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Novel Strategies to Enhance Lateral Flow Immunoassay Sensitivity for Detecting Foodborne Pathogens: A Review Weihua Lai J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/jf5046415 • Publication Date (Web): 24 Dec 2014 Downloaded from http://pubs.acs.org on December 30, 2014
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Novel Strategies to Enhance Lateral Flow Immunoassay Sensitivity for Detecting Foodborne Pathogens: A Review
Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:
Journal of Agricultural and Food Chemistry jf-2014-046415.R1 Review 23-Dec-2014 Shan, Shan; State key Laboratory of Food Science and Technology, Lai, Weihua; State Key Laboratory of Food Science and Technology, Nanchang University, Xiong, Yonghua; 2Jiangxi-OAI Joint Research Institute, Nanchang University, Wei, Hua; State Key Laboratory of Food Science and Technology, Nanchang University, Xu, Hengyi; State Key Laboratory of Food Science and Technology, Nanchang University,
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Novel Strategies to Enhance Lateral Flow Immunoassay Sensitivity
2
for Detecting Foodborne Pathogens: A Review
3
Shan Shan, Weihua Lai*, Yonghua Xiong, Hua Wei, Hengyi Xu
4
State Key Laboratory of Food Science and Technology, Nanchang University,
5
Nanchang 330047, China
6 7 8 9 10 11 12 13 14
*Send correspondence to:
15
Weihua Lai, Ph.D, Professor
16
State Key Laboratory of Food Science and Technology, Nanchang University
17
Address: 235 Nanjing East Road, Nanchang 330047, China
18
Tel: 0086-13879178802; Fax: 0086-791-88157619;
19
E-mail:
[email protected] 20
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Abstract: Food contaminated by foodborne pathogens causes diseases, affects
22
individuals, and even kills these affected individuals. As such, rapid and sensitive
23
detection methods should be developed to screen pathogens in food. One of current
24
detection methods is lateral flow immunoassay, an efficient technique because of
25
several advantages, including rapidity, simplicity, stability, portability, and sensitivity.
26
In this review, we present the format and principle of lateral flow immunoassay strip
27
and the development of conventional lateral flow immunoassay for detecting
28
foodborne pathogens. Further, we focus on novel strategies that can be applied to
29
enhance the sensitivity of lateral flow immunoassay to detect foodborne pathogens;
30
these strategies include innovating new labels application, designing new formats of
31
lateral flow immunoassay, combining with other methods, and developing
32
signal-amplification systems. With these advancements, detection sensitivity and
33
detection time can be greatly improved.
34
Key words: lateral flow immunoassay, foodborne pathogens, sensitivity enhancement
35
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1. Introduction
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Food safety is an important public health priority. Foodborne diseases are serious
38
global problems and impose a significant burden not only on infected individuals but
39
also on the economy. Although a majority of cases are mild, occasional cases can lead
40
to serious or long-term conditions or even death. The Centers for Disease Control and
41
Prevention estimated that each year roughly one in six Americans (or 48 million
42
people) became adversely affected by foodborne diseases.1 The World Health
43
Organization estimated that approximately a million people suffered from foodborne
44
diseases each year in the UK.2 Foodborne pathogens contaminate food at some parts
45
of a food chain from farm to fork. Salmonella, enterohemorrhagic Escherichia coli
46
(EHEC),
47
Cronobactor, Vibrio, and Shigella are reported frequently. For instance, Salmonella is
48
one of the most common causes of food poisoning in the world and can cause more
49
serious illnesses in older adults, infants, and persons with chronic diseases. Every year,
50
Salmonella is estimated to cause approximately 1.2 million illnesses in the United
51
States.3 Another case involves EHEC; this pathogen is often reported as one of the
52
most common causes of foodborne outbreaks. For example, approximately 265,000
53
EHEC infections occur each year in the United States. EHEC O157 also causes
54
approximately 36% of these infections, and non-O157 EHEC accounts for the
55
remaining cases.4 In other instances, L. monocytogenes primarily affects older adults,
56
pregnant women, newborns, and adults with weakened immune systems. In the
57
United States, L. monocytogenes causes nearly 2,500 cases of listeriosis per year.5
Listeria
monocytogenes,
Staphylococcus
aureus,
Campylobacter,
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Foods that are most frequently associated with foodborne illnesses include meat, fish,
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poultry, vegetables, and fruits. These foods may cause illnesses when they are eaten.
60
In traditional method, analytes are initially cultured in enrichment media for 18 h
61
to 24 h and then cultured in selective media for 18 h to 24 h; biochemical and
62
serological tests are subsequently performed to confirm suspected colonies for another
63
3 d to 5 d. Conventional detection methods require 5 d to 7 d to identify target
64
bacteria. The methods of conventional isolation and culture are time-consuming and
65
laborious. Therefore, a simple, specific, and sensitive method that can be used to
66
detect pathogens should be developed. Several methods based on nucleic detection,
67
such as real-time polymerase chain reaction (PCR)6-8, multiplex PCR9-11, and
68
loop-mediated isothermal amplification (LAMP) method12-14, have been developed
69
rapidly. Enzyme-linked immunosorbent assay (ELISA)15-17, biosensing18-21, and
70
electrochemical strategies22-23 have also been extensively investigated. On the one
71
hand, these methods are sensitive and specific; on the other hand, these methods
72
require technical expertise, laborious procedures, and expensive instrumentation. In
73
addition, these technologies are inappropriate for on-site tests. With these drawbacks,
74
detection techniques have been developed and improved. For instance, lateral flow
75
immunoassay (LFI) has been considered as a promising diagnostic tool extensively
76
applied to detect drugs24-27, toxins28,29, hormones30,31, heavy metals32,33, pesticides34,35,
77
and pathogens36,37. LFI provides several advantages, including rapid procedure,
78
convenient operational requirements, sensitivity, and cost-effective equipment.
79
Review papers have described the strengths, weaknesses, opportunities, and threats of
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LFI38, as well as its advanced ability to detect mycotoxins, phycotoxins, and
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pesticides39,40. In this paper, recent advances on LFI applied to detect foodborne
82
pathogens are described. Furthermore, sensitivity enhancement methods are presented
83
and discussed.
84 85
2. Format and principle of traditional LFI
86
2.1 Components of the LFI format
87
Immunochromatography test paper technology, which was developed in the early
88
1980s, involves rapid detection based on immunological techniques. A LFI device
89
consists of a sample pad, a conjugate pad, a nitrocellulose membrane, and an
90
absorbent pad. A test zone and a control zone are found on the nitrocellulose
91
membrane (Fig. 1A).
92 93
Sample pads are used to filter specific sample components; these pads can also change the pH of a sample and release analytes with high efficiency.
94
Conjugate pads are used to accept recognition labels and keep these labels stable
95
during their entire shelf-life period. Recognition labels are added to a treated
96
conjugate pad by impregnating them in a conjugate suspension or dispensing them
97
with quantitative non-contact dispensers.
98
After a sample is added to a strip, this sample migrates through a sample pad to a
99
conjugate pad, where recognition labels are immobilized. Analytes in a given sample
100
then interact with recognition labels when they migrate to an analytical region.
101
Specific biological components are then bound at a test line and a control line via
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quantitative dispensers. Absorbent pad resembles an engine of an LFI device; absorbent pad pulls fluid
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and retains this fluid during the whole assay.
105
2.2 LFI principle to detect foodborne pathogens
106
Competitive assay and sandwich assay are the two kinds of formats commonly
107
used in LFI. Competitive assay is preferred when target analytes exhibit low
108
molecular weight or present single specific antigen. Sandwich assay is performed
109
when target analytes present several epitopes, such as pathogens. In a sandwich assay,
110
the first analyte ligand (monoclonal antibodies are commonly used) is used to label
111
markers. The second ligand (polyclonal antibodies are widely used) is placed on the
112
test zone of a nitrocellulose membrane and assigned as the test line. The specific
113
antibodies coating the control line capture excess labeled antibodies and conjugates.
114
As sample extract is applied to the LFI device, the labeled first ligand initially binds
115
to the target analyte to form an analyte-ligand complex; this complex then migrates
116
forward by capillary force to the test zone. The second ligand in the test zone
117
subsequently binds to the target analyte. Detectable signal enhancement in the test
118
zone directly corresponds to the amount of a target analyte in a sample. In sandwich
119
assay, detectable signal enhancement is observed as target analyte concentration
120
increases.
121 122 123
3. Conventional LFI to detect foodborne pathogens Colloidal gold lateral flow strip is considered as one of the most widely used
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tools to detect foodborne pathogens. Colloidal gold is essentially inert and forms
125
spherical particle. Antibodies bind to the surfaces of these gold particles with
126
enormous strength when correctly coupled, thus providing a high degree of long-term
127
stability.41,42 Hence, ligands exhibit good compatibility with biomolecules and can
128
retain biochemical activities of tagged biomolecules. Colloidal gold-based LFI has
129
been applied to detect foodborne pathogens.43-50 Furthermore, a LFI device has been
130
used to determine the analytical sensitivity of E. coli O157 of approximately 105
131
CFU/mL51,52. Preechakasedkit et al.53 also used colloidal gold-based LFI strip to
132
detect S. typhimurium, and the limit of detection was 1.14 × 105 CFU/mL. Ueda et
133
al.54 applied the colloidal gold LFI strip to detect L. monocytogenes, and the detection
134
limit of this assay was 106 CFU/mL. The detection limit of Vibrio harveyi was 106
135
CFU/mL when a colloidal gold LFI was used.55 Wiriyachaiporn et al.56 further
136
developed an LFI device to detect Staphylococcus aureus, and the result showed a
137
detection limit of 106 CFU/mL. Colloidal gold-based LFI has also been widely
138
applied in commercial detection. Table 1 summarizes the information regarding
139
conventional colloidal gold LFI applied to detect foodborne pathogens in industries
140
from some manufacturers. Although this summary includes only several commercial
141
products, the main trends of colloidal gold LFI in the detection of foodborne
142
pathogens are shown.
143 144 145
4. Recent advancements in LFI to detect foodborne pathogens Researchers have devoted efforts to develop a rapid and sensitive lateral flow
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assay platform to screen foodborne pathogens (Table 2). New advancements in lateral
147
flow assay have also been developed to detect foodborne pathogens; these
148
advancements include innovating novel labels, designing new formats of LFI,
149
combining with other methods, and developing signal-amplification systems.
150
4.1 Innovation of novel label application
151
An ideal label should be stable, low non-specific binding, cost effective, and
152
easy for conjugation; an ideal label could also be used for multi-analyte detection.
153
Many efforts have also been devoted to discover novel labels to enhance detection
154
sensitivity. In this paper, the types of the new labels successfully applied to enhance
155
the sensitivity of LFI to detect foodborne pathogens were described; these new labels
156
include colored labels, luminescent nanoparticles (NPs), and labels based on other
157
properties.
158
4.1.1 Colored labels
159
Colored labels are visible to the naked eyes. They have been widely used in a lot
160
of fields owing to their convenience, such as colloid gold and colored latexes are the
161
most common labels in LFI. Carbon nanoparticles have also been applied as labels
162
owing to their low price, nontoxicity and high signal intensity, and they can easily
163
bind ligands to their large surface area, and they are suitable for LFI detection because
164
of their “black-on-white” test results.57,58
165
Blažková59 used carbon as a label to develop a nucleic acid test strip to detect
166
Cronobacter spp. The PCR products of the analytes were conjugated with biotin on
167
one side and digoxigenin on the other side. Carbon NPs were labeled with neutravidin
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as capture conjugate. Anti-digoxigenin antibodies and biotin-protein conjugates were
169
sprayed on NC membrane as test line and control line, respectively. The detection
170
limit of PCR product was 8 ng.
171
4.1.2 Luminescent NPs
172
Luminescent NPs have been extensively investigated because these NPs exhibit
173
high sensitivity and spectral characteristics. Fluorescent dyes60, quantum dots61, and
174
up-converting emitters62 have been applied as labels to detect Campylobacter jejuni, E.
175
coli O157:H7, and Vibrio in an LFI system.
176
Fluorescent dyes display low photo-stability and are highly quenched by the
177
environment and concentration63. To overcome these drawbacks, researchers should
178
couple multiple fluorescent dyes to a carrier or produce particles loaded with
179
fluorescent dyes. Xie et al.64 applied LFI based on fluorescent microspheres as labels
180
to detect E. coli O157:H7 with the reader which was from Shanghai Huguo Science
181
Instrument Co, Ltd (excitation wavelength = 470 nm; emission wavelength = 520 nm).
182
The results showed that the sensitivity of this LFI system was 104 CFU/mL while the
183
sensitivity of LFI based on colloidal gold was 105 CFU/mL.
184
Quantum dots have been applied to bioassays and biosensors due to excellent
185
signal brightness, size-tunable light emission, broad excitation spectrum, narrow
186
emission spectrum, and excellent stability resisting photobleaching.65-67 Bruno68
187
compared colloidal gold with quantum dot versions in lateral flow test strips to detect
188
E. coli O157:H7. A handheld long wave UV light (365 nm) and an orange Schott
189
glass filter were used for illumination and enhancing visual detection of quantum dot
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particles. The limit of detection of E. coli O157:H7 were 6,000 and 600 cells per test
191
in a buffer by using colloidal gold lateral flow assay system and quantum dot lateral
192
flow assay system, respectively.
193
Up-converting emitters are reported more sensitive than conventional labels such
194
as colloidal gold or colored latex beads.69 Up-converting emitters exhibit several
195
advantages, including long lifetime and sequential two-photon absorption possible in
196
a microsecond time scale. With these emitters, background absorption can be
197
minimized because of anti-stokes luminescence, and biomolecule photodegradation
198
does not occur because of excitation occurring in the infrared area and the ease of
199
production.70 An ultrasensitive up-conversion fluorescent lateral flow strip based on
200
NaYF4:Yb,Er NPs was applied to detect Vibrio anguillarum.62 Carboxyl-modified
201
β-NaYF4:Yb,Er NPs were synthesized by a facile one-pot solvothermal approach and
202
then coupled with mAb as a conjugate probe in lateral flow assay. Continuous wave
203
near infrared spectrum laser at λ = 980 nm was used to excite the upconversion
204
nanoparticles on the strip by the Up-converting nanoparticles-LFI biosensor. The
205
sensitivity of the Up-converting fluorescent strip was 102 CFU/mL and 100 times
206
higher than that in enzyme-linked immunosorbent assays, exhibited good specificity
207
attributed to no cross reaction with eight other pathogens.
208
4.1.3 Other labels
209 210 211
In addition to colored and luminescent particles, new labels, including magnetic beads, enzymes, and liposomes, are also applied in LFI. Magnetic beads have been widely used to separate target analytes; these
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magnetic beads have also been used as carriers and labels to detect pathogens.71-74
213
Wang et al.75 developed a super-paramagnetic lateral flow assay to detect Bacillus
214
anthracis spores.
215
super-paramagnetic beads to capture bacterial spores. Different anti-B. anthracis
216
mAbs were sprayed onto a nitrocellulose membrane as a test line, and goat anti-mouse
217
IgG was used as a control line. The detection system yielded a linear range of 4 × 103
218
CFU/mL to 4 × 106 CFU/mL, and reproducible detection limits were 200 spores mg–1
219
in milk powder. These super-paramagnetic beads were larger and reaction time was
220
longer than other types of strip tests because spores were larger (approximately 1 µm
221
in diameter) than other analytes (such as proteins). The conjugates that run through a
222
membrane required a long reaction time when the conjugates of magnetic beads and
223
antibodies captured the spores. Small-diameter particles were also used as labels.
224
With these particles, flow rate was higher and reaction time was shorter; however, the
225
use of these materials reduced detection sensitivity. Yan et al.76 discussed the effect of
226
the characteristics of Fe3O4 NPs (including particle size, size distribution, surface
227
biomodification, magnetic property, and colloidal stability) on LFI when these Fe3O4
228
NPs were used as labels to detect V. parahaemolyticus. To increase sensitivity and
229
decrease false positive reaction, they chose Fe3O4 NPs which had small size and good
230
colloidal stability as the labels. The 200nm Fe3O4 nanoparticles were selected for the
231
detection system.
Anti-B.
anthracis
mAbs
were
conjugated
to
300
nm
232
In another approach, enzymes as labels were introduced to lateral flow assay to
233
decrease the limit of detection. Enzymes can be linked to biomolecules to form
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complexes and exhibit enzymatic and immunological activities. The reaction between
235
enzymes and substrates amplifies optical signals. Enzyme-linked immunosorbent
236
assay (ELISA) based on the reaction between enzymes and substrates are considered
237
as one of the most outstanding assays. Hence, ELISA combined with lateral flow
238
assay to detect analytes could be an excellent strategy to amplify signals.77 Park et
239
al.78 fabricated an LFI based on sandwich ELISA to detect E. coli O157:H7. Optical
240
signals were generated by the reaction between horseradish peroxidase (HRP) and its
241
substrate, which could be read directly by naked eyes. The lower limit of detection
242
was 103 CFU/mL.78 A similar method was applied to detect S. typhimurium. An
243
immunostrip was used to quantitatively determine S. typhimurium in the range of 9.2
244
× 103 CFU/mL to 9.2 × 106 CFU/mL based on chemiluminescent signals.79
245
Liposomes are formed from amphiphilic molecules and can encapsulate and
246
immobilize molecular markers, such as dyes, via covalent or hydrophobic interactions.
247
Khreich et al.80 encapsulated sulfordamine B in liposomes as labels to detect
248
fluorescent signals and obtained a 15-fold increase in sensitivity compared with
249
optical detection sensitivity of colored labels. Immunoliposome-encapsulated
250
sulfordamine B has also been applied as a capture label to detect S. typhimurium and
251
compared with traditional colloidal gold lateral flow strip.81-83 The result showed that
252
the sensitivity of immunoliposome immunoassay strip was higher than that of
253
traditional colloidal gold immunoassay strip; this high sensitivity was appropriate to
254
detect food matrix.
255
4.2 New formats of LFI
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The application of antibodies in a LFI system exhibits several weaknesses,
257
including difficult preparation and poor stability. A new ligand should be discovered
258
to replace antibody. With advances in nucleic acid- and genomic sequence-based
259
techniques, various detection methods have been developed using nucleic acid rather
260
than antibodies. A rapid and sensitive detection system, which is called nucleic acid
261
LFI84-86, has been developed. In the detection of S. aureus by nucleic acid LFI,
262
quantum dot-streptavidin conjugate was used as a capture probe. Anti-digoxigenin
263
antibody and BSA-biotin conjugate were sprayed on a membrane as a control line and
264
a test line, respectively.87 This detection system was based on two reaction systems,
265
including digoxigenin and antidigoxigenin, streptavidin and biotin. DNA extracted
266
from the analytes in a sample was labeled with digoxigenin on one side and with
267
biotin on the other side. The schematic of detection principles was shown in Fig. 1B.
268
The limits of detection of S. aureus were 3 CFU/mL and 30 CFU/g in spiked milk
269
powder and meat samples, respectively. Extracted DNA from analytes in a sample is
270
usually amplified by PCR. LAMP was applied to perform rapid and sensitive nucleic
271
acid amplification combined with lateral flow assay.88-90 A shorter time is needed to
272
amplify target DNA by LAMP than by PCR because a set of four primers that
273
recognize a total of six distinct sequences on the target DNA is used in LAMP; LAMP
274
also relies on the auto-cycling strand displacement of DNA synthesis performed by
275
the large DNA polymerase under isothermal conditions.
276
Aptamers are another choice to replace antibodies as recognition molecules used
277
for bacterial detection methods to bind specific analytes.91-94 Yonekita et al.95 used
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antimicrobial peptides (AMPs) as recognition probes for bacteria in a LFI.
279
Streptavidin-colloidal gold was conjugated with biotinylated AMP, which was used as
280
a capture probe. Anti-analyte antibodies were then sprayed on a test line while
281
anti-streptavidin antibodies were sprayed on a control line. Moreover, LFI was
282
applied to simultaneously detect and identify Shiga toxin-producing E. coli O157,
283
O26, and O111 (Fig. 3). Studies have focused on the simultaneous detection by
284
LFI.96-97 Simultaneous detection system exhibits several advantages, such as simple,
285
rapid, and sample-saving, compared with single detection. Yu et al.98 developed a
286
colloidal gold LFI to simultaneously detect V. cholerae serogroups O1 and O139,
287
which could cause epidemic and pandemic cholera. The result showed that 100%
288
sensitivity and 100% specificity were obtained for O1 and O139 in all of the tested
289
strains. The loss of intensity in the test line after 21 weeks was not significant
290
compared with that after 4 weeks; LFI remained stable when stored dry at room
291
temperature for at least three months.
292
4.3 Combined with other methods
293
The sensitivity and specificity of methods used to detect foodborne pathogens
294
should be improved. Immunomagnetic separation (IMS) performs an increasingly
295
important function in the isolation and concentration of foodborne pathogens in food
296
sample.99-101 A detection method of combined IMS and LFI can significantly improve
297
sensitivity, decrease detection time, and eliminate interference from food
298
matrices.102-104 Cui et al.105 used IMS and traditional colloidal gold LFI to detect E.
299
coli O157:H7. The results showed that the detection limit was 7.6 × 103 CFU/mL;
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thus, the result was 10 times higher than that of colloidal gold lateral flow assay. The
301
schematic was shown in Fig. 3. Chen et al.106 developed an LFI test to detect
302
Cronobacter and used silica-coated magnetic NPs to separate nucleic acid from
303
Cronobacter lysate. The detection limit of this technique was 107 CFU/mL in pure
304
culture, and 1 mL of 105 CFU Cronobacter was detected as positive one in pure
305
culture after silica-coated magnetic NP treatment was administered.
306
In a food matrix, some non-target analytes may cause false-positive results. In
307
the primary enrichment phase, the growth of closely related non-target bacteria should
308
be suppressed to improve detection sensitivity. Muldoon and co-workers107 applied
309
specific bacteriophages, which could suppress non-Salmonella bacterial growth for
310
the control of cross-reactive non-Salmonella bacteria during primary sample
311
enrichment. The bacteriophage cocktail that was added into primary enrichment
312
significantly reduced the false positives of immunochromatographic test strip. False
313
positives were reduced from 32 of 115 samples tested to zero in naturally
314
contaminated beef samples.
315
4.4 Development of the signal-amplification system
316
4.4.1 Application of a biotin-streptavidin system
317
Several approaches have been used to amplify signals of traditional gold
318
colloidal LFI. These approaches aim to improve LFI sensitivity. To increase
319
sensitivity effectively, a biotin-streptavidin system has been applied in lateral flow
320
assay. Biotin-streptavidin system exhibits good reaction specificity and high affinity;
321
hence, this system has been widely applied in many field detections.108,109
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Furthermore, this system can greatly contribute to signal amplification and
323
background
324
immunochromatographic strip based on a biotin-streptavidin system to detect E. coli
325
O157:H7. Colloidal gold labeled anti-E. coli O157 monoclonal antibodies and
326
biotinylated anti-E. coli O157 polyclonal antibodies were impregnated in a conjugated
327
pad and a sample pad, respectively. Streptavidin and goat anti-mouse antibodies were
328
immobilized in a test line and a control line of nitrocellulose membrane, respectively.
329
E. coli O157 bound to biotinylated antibody and colloidal gold-labeled antibody to
330
form a biotinylated antibody-analyte-colloidal gold-labeled antibody complex. This
331
complex migrated to the detection zone by capillary action. The test line became red
332
when the complex was captured by streptavidin via high-affinity biotin-streptavidin
333
interaction (Fig. 4). Streptavidin coated in the test line could reduce the background
334
signal and amplify detection signal owing to the high affinity between biotin and
335
streptavidin. This method could detect E. coli O157 at a minimum of 2.3 × 103
336
CFU/mL without enrichment. Sensitivity was near 100 times higher than that of
337
traditional lateral flow assay.52
338
4.4.2 Enzyme amplification
activity
reduction.110
Zhao
et
al.111
prepared
colloidal
gold
339
In another approach involving signal amplification, enzymes are introduced to
340
lateral flow assay. Cho et al.112 prepared an antibody-enzyme-magnetic NP complex
341
as a label to detect L. monocytogenes by employing lateral flow assay. In this strategy,
342
magnetic properties and enzyme catalytic amplifications were combined, thereby
343
generating a darker signal on magnetic beads because of enzyme-substrate reaction
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(Fig. 5). The study included three steps as follows: 1) magnetic separation and
345
concentration; 2) lateral-flow immunochromatography; and 3) signal generation by
346
placing HRP substrates on the absorption pad on the lateral side of a nitrocellulose
347
membrane pad (Fig. 5). Signals were captured as images using a scanner. The optical
348
densities of the images were digitized by using image analysis program (Multianalyst
349
version 1.1, Bio-Rad Laboratories, Hercules, CA) to obtain the signal value. After
350
signal was amplified, the limit of detection of 95 CFU/mL was obtained in buffer
351
solution.
352
4.4.3 Silver enhancement
353
Silver nucleation on gold has been applied to perform sensitive biosensing and
354
achieve signal amplification.113-116 Silver precipitates on gold and can enhance
355
electrochemical and colorimetric signals; silver can also vary the resistance or
356
conductance of a substrate.117 Color is greatly enhanced on the basis of the reaction
357
between silver salt and reducing agent when silver is deposited on gold. Liu et al.118
358
utilized a 16S rDNA/rRNA probe-conjugated AuNPs and silver enhancement method
359
based on sandwich LFI to detect Salmonella. In this assay, anti-avidin antibody was
360
sprayed onto a nitrocellulose membrane to capture avidin. A biotinylated capture
361
probe was conjugated with avidin and 16S rDNA/rRNA probe-conjugated AuNPs as a
362
capture probe to capture Salmonella-targeted nucleic acid. Both biotinylated probe
363
and probe-conjugated AuNPs could capture Salmonella-targeted nucleic acid; these
364
probes then constituted sandwich LFI. To improve detection sensitivity and intensify
365
AuNP signal, researchers soaked test strips in silver enhancer reagents. Nucleic acids
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were detected from 107 bacterial cells. The detection limit was as low as 104 cells
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after silver enhancement was performed (Fig. 6).
368 369
5. Conclusions
370
Traditional lateral flow assay has been widely applied to detect foodborne
371
pathogens. However, detection sensitivity is limited. As such, future studies should
372
focus on enhancing detection sensitivity and decreasing detection time of LFI to
373
detect foodborne pathogens. Scientists have devoted efforts to attain these goals. This
374
review described recent LFI advancements, including using new labels, applying new
375
formats, combining other methods, and utilizing signal-amplification systems, in
376
foodborne pathogen detection. With these advancements, detection sensitivity and
377
detection time can be greatly improved. In the future, hard word should be done to
378
further improve the stability and convenience of these sensitive lateral flow
379
immunoassays.
380 381
Acknowledgements
382
We are grateful to the Research Program of State Key Laboratory of Food Science
383
and Technology, Nanchang University (Project No. SKLF-ZZB-201307), earmarked
384
fund for Jiangxi Agriculture Research System (JXARS-03), and the Nanchang
385
Technological Program (2012-CYH-DW-SP-001) for financial support.
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788
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789
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790
Figure Captions
791
Figure 1 Components of LFI strip format (A). Schematic of detecting S. aureus
792
principles of nucleic acid LFI (B) (Adapted from Chen et al87).
793 794
Figure 2 Schematic of the multiplex LFI strip. Anti-analytes antibodies were sprayed
795
on test lines to capture E. coli O157 (A), E. coli O26 (B), and E. coli O111 (C). Analytes
796
were mixed with streptavidin-colloidal gold conjugate solution, and the mixtures were
797
added to the sample pad of the LFI strip. After 15 min, positive or negative results
798
could be determined (Adapted from Yonekita et al95).
799 800
Figure 3 Schematic of immunomagnetic separation combined with colloidal gold LFI
801
(Adapted from Cui et al105).
802 803
Figure 4 Schematic of the colloidal gold LFI based on biotin-streptavidin system to
804
detect E. coli O157:H7
805 806
Figure 5 Schematic of LFI based on enzyme combined with magnetic separation step
807
to detect L. monocytogenes (Adapted from Cho et al112)
808 809
Figure 6 Schematic of LFI. As immunoreaction was completed, test strip was soaked
810
in silver enhancer reagent to intensify the signal of gold nanoparticles (Adapted from
811
Liu et al 118).
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813
Tables
814
Table 1 Commercial colloidal gold LFI applied to detect foodborne pathogens Company
Web site
Analytes
www.kwinbon.com
E. coli O157:H7, Listeria monocytogens
Biocontrol
www.biocontrolsys.com
EHEC, Salmonella, Listeria monocytogens
DuPont
www.dupont.com
E.
Beijing
Kwinbon
Biotechnology
coli
O157:H7,
Salmonella,
Listeria
monocytogens Invisible sentinel
invisiblesentinel.com
E.
coli
O157:H7,
STEC,
Salmonella,
Listeria .spp, Campylobacter Merck KGaA/EMD
www.merck.com
E. coli O157:H7, Salmonella,Campylobacter
www.neogen.com
E.
Chemicals Neogen company
coli
O157:H7,
Salmonella,
Listeria
monocytogens New
Horizons
www.nhdiag.com
E. coli O157:H7, Salmonella.spp
www.nipponham.co.jp
E.
Diagnostic Nippon
Meat
Packers Quicking Biotech
coli
O157,
O26,
O111,
Salmonella
enteritidis, Listeria .spp, Campylobacter jejuni www.quicking.cn
E.
coli
O157:H7,
Salmonella,
Listeria
Salmonella,
Listeria
Salmonella,
Listeria,
monocytogens Romer labs
www.romerlabs.com
E.
coli
O157:H7,
monocytogens Ubio Biotechnology www.ubio.in
E.
coli
O157:H7,
System
Shigella, Vibrio cholera, V. parahemolyticus
815 816
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817 818
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Table 2 Novel strategies applied in LFI for detecting foodborne pathogens Methods enhance detection sensitivity Colored Labels
819 New label application
Luminescent NPs
Other Labels New formats of LFI Combined with other methods Signal-amplification system
Nucleic Acid LFI AMPs LFI&Simultaneous detection Combined with IMS Biotin-streptavidin system Enzyme amplification Silver enhancement
Target Shiga toxin-producing E. coli Cronobacter spp E. coli O157:H7 E. coli O157:H7 V. anguillarum B. anthracis E. coli O157:H7 S. typhimurium S. aureus Shiga toxin-producing E. coli E. coli O157:H7 E. coli O157:H7 L. monocytogenes Salmonella
Limit of detection 4
5
10 /10 CFU/mL 8 ng 104 CFU/mL 6×102 CFU/mL 102 CFU/mL 200 spores/mg 103 CFU/mL 102 CFU/mL 3 CFU/mL 104 CFU/mL 7.6×103 CFU/mL 2.3×103 CFU/mL 95 CFU/mL 104 CFU/mL
Ref 58 59 64 68 62 75 78 81 87 95 105 111 112 118
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820
Figures
821
822 823
Figure 1
824
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825
826 827
Figure 2
828
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829
830 831
Figure 3
832
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833
834 835
Figure 4
836
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837
838 839
Figure 5
840
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841
842 843
Figure 6
844
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TOC graphic
847
The novel strategies can be applied in LFI to improve sensitivity. The labels of LFI include
848
colored particles, fluorescent microspheres, quantum dots, up-converting nanoparticles, magnetic
849
beads, enzymes, and liposomes. New formats of LFI include selecting DNA/RNA or aptamer as
850
the ligands, simultaneously detecting many kinds of analytes. The immunomagnetic
851
separation-LFI system and the signal-amplification systems including biotin-streptavidin system,
852
enzyme amplification, and sliver enhancement are also applied recently.
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