images

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identifying the biological effects of different treatments using ultrasound and ozone on bacterial

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cell walls. Results demonstrated that treatment with ozone alone (1mg/l) resulted in a significant

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reduction (93%) in the number of live cells after 16 minutes treatment whereas ultrasound alone

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showed only a small reduction (24%). However a combination of ozone and ultrasound showed a

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synergistic effect and enhanced the inactivation to 99%after 4 minutes.

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Conclusion: A combined ultrasound and ozone treatment of bacterial suspensions using a

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commercial system (USS, Ultrasonic Systems Gmbh) affords a promising method for water

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disinfection that is better than treatment using either method alone. Standard viable plate count

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analysis is normally used to assess the effectiveness of disinfection treatments; however flow

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cytometry proved to be a more sensitive method to determine the actual effects in terms of not

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only live and dead cells but also damaged cells. This type of analysis (cell damage) is difficult if

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not impossible to achieve using traditional plate counting methodology.

Aim of the study: To assess the synergetic effect of combined ultrasound and ozone treatment on

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Introduction

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replacement for chlorination in that it reduces disinfection by-products [1]. It is an unstable tri-

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oxygen molecule which easily releases oxygen atoms in aqueous solutions which subsequently

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reacts with hydrogen ions in water leading to increased levels of hydroxyl radicals (OH●) in the

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medium [2]. Joret showed that ozone had lethal effects on a wide range of microbes, such as

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bacteria, viruses, parasites, Candida, polio virus and Mycobacterium through oxidizing the cell

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membrane resulting in cell disruption [3]. Disinfection by ozone is strongly dependent on water

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characteristics such as dissolved organic carbon (DOC), pH, temperature and bromide

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concentrations [4][5].

Ozone is an important disinfection gas for drinking water treatment and is a suitable

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Over the last 20 years researchers and engineers have found that a combination of two or more

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disinfection methods involving oxidation (Advanced Oxidation Processes - AOPs) is very

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effective in water purification. AOPs help to improve water quality and reduce treatment costs

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[6]. Combinations of ultrasound with other oxidants such as ozone, chlorine and hydrogen

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peroxide (H2O2) have been investigated [4]. These studies concluded that employing a

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combination of ozone and ultrasound treatments in the secondary stage of wastewater treatment

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resulted in reductions in the required contact time to inactivate microorganisms.

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The collapse of acoustic cavitation bubbles generated during the application of ultrasound to

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aqueous systems affords several routes to disinfection [7][8]. Cavitation bubble collapse creates

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a “hot spot” where extremely high temperatures, pressures and free radicals form [9]. The

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possible effects of sonication on microorganisms are:

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High temperatures in and around the collapsing bubble resulting in enzyme denaturation

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High hydro-mechanical pressures generated on bubble collapse resulting in shear forces and liquid jets which lyse cells



Production of a highly oxidative species of OH● and H2O2from the hot spots which can attack and weaken cell membranes by attacking their chemical structure [10] [11].

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The impact of ultrasound on bacteria has been reported in previous studies that focus on cell

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inactivation, the release of cell contents and the production of a temporary permeability of cell

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walls. In addition, the effect of ultrasound on inactivation of microorganisms using varying

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treatment times for many pure cultures of different bacteria, yeast, fungi and viruses has been

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reported [12][13].

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Microorganism inactivation has also been investigated using a combination of ultrasound with

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ozone or UV radiation [14][15][16][17], high pressures and temperatures, or different ultrasonic

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parameters [18][19].

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Previous investigations of combined treatments with ozone and ultrasound have indicated that

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the synergistic effect of sonication and ozone significantly increases bacterial inactivation rates.

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Jyoti and Pandit indicated that microbial damage resulting from hydro-mechanical forces is

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greater than sonochemical effects [20]. It was reported that the combination of hydrodynamic

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acoustic cavitation and ozone reduced power consumption, in addition to reducing the formation

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of disinfection by-products (DBP) during treatment [21].

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Jyoti and Pandit used the viable plate count (VPC) analysis method to assess the effect of

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acoustic cavitation and ozone on microbial disruption [22]. However, culture based methods are

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time consuming and do not identify sub-lethally damaged cells. Such cells retain some metabolic

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activity but are unable to produce a viable colony on solid growth media (agar). This can occur

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when bacteria undergo stress but are not completely inactivated or killed by water treatment

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methods such as chlorination. However, if sufficient numbers of these organisms are ingested

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they have the potential to cause illness [23]. Therefore, there is an increasing interest in finding

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an analytical technique which can yield more information on the physiological status of bacteria

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than can be deduced from standard culture methods. Additionally, different environmental

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conditions can affect the viable counting method [24]. Various characteristics must be

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considered when evaluating microbial viability which includes membrane integrity, cell growth

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and metabolic activity; however membrane integrity has received the majority of research [25].

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Flow cytometry (FCM) is capable of quantifying features of cells primarily by visual means.

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FCM records measurements from individual cells but can process thousands of cells (10,000

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events) in seconds. In conjunction with fluorescent staining it can provide information relating to

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the estimation and discrimination of different physiological characteristics of cells [24].The aim

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of this study was to evaluate the application of FCM with dual staining for a rapid, accurate and

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reliable assessment of cell viability after ultrasound, ozone and combined treatments using the

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USS system. The results were compared with those obtained using a traditional CFU culture

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method.

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Material and methods Preparation and viability assessment of bacterial suspensions

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A stock suspension of Escherichia coli was prepared according to [26]. 1 ml of bacterial

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suspension was taken after 0, 2, 4, 8 and 16 minutes treatment and added to 9 ml of 0.9% saline

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(9 g/L sigma Aldrich Ltd.) containing sodium thiosulfate Na2S2O3 (2 g/l) to neutralise any

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residual ozone in the sample. Samples were serially diluted to obtain 2g/l [14]. Each dilution was

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cultured on nutrient agar plates in triplicates and incubated at 37°C for 24 hours. The results

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from plate counts were converted into CFU/ml:

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CFU’s/ml = No. of colonies * volume of cultured sample * dilution of sample

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Ultrasound and ozone system (USO 3 )

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A large-scale water treatment device from Ultrasonic Systems Gmbh, Germany, was employed

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to deliver ultrasound and ozone for water treatment using a technology known as USO3. The

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system consisted of an ozone generator, 20 ultrasonic transducers at maximum power setting

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(100 Watt each/612 kHz), a mixer and external pump that was capable of treating up to a

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maximum capacity of 4 m3/h = 66.67 L/min. Internally, the system consisted of long steel tubes

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with a diameter of 108.3 mm, L = 3.02 m, V = 28 L = 0.028 m3), retention time: 0.8 min. The

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bacterial suspension was pumped into the USO3 kit using an external pump (flow rate 35 L/min).

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Experiments were performed at a constant flow rate of 35 L/min, delivering a constant ozone

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supply capable of generating a concentration which accumulated to 1 mg/L. The aqueous

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suspension of the bacteria was treated at this flow rate in a recycling system for 16 minutes with

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pulsed ultrasonic treatment on for 5 seconds and off for 5 seconds. The ozone concentration was

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measured using the indigo colorimetric method [27].

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Transmission electron microscopy

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A transmission electron microscope TEM was used to provide high resolution images, (2010F,

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JEOL Ltd., Japan). The instrument used a 200kV field emission gun instrument in cryo-

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conditions and was fitted with a GatanUltraScan™ 4000 camera which provided the spatial

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resolution necessary for detailed biological structural analysis and very high magnification of

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samples down to 0.5nm with three dimensional imaging.

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Experiment design

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75 L of Escherichia coli bacterial suspension (OD 0.01, 2x105 cell/ml) was treated with

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ultrasound alone, ozone alone (1mg/l, power 160 W, 10% gas flow) and a combination of

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ultrasound and ozone for 16 minutes. Samples were taken after 0, 2, 4, 8 and 16 minutes

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treatment and the residual ozone in sample was neutralised using 2% sodium thiosulfate (Sigma

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Aldrich Ltd.) in sterile saline (0.9% NaCl Sigma Aldrich Ltd.). An in-built cooling system was

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used to maintain sample temperature below 25ºC. Samples were analysed by viable plate counts

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to measure CFU/ml and flow cytometry. All data was analysed using ANOVA test.

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Bacterial staining and flow cytometry

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The use of double staining techniques involving nucleic acid stains (SYTO9) (green

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fluorescence) and Propidium Iodide (PI) (red fluorescence) provide a valuable method for the

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estimation of bacteria viability [185]. The fluorescent probes/stains and counting kit for flow

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cytometry were purchased from Invitrogen (LIVE/DEAD® BaclightTM bacterial viability and

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counting kit) which consisted of two stains SYTO9 (nucleic acid stain) at a dose of 200 µl (3.34

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mM in DMSO) specific for live cells, Propidium Iodide (PI) at a dose of 200 µl (20 mM in

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DMSO) for dead cells. The calibration beads were microsphere standard beads 6.0 µm diameter

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with a concentration of 1×108 beads/ml in deionised water (10ml) containing 2 mM sodium

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azide.

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Flow cytometry measures several parameters at the same time for each cell:

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Low angle forward scatter intensity (FSC) is proportional to cell diameter.

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Orthogonal (90º) scatter intensity (SSC) relates to the quantity of granular structures within the

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cell. Fluorescence intensities at several wavelengths can be observed using different fluorescence

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channels.

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Estimation of cell integrity or viability using flow cytometry

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500 µl bacterial suspensions were collected in an Eppendorf tube (1.5 ml). Samples were

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simultaneously stained with SYTO9 and Propidium Iodide (PI). Fluorochromes were added

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according to the manufacturer’s instructions. Dyes were purchased as a BaclightTM bacterial

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viability and counting kit from Invitrogen Ltd. 1.5 µl of fluorescent stain was added to each

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sample, vortexed and analysed after 10-30 minutes incubation at room temperature. Excitation of

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SYTO9 emitted green fluorescence at 525-550 nm and excitation of PI at 536nm produced red

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fluorescence at 600nm. Viable cells (with intact membranes) are only permeable to SYTO9 but

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not PI; hence viable cells emit green fluorescence. In contrast, dead cells (with permeabilised

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membranes) were stained with PI and emit red fluorescence. BD FACS Calibur flow cytometry

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was optimised by FACS COMP standard fluorescence beads (2 µm) to verify the instrument

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performance. Bacterial populations were positioned so that they were all located in scale on a

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FSC V’s SSC plot. For fluorescence measurements, FL1, FL2 and FL3 voltages were adjusted to

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place the unstained populations in the lower left quadrant of two parameter plots. Flow

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cytometry instruments settings were as follows: threshold (FSC), FSC (E02), PMT 4.75

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(logarithmic amplification (LA), SSC-597 V (LA), FL1 617/550 V (LA), FL2 531 V (LA) and

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FL3 623 V (LA). 10,000 cells were acquired per acquisition and bacterial populations were gated

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using a combination of FSC, SSC, live/dead cells and discriminated using FL1 V’s FL3. Live (3

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hour bacterial culture) and dead (heating bacterial cells to 75°C for 30 minutes) controls were

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also undertaken to calibrate results. To ensure continuity of results all experiments were run for a

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total of 16 minutes.

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Effect of ultrasound and ozone separately a nd in combination on the biological structure of bacterial cell walls using CRYO TEM analysis

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Escherichia coli bacterial suspension was placed in a suitable container and treated with

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ultrasound alone ozone alone and a combination of ultrasound and ozone for 16 minutes using

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the USO3 system. Samples were taken at 0, 2, 4, 8 and 16 minutes and analysed using TEM

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microscopy. To determine the percentage inactivation by TEM, a protocol was followed where

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intact cells were considered to be robust and cells displaying a loss of cell integrity were

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considered dead. For each TEM calculation 100 cells within a grid were observed following each

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treatment. Cells which appear to show damage were then converted to a percentage.

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Supporting Information

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Figure 1 Schematic diagram of the USO3 system taken from the manual provided by USS, (20

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ultrasonic transducers at maximum power setting (100 Watt each/612 kHz)

Output

Input

205 Ultrasonic transducer

Opti-mixer

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Measurement of dissolved ozone in water

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NaCl (0.9 g/l) was added to 75 L of fresh water to convert the water to saline which was

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circulated through the ultrasound and ozone (USO3) system for 16 minutes. 50ml of sample was

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collected and analysed using the UV-Vis spectrophotometer (Shimadzu, UK 5030). Results

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illustrate that the absorbance of water samples at 254 nm increased over 16 minutes treatment,

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which is due to the accumulation of ozone in water over 16 minutes treatment (Figure 2).

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Figure 2 Accumulation of residual ozone in 75 L water using the ultrasound and ozone system

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over 16 minutes analysed using UV-Vis spectrophotometry

Control

Ozone (ppm)

Abs.

Time [min]

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Results

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Effect of ultrasound on the viability of Escherichia coliusing theUSO 3 system.

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A single bacterial suspension of Escherichia coli was treated with ultrasound for 16 minutes.

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Results illustrate a small inactivation effect of 24% on the number of live cells over 16 minutes

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treatment compared to controls (Figure 3). This is in accord with other results obtained at a

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laboratory scale using higher frequencies [26] [28]. 512 kHz was used to treat different types of

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bacteria for 15 and 60 minutes which achieved less than 1 log reduction at the end of the

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treatment time.

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Figure 3 Effect of ultrasound on the viability of Escherichia coli, (OD 0.01, 2x105 cell/ml) using

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the USO3 system for 16 minutes analysed using viable plate counts Control

CFU/ml

US

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Time [min]

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The bacteria were also analysed by flow cytometry using the Invitrogen BaclightTM bacterial

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viability and counting kit. The dot plots illustrated in Figure 4 represent bacterial populations

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that are divided into two populations; live cells appearing in the lower right (LR) quadrant and

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dead cells in the lower left (LL) quadrant. Flow cytometry data illustrates small reductions of

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26% in the number of live cells after 16 minutes treatment. Flow cytometry data supports viable

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plate count results, but some cells were observed in the live cells position after treatment.

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Although there were still some cells observed in the position expected for live cells the majority

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of cellular material after treatment appeared in an intermediate position between that for live

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cells and that for dead cells. This is where one would normally find viable but non culturable

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(VBNC) cells. Cell wall fragments may up take some of the background fluorescent stain.

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Figure 4 Effect of ultrasound on the viability of Escherichia coli treated with the USO3 system

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for 16 minutes analysed using flow cytometry

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Control

16 min

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Effect of ozone on 75L Escherichia coli treated with theUSO 3 system for 16 minutes

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A single bacterial suspension of Escherichia coli was treated with ozone using the USO3

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system for 16 minutes. Results demonstrate a 93% reduction in number of live E. coli cells over

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16 minutes treatment time (Figure 5). The results obtained for reductions in E. coli are in accord

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with those of other researchers who achieved 1 log reduction for E. coli [30].

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Figure 5 Effect of ozone on Escherichia coli using the USO3 system for 16 minutes analysed

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using viable plate counts

Control

CFU/ml

O3

Time [min]

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Escherichia coli were also analysed using flow cytometry as outlined above. The data illustrates

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a significant reduction of 85% (ANOVA P value 0.01) in the number of live cells within 16

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minutes of treatment. However, some cells still remained in the lower right (LR) quadrant of the

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graph after treatment. The appearance of live bacterial cells is due to fragments or parts of cell

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walls that can uptake fluorescent stains specific for live cells (SYTO9) and so appear in live cell

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position.

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Figure 6 Effect of ozone on Escherichia coli treated with the USO3 system for 16 minutes

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analysed using flow cytometry

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Control

16 min

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Combined effect of ultrasound and ozone on the viability of Escherichia coli treated using the USO 3 system

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A single bacterial suspension of Escherichia coli was simultaneously treated with ultrasound and

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ozone for 16 minutes. Results illustrate a significant inactivation effect of 99% occurred after 4

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minutes treatment (Figure 7).

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Figure 7 Effect of ultrasound and ozone on the viability of Escherichia coli using the USO3

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system for 16 minutes analysed using viable plate counts Control

CFU/ml

USO3

Time [min]

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Complete inactivation was achieved within 4 minutes, which can be ascribed to a synergistic effect of ultrasound and ozone.

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Flow cytometry data showed a significant reduction of 99% (ANOVA P value 0.01) in the

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number of live cells along with an increase in number of dead cells over 16 minutes treatment

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with ultrasound and ozone (Figure 8). However, a small number of cells (0.6%) appeared to

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remain in the lower right quadrant following treatment.

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Figure 8 Effect of ultrasound and ozone on the viability of Escherichia coli using the USO3

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system for 16 minutes analysed using FCM

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Control

16 min

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Discussion Effect of ultrasound alone on Escherichia coli treated with the USO3 system for 16 minutes

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Bacterial cells treated with ultrasound alone using the USO3 system demonstrated a small

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inactivation effect 24% according to VPC and 26% according to FCM (ANOVA P value 0.05)

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after 16 minutes treatment. This is in accord with previous findings at 512 kHz in that higher

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frequencies at low power have low inactivation effects [26].FCM illustrated a good correlation

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with VPC results supporting this evidence of poor inactivation rates [30] [32].

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Effect of ozone alone on Escherichia coli treated with the USO3system for 16 minutes

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Ozone treatment clearly demonstrated a significant reduction (93% according to VPC and 85%

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according to FCM) (ANOVA P value 0.01) in bacterial cells following 16 minutes treatment.

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Flow cytometry data showed a significant reduction in the number of live cells along with

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increases in the number of dead cells. A study by Sharma reported a promising technique to

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oxidize bacteria in the food and water industry, resulting in more than a 3.5 log reduction in

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viable cells after treatment with 1 ppm ozone for 60 minutes using a selection of bacterial strains

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including the spore forming bacteria Bacillus subtilis [31].

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Effect of combined treatment ultrasound and ozone on Escherichia coli treated with the USO3system for 16 minutes Ultrasound can enhance the effect of ozone on bacteria through a number of processes: 

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De-clumping of bacterial clusters to disperse bacteria as single cells which are more susceptible to oxidation by ozone



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Formation and collapse of cavitation bubbles leads to weakening of the bacterial cells due to breaking chemical bonds in the cell membrane

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Sonication enhances ozone decomposition rates in water during treatment

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Sonication increases the level of free radicals and hydrogen peroxide (H2O2) in the

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medium which has bactericidal effects on various microorganisms [25]. Combined

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treatment using the USO3 system produced complete inactivation (99% according to

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VPC and 99% according to FCM) in bacterial cells within 4 minutes treatment.

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Ultrasound clearly enhanced the effect of ozone on bacterial inactivation (ANOVA P

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value 0.01).

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It is clear that the application of (USO3) could significantly contribute in water treatment

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plants. The combination of ozone and ultrasound provides synergetic effects which can

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substantially reduce the cost of treatment and improves health and safety issues associated with

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ozone production. In addition, by employing combined techniques (USO3) can rapidly achieve a

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reliable and safe quality of water for consumers.

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(TEM) evidence of the effect of ultrasound alone for 16 minutes on the biological structure of

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Escherichia coli treated with the USO3system

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TEM imaging was used to show any structural changes on the cells when a Escherichia coli

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bacterial suspension was treated with ultrasound alone. Results showed cell shrinkage and cracks

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in some cell walls and in some cases destruction of the entire cell. Overall approximately 24% of

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bacterial cells had an irregular shape after treatment. An example of a normal and damaged cell

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is shown in (Figure 9).

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Figure 9 (TEM) images showing the effect of ultrasound on the biological structure of

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Escherichia coli treated with the USO3 system (A) control (untreated 24 hour culture, bacterial

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cell with intact cell wall) (B) 16 minutes ultrasound treatment (bacterial cell wall with irregular

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surface and holes due to sonication treatment)

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(A) Control

(B) 16 min treatment

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(TEM) evidence of the effect of ozone alone (1mg/L) on the biological structure of Escherichia coli treated with the USO3 system

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Escherichia coli bacterial suspension was treated with ozone alone to determine the effect of

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ozone on the biological structure of bacterial cells. Results show that ozone produces holes in the

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cell walls as a result of oxidation effects. Overall approximately 86% of bacterial cells had such

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holes in their cell wall and some of the cells had lost their entire cell wall after treatment (Figure

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10).These results are in accord with those of Sharma who demonstrated bactericidal effects of

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ozone result from the disruption of envelope integrity through peroxidation of phospholipids and

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lipoproteins [31].Overall approximately 86% of bacterial cells lost their cell wall after treatment

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(Figure 10).

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Figure 10 (TEM) images showing the effect of ozone alone on the biological structure of

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Escherichia coli treated with the USO3 system (A) control (B) 16 minutes ozone treatment

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(A) Control

(B) 16 min treatment

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Transmission Electron Microscopy (TEM) evidence of the effect of combined treatments of

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ultrasound and ozone on the biological structure of Escherichia coli treated with the USO3

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system. Escherichia coli were subjected to combined treatment with ultrasound and ozone for 16

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minutes using the USO3system. Results illustrate that combined treatment with ultrasound and

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ozone had a significantly greater inactivation effect than separate treatment. TEM images show

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that 95% of the bacteria lost the entire cell walls after treatment, which may be due to the

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synergistic effect of ultrasound and ozone over 16 minutes treatment (Figure 11).

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Figure 11 (TEM) images showing the effect of ultrasound and ozone on the biological structure

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of Escherichia coli treated with the USO3 system (A) control (B) 16 minutes treatment with

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ultrasound and ozone

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(A) Control

(B) 16 min treatment

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From the above TEM images it is clear that ultrasound combined with ozone had a great

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impact on the biological structure of Escherichia coli bacteria. Ozone alone has a significant

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effect on the bacterial cells but ultrasound enhanced this effect resulting in cell shrinkage due to

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cell wall removal and the subsequent release of cell contents. This may be due to the combined

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ultrasonic effects (hydro-mechanical pressures resulting in shear forces and liquid jets which lyse

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cells, high temperatures and chemical effects due to the production of highly oxidative species of

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OH● and H2O2), which have a strong anti-microbial action on cells [26].Summary of the (TEM)

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results for bacterial inactivation. The percentage inactivation by TEM was determined using the

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protocol outlined in the material and methods section and the results are shown (Table 1).

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Table 1 Percentage inactivation in E. coli bacteria 1011 cell/ml treated using the USO3 system for

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16 minutes analysed by TEM

395 Bacterial Ultrasound Strain (612 kHz) E. coli E. coli YES E. coli YES

Ozone (0.5 mg/l) YES YES

Treatment time(min) 16 16 16

% inactivation 86 24 95

396 397 398

Acknowledgment

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The authors would like to thank the Iraqi Ministry of Higher Education who supported this

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research, Dr. Afthab Hussain at Coventry University for assistance with flow cytometry and Mr.

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Ian Portman director of the imaging suite at Warwick University for support in TEM data

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analysis.

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