Isolation and detection of Listeria monocytogenes using fluorogenic and chromogenic substrates for phosphatidylinositol-specific phospholipase C

Listeria monocytogenes in foods-From culture identification to whole-genome characteristics

Listeria monocytogenes is an important foodborne pathogen, which is able to persist in the food production environments. The presence of these bacteria in different niches makes them a potential threat for public health. In the present review, the current information on the classical and alternative methods used for isolation and identification of L. monocytogenes in food have been described. Although these techniques are usually simple, standardized, inexpensive, and are routinely used in many food testing laboratories, several alternative molecular-based approaches for the bacteria detection in food and food production environments have been developed. They are characterized by the high sample throughput, a short time of analysis, and cost-effectiveness. However, these methods are important for the routine testing toward the presence and number of L. monocytogenes, but are not suitable for characteristics and typing of the bacterial isolates, which are crucial in the study of listeriosis infections. For these purposes, novel approaches, with a high discriminatory power to genetically distinguish the strains during epidemiological studies, have been developed, e.g., whole-genome sequence-based techniques such as NGS which provide an opportunity to perform comparison between strains of the same species. In the present review, we have shown a short description of the principles of microbiological, alternative, and modern methods of detection of L. monocytogenes in foods and characterization of the isolates for epidemiological purposes. According to our knowledge, similar comprehensive papers on such subject have not been recently published, and we hope that the current review may be interesting for research communities.

Small Produce Farm Environments Can Harbor Diverse Listeria monocytogenes and Listeria spp. Populations

ABSTRACT

A comprehensive understanding of foodborne pathogen diversity in preharvest environments is necessary to effectively track pathogens on farms and identify sources of produce contamination. As such, this study aimed to characterize Listeria diversity in wildlife feces and agricultural water collected from a New York state produce farm over a growing season. Water samples were collected from a pond (n = 80) and a stream (n = 52). Fecal samples (n = 77) were opportunistically collected from areas <5 m from the water sources; all samples were collected from a <0.5-km2 area. Overall, 86 (41%) and 50 (24%) of 209 samples were positive for Listeria monocytogenes and Listeria spp. (excluding L. monocytogenes), respectively. For each positive sample, one L. monocytogenes or Listeria spp. isolate was speciated by sequencing the sigB gene, thereby allowing for additional characterization based on the sigB allelic type. The 86 L. monocytogenes and 50 Listeria spp. isolates represented 8 and 23 different allelic types, respectively. A subset of L. monocytogenes isolates (n = 44) from pond water and pond-adjacent feces (representing an ∼5,000-m2 area) were further characterized by pulsed-field gel electrophoresis (PFGE); these 44 isolates represented 22 PFGE types, which is indicative of considerable diversity at a small spatial scale. Ten PFGE types were isolated more than once, suggesting persistence or reintroduction of PFGE types in this area. Given the small spatial scale, the prevalence of L. monocytogenes and Listeria spp., as well as the considerable diversity among isolates, suggests traceback investigations may be challenging. For example, traceback of finished product or processing facility contamination with specific subtypes to preharvest sources may require collection of large sample sets and characterization of a considerable number of isolates. Our data also support the adage "absence of evidence does not equal evidence of absence" as applies to L. monocytogenes traceback efforts at the preharvest level.

HIGHLIGHTS

Delivery of selective agents via time-delayed release tablets improves recovery of Listeria monocytogenes injured by acid and nitrite

Listeria selective enrichment media are designed to enhance the isolation of the organism and increase the chances of detection. Drawbacks include the requirements for prolonged sample incubation (48 to 72 h) and manual addition of selective agents, which may be a source of contamination. Modified Listeria recovery broth (mLRB) is a proprietary enrichment medium formulated to facilitate the recovery of injured cells; its selective agents are incorporated into a format that allows delayed release until 6 h of incubation. We evaluated the change in cell populations over time for acid- and nitrite-injured Listeria monocytogenes in mLRB with the selective agents added manually at 0 h (mLRBS0) and 6 h (mLRBS6). Recovery of injured cells in mLRB plus time-delayed tablets (mLRBTD) was also compared with that in enrichment media recommended by the U.S. Department of Agriculture (University of Vermont broth), the U.S. Food and Drug Administration (buffered Listeria enrichment broth), and the International Organization for Standardization (demi-Fraser broth). Nitrite- or acid-injured Listeria at approximately 10 CFU/ml were inoculated into each broth medium, and Listeria populations were enumerated at various times from 12 to 48 h of incubation at 37°C. Analysis of variance revealed that acid-injured Listeria populations in mLRBS6 at 24 h were significantly higher (P < 0.05) than those in mLRBS0; however, the differences in populations on these two media were not significant for nitrite-injured cells. Cell populations of four strains of Listeria inoculated into mLRBTD were significantly higher at 24 h than when those strains were enriched in buffered Listeria enrichment broth, demi-Fraser broth, and University of Vermont broth. Comparison between artificially contaminated milk and meat samples with a four-strain cocktail of Listeria resulted in cell populations that were significantly higher (P < 0.05) at 24 h on mLRBTD for contaminated meat than on mLRB for contaminated milk. Delivery of selective agents via time-delayed release tablets into mLRB maximizes recovery of acid- and nitrite-injured Listeria and saves analyst time during food sample analysis.

Development of a novel selective and differential medium for the isolation of Listeria monocytogenes

A new medium (lecithin and levofloxacin [LL] medium) is described for the isolation of Listeria monocytogenes from food samples. LL medium includes lecithin from soybeans for the detection of phosphatidylinositol-specific phospholipase C (PI-PLC) and phosphatidylcholine-specific phospholipase C (PC-PLC) produced by L. monocytogenes. Levofloxacin is incorporated to inhibit the growth of microorganisms other than L. monocytogenes, especially Bacillus cereus, shown to possess PI-PLC and PC-PLC activities. L. monocyogenes produced white colonies with a halo on LL medium, whereas Listeria innocua appeared as white colonies without a halo. Levofloxacin at 0.20 mg/liter completely inhibited the growth of B. cereus, while the growth of L. monocytogenes was unaffected. In the second phase of the study, the sensitivity and the specificity of LL medium were compared to those of modified Oxford agar (MOX) and two chromogenic media (Brilliance Listeria agar and CHROMagar Listeria), using a total of 250 food samples. From 200 unspiked food samples, the specificity of LL medium (96.0%) was superior to that of MOX (72.0%) and similar to the specificities of Brilliance Listeria agar (96.5%) and CHROMagar Listeria (94.5%). From 50 spiked food samples, LL medium and CHROMagar Listeria represented the highest sensitivities (96.0%), followed by Brilliance Listeria agar (92.0%) and MOX (54.0%). Also, LL medium showed the highest confirmation rate (98.8%), followed by Brilliance Listeria agar (98.7%), CHROMagar Listeria (98.3%), and MOX (52.0%). On the basis of its good specificity and cost effectiveness, LL medium is useful for the isolation of L. monocytogenes from food samples.

Silage collected from dairy farms harbors an abundance of listeriaphages with considerable host range and genome size diversity

Since the food-borne pathogen Listeria monocytogenes is common in dairy farm environments, it is likely that phages infecting this bacterium ("listeriaphages") are abundant on dairy farms. To better understand the ecology and diversity of listeriaphages on dairy farms and to develop a diverse phage collection for further studies, silage samples collected on two dairy farms were screened for L. monocytogenes and listeriaphages. While only 4.5% of silage samples tested positive for L. monocytogenes, 47.8% of samples were positive for listeriaphages, containing up to >1.5 × 10(4) PFU/g. Host range characterization of the 114 phage isolates obtained, with a reference set of 13 L. monocytogenes strains representing the nine major serotypes and four lineages, revealed considerable host range diversity; phage isolates were classified into nine lysis groups. While one serotype 3c strain was not lysed by any phage isolates, serotype 4 strains were highly susceptible to phages and were lysed by 63.2 to 88.6% of phages tested. Overall, 12.3% of phage isolates showed a narrow host range (lysing 1 to 5 strains), while 28.9% of phages represented broad host range (lysing ≥11 strains). Genome sizes of the phage isolates were estimated to range from approximately 26 to 140 kb. The extensive host range and genomic diversity of phages observed here suggest an important role of phages in the ecology of L. monocytogenes on dairy farms. In addition, the phage collection developed here has the potential to facilitate further development of phage-based biocontrol strategies (e.g., in silage) and other phage-based tools.

Diversity of Listeria species in urban and natural environments

A total of 442 Listeria isolates, including 234 Listeria seeligeri, 80 L. monocytogenes, 74 L. welshimeri, 50 L. innocua, and 4 L. marthii isolates, were obtained from 1,805 soil, water, and other environmental samples collected over 2 years from four urban areas and four areas representing natural environments. Listeria spp. showed similar prevalences in samples from natural (23.4%) and urban (22.3%) environments. While L. seeligeri and L. welshimeri were significantly associated with natural environments (P ≤ 0.0001), L. innocua and L. monocytogenes were significantly associated with urban environments (P ≤ 0.0001). Sequencing of sigB for all isolates revealed 67 allelic types with a higher level of allelic diversity among isolates from urban environments. Some Listeria spp. and sigB allelic types showed significant associations with specific urban and natural areas. Nearest-neighbor analyses also showed that certain Listeria spp. and sigB allelic types were spatially clustered within both natural and urban environments, and there was evidence that these species and allelic types persisted over time in specific areas. Our data show that members of the genus Listeria not only are common in urban and natural environments but also show species- and subtype-specific associations with different environments and areas. This indicates that Listeria species and subtypes within these species may show distinct ecological preferences, which suggests (i) that molecular source-tracking approaches can be developed for Listeria and (ii) that detection of some Listeria species may not be a good indicator for L. monocytogenes.

Longitudinal monitoring of Listeria monocytogenes contamination patterns in a farmstead dairy processing facility

Contamination of dairy products with Listeria monocytogenes is a concern because multiple human listeriosis outbreaks have been linked to contaminated cheese and dairy products. Dairy production on farmstead operations may be a particular concern because L. monocytogenes is also an animal pathogen that can be shed by ruminants with and without clinical symptoms; physical proximity between production animal and dairy processing facilities may thus provide a higher risk for introduction of L. monocytogenes into the dairy production process. To better understand the risks of L. monocytogenes contamination associated with farmstead dairy production, samples from a farmstead dairy processing operation and the milking barn of the directly adjacent dairy sheep operation were tested for L. monocytogenes over a 3-yr period. Prevalence of L. monocytogenes for samples collected on the farm (n = 85) and the dairy production facility (n = 674) was 9.4 and 2.7%, respectively. Molecular subtyping using automated EcoRI ribotyping of L. monocytogenes isolates revealed that distinct subtypes were associated with the dairy production facility and the farm's milking parlor. Although a total of 5 and 4 different ribotypes were identified among isolates obtained from the dairy production facility and the milking parlor, respectively, only 1 ribotype (DUP-1030A) was isolated from both. Different ribotypes were predominant among isolates from the dairy production facility (ribotype DUP-1052A, representing 15 of 18 isolates) and the farm's milking parlor (ribotype DUP-1039A, representing 4 of 8 isolates); each of these ribotypes appeared to persist over time in the respective area. Our data support that i) in farmstead dairy processing facilities, L. monocytogenes present on the farm can largely be prevented from being introduced into the processing facility; and ii) L. monocytogenes can persist on farm and in processing areas, providing a potential high-risk source for contamination. Preventing cross contamination between dairy production and processing facilities and control of persistent L. monocytogenes are thus critical to assuring the microbial safety of farmstead dairy products.

Toward an improved laboratory definition of Listeria monocytogenes virulence

Listeria monocytogenes is an opportunistic foodborne pathogen that encompasses a diversity of strains with varied virulence. The ability to rapidly determine the pathogenic potential of L. monocytogenes strains is integral to the control and prevention campaign against listeriosis. Early methods for assessing L. monocytogenes virulence include in vivo bioassays and in vitro cell assays. While in vivo bioassays provide a measurement of all virulence determinants of L. monocytogenes, they are not applied routinely due to their reliance on experimental animals whose costs have become increasingly prohibitive. As a low cost alternative, in vitro cell assays are useful for estimating the virulence of L. monocytogenes strains. However, these assays are often slow, and at times variable. Prior attempts to ascertain L. monocytogenes virulence by targeting virulence-associated proteins and genes have been largely unsuccessful, since many of the assay targets are present in both virulent and avirulent strains. Recent identification of novel virulence-specific genes (particularly internalin gene inlJ) has opened a new avenue for rapid, sensitive, and precise differentiation of virulent L. monocytogenes strains from avirulent strains. The application of DNA sequencing technique also offers an additional tool for assessing L. monocytogenes virulence potential. By providing an update on the laboratory methods that have been reported for the determination of L. monocytogenes pathogenicity, this review discusses future research needs that may help achieve an improved laboratory definition of L. monocytogenes virulence.

Listeria monocytogenes fecal shedding in dairy cattle shows high levels of day-to-day variation and includes outbreaks and sporadic cases of shedding of specific L. monocytogenes subtypes

Fecal shedding of Listeria monocytogenes poses a risk for contamination of animal feed and agricultural environments and raw food at the pre-harvest stages of food production. To be able to reduce these risks it is critical to improve understanding of the epidemiology of L. monocytogenes shedding in feces. The objective of this study was to assess the daily variability of fecal shedding and its association with individual animal (lactation number and the day of current lactation) and environmental (feed) risk factors. That was achieved by application of longitudinal daily sample collection in a herd of dairy cattle and molecular characterization of isolated L. monocytogenes. Fecal samples (25) and silage samples (2) were collected daily during two 2-week periods and one 5-day period. L. monocytogenes was isolated from 255 out of 825 (31%) fecal samples on 24 out of 33 (73%) days, and from 25 out of 66 (38%) silage samples on 16 out of 33 (48%) days. Ninety-four percent of cows excreted L. monocytogenes in feces at least once during the study period. Our data analyses indicated that (i) the prevalence and incidence risk of L. monocytogenes fecal shedding in cattle vary considerably over time, from 0 to 100%, and both are associated with contamination of silage, (ii) L. monocytogenes fecal shedding in cattle could occur as part of an outbreak or as an isolated sporadic case, (iii) L. monocytogenes subtypes associated with human infections are commonly isolated from cattle feces and silage, and (iv) a single cow can harbor more than one L. monocytogenes subtype on any given day. Although limited to a single dairy cattle herd, these findings provide a significant advancement in the understanding of the epidemiology of L. monocytogenes fecal shedding in dairy cattle.

Daily variability of Listeria contamination patterns in a cold-smoked salmon processing operation

An understanding of Listeria transmission and contamination patterns in processing environments of ready-to-eat foods is critical for improving control of Listeria monocytogenes. A cold-smoked fish processing operation was the site used to study variability in Listeria contamination in a processing environment associated with a ready-to-eat food product throughout one production week (five consecutive days). Intensive testing was conducted on finished products and environmental samples collected at the beginning, middle, and end of each working day. A total of 20 finished products and 22 to 36 environmental samples were collected at each sampling time, and an additional 12 environmental samples were collected on days 4 and 5. Overall, a total of 782 samples, 300 finished products and 482 environmental samples, were tested. All samples were collected from processing steps after smoking, including skinning, trimming, slicing, staging, and packing. A total of 28 finished and 57 environmental samples (9.3 and 11.8%, respectively) were positive for Listeria spp. (including 1 and 5 samples positive for L. monocytogenes, respectively). DNA sequencing of the sigB gene allowed differentiation of eight Listeria subtypes. Listeria prevalence varied significantly between days, and a high prevalence in both environmental samples and finished products on day 3 was likely associated with a point source contamination event by a single Listeria welshimeri subtype. There were no consistent differences in Listeria prevalence among samples collected from the beginning, middle, and end of the production day, but subtype data often revealed unique contamination patterns for samples collected at different times of a given day. Listeria contamination patterns and prevalences were highly variable between days and within a given day. These findings indicate that chance events play an important role in the contamination of finished products, thus complicating efforts to define Listeria transmission patterns in processing environments associated with ready-to-eat foods.

Molecular characterization of Listeria monocytogenes from natural and urban environments

Characterization of 80 Listeria monocytogenes isolates from urban and natural environments differentiated 7 and 26 EcoRI ribotypes, respectively. Whereas the majority of isolates from the natural environment represented L. monocytogenes lineage II (12 of 13 isolates), urban isolates grouped evenly into lineages I and II (32 and 33 isolates, respectively) and included two lineage III isolates. Multilocus sequence typing of all natural isolates and a randomly selected subset of 30 urban isolates showed a higher overall diversity (Simpson index of discrimination [D] of 0.987 and 0.920, respectively) than did EcoRI ribotyping (D = 0.872 and 0.911, respectively). Combined analysis with ribotype and lineage data for 414 isolates from farm sources, 165 isolates from foods and food-processing environments, and 342 human clinical isolates revealed that lineage I was significantly more common among human (P < 0.0001) isolates, whereas lineage II was more common among isolates from the natural environment, farms, and foods (P < or = 0.05). Among a total of 92 ribotypes, 31 showed significant associations with specific isolate sources. One ribotype (DUP-1039C) was significantly associated with both natural environments and farms. A spatial analysis showed a marginal association between locations in the natural environment positive for L. monocytogenes and a proximity to farms. Our data indicate that (i) L. monocytogenes strains from different sources show a high level of diversity; (ii) L. monocytogenes subtypes differ significantly in their associations with different environments, even though populations overlap; and (iii) a higher proportion of isolates from environmental sources than from human clinical cases can be classified into L. monocytogenes lineage II, which supports the classification of this lineage as an environmentally adapted subgroup.

Methods for the isolation and identification of Listeria spp. and Listeria monocytogenes: a review

Listeria monocytogenes is an important food-borne pathogen and is widely tested for in food, environmental and clinical samples. Identification traditionally involved culture methods based on selective enrichment and plating followed by the characterization of Listeria spp. based on colony morphology, sugar fermentation and haemolytic properties. These methods are the gold standard; but they are lengthy and may not be suitable for testing of foods with short shelf lives. As a result more rapid tests were developed based on antibodies (ELISA) or molecular techniques (PCR or DNA hybridization). While these tests possess equal sensitivity, they are rapid and allow testing to be completed within 48 h. More recently, molecular methods were developed that target RNA rather than DNA, such as RT-PCR, real time PCR or nucleic acid based sequence amplification (NASBA). These tests not only provide a measure of cell viability but they can also be used for quantitative analysis. In addition, a variety of tests are available for sub-species characterization, which are particularly useful in epidemiological investigations. Early typing methods differentiated isolates based on phenotypic markers, such as multilocus enzyme electrophoresis, phage typing and serotyping. These phenotypic typing methods are being replaced by molecular tests, which reflect genetic relationships between isolates and are more accurate. These new methods are currently mainly used in research but their considerable potential for routine testing in the future cannot be overlooked.

Molecular Subtyping and Tracking of Listeria monocytogenes in Latin-Style Fresh-Cheese Processing Plants

Latin-style fresh cheeses, which have been linked to at least 2 human listeriosis outbreaks in the United States, are considered to be high-risk foods for Listeria monocytogenes contamination. We evaluated L. monocytogenes contamination patterns in 3 Latin-style fresh-cheese processing plants to gain a better understanding of L. monocytogenes contamination sources in the manufacture of these cheeses. Over a 6-mo period, 246 environmental samples were collected and analyzed for L. monocytogenes using both the Food and Drug Administration (FDA) method and the Biosynth L. monocytogenes detection system (LMDS). Finished cheese samples from the same plants (n = 111) were also analyzed by the FDA method, which was modified to include L. monocytogenes plating medium (LMPM) and the L. monocytogenes confirmatory plating medium (LMCM) used in the LMDS method. Listeria monocytogenes was detected in 6.3% of cheese and 11.0% of environmental samples. Crates, drains, and floor samples showed the highest contamination rates, with 55.6, 30.0, and 20.6% L. monocytogenes positive samples, respectively. Finished products and food contact surfaces were positive in only one plant. The FDA method showed a higher sensitivity than the LMDS method for detection of L. monocytogenes from environmental samples. The addition of LMPM and LMCM media did not further enhance the performance of the FDA method for L. monocytogenes detection from finished products. Molecular subtyping (PCR-based allelic analysis of the virulence genes actA and hly and automated ribotyping) was used to track contamination patterns. Ribotype DUP-1044A, which had previously been linked to a 1998 multistate human listeriosis outbreak in the United States, was the most commonly identified subtype (20/36 isolates) and was isolated from 2 plants. This ribotype was persistent and widespread in one factory, where it was also responsible for the contamination of finished products. We hypothesize that this ribotype may represent a clonal group with a specific ability to persist in food processing environments. While previous listeriosis outbreaks were linked to Latin-style fresh cheeses made from unpasteurized milk, the presence of this organism in pasteurized cheese products illustrates that persistent environmental contamination also represents an important source of finished product contamination.

Longitudinal studies on Listeria in smoked fish plants: impact of intervention strategies on contamination patterns

Four ready-to-eat smoked fish plants were monitored for 2 years to study Listeria contamination patterns and the impact of plant-specific Listeria control strategies, including employee training and targeted sanitation procedures, on Listeria contamination patterns. Samples from the processing plant environment and from raw and finished product were collected monthly and tested for Listeria spp. and Listeria monocytogenes. Before implementation of intervention strategies, 19.2% of raw product samples (n = 276), 8.7% of finished product samples (n = 275), and 26.1% of environmental samples (n = 617) tested positive for Listeria spp. During and after implementation of Listeria control strategies, 19.0% of raw product samples (n = 242), 7.0% of finished product samples (n = 244), and 19.5% of environmental samples (n = 527) were positive for Listeria spp. In one of the four fish plants (plant 4), no environmental samples were positive for L. monocytogenes, and this plant was thus excluded from statistical analyses. Based on data pooled from plants 1, 2, and 3, environmental Listeria spp. prevalence was significantly lower (P < 0.05) for nonfood contact surfaces and the finished product area and for the overall core environmental samples after implementation of control strategies. Listeria prevalence for floor drains was similar before and after implementation of controls (49.6 and 54.2%, respectively). Regression analysis revealed a significant positive relationship (P < 0.05) between L. monocytogenes prevalence in the environment and in finished products before implementation of control strategies; however, this relationship was absolved by implementation of Listeria control strategies. Molecular subtyping (EcoRI ribotyping) revealed that specific L. monocytogenes ribotypes persisted in three processing plants over time. These persistent ribotypes were responsible for all six finished product contamination events detected in plant 1. Ribotype data also indicated that incoming raw material is only rarely a direct source of finished product contamination. While these data indicate that plant-specific Listeria control strategies can reduce cross-contamination and prevalence of Listeria spp. and L. monocytogenes in the plant environment, elimination of persistent L. monocytogenes strains remains a considerable challenge.

A review of conventional detection and enumeration methods for pathogenic bacteria in food

With continued development of novel molecular-based technologies for rapid, high-throughput detection of foodborne pathogenic bacteria, the future of conventional microbiological methods such as viable cell enumeration, selective isolation of bacteria on commercial media, and immunoassays seems tenuous. In fact, a number of unique approaches and variations on existing techniques are currently on the market or are being implemented that offer ease of use, reliability, and low cost compared with molecular tools. Approaches that enhance recovery of sublethally injured bacteria, differentiation among species using fluorogenics or chromogenics, dry plate culturing, differentiation among bacteria of interest using biochemical profiling, enumeration using impedence technology, techniques to confirm the presence of target pathogens using immunological methods, and bioluminescence applications for hygiene monitoring are summarized here and discussed in relation to their specific advantages or disadvantages when implemented in a food microbiology setting.Key words: food pathogen, detection, enumeration methods, food safety.

New chromogenic plating media for detection and enumeration of pathogenic Listeria spp.—an overview

In recent years a number of selective chromogenic plating media for pathogenic Listeria spp. have been developed and marketed. Their advantages are direct detection and enumeration of pathogenic Listeria spp. utilizing cleavage of substrates by the virulence factor phosphatidylinositol-phospholipase C (PI-PLC) and, to a lesser extent, by phosphatidylcholin-phospholipase C (PC-PLC). There are two groups of such media: the first utilizes cleavage by PI-PLC of L-alpha-phosphatidyl-inositol, forming a white precipitation zone around the colony, combined with the chromogenic substrate 5-bromo-4-chloro-3-indoxyl-beta-D-glucopyranoside for detection of beta-d-glucosidase, which occurs in all Listeria spp. All Listeria spp. produce turquoise colonies on these media which include ALOA , CHROMagar Listeria, BBL CHROMagar Listeria, and OCLA. The second group of media utilizes 5-bromo-4-chloro-3-indoxyl-myoinositol-1-phosphate, forming blue-turquoise colonies of pathogenic Listeria spp. and white colonies of non-pathogenic Listeria spp. BCM trade mark Listeria monocytogenes plating medium, Rapid'L.mono and LIMONO-Ident-Agar belong to this group. Selective chromogenic L. monocytogenes plating media offer the attraction of rapid economic detection and enumeration of pathogenic Listeria spp. within 24 or 48 h of incubation at 36+/-1 degrees C. This overview summarises the characteristics of these chromogenic plating media, reviews important evaluations, and focuses on replacement of conventional by these chromogenic plating media, particularly for applications in the food industry.

Ecology and transmission of Listeria monocytogenes infecting ruminants and in the farm environment

A case-control study involving 24 case farms with at least one recent case of listeriosis and 28 matched control farms with no listeriosis cases was conducted to probe the transmission and ecology of Listeria monocytogenes on farms. A total of 528 fecal, 516 feed, and 1,012 environmental soil and water samples were cultured for L. monocytogenes. While the overall prevalence of L. monocytogenes in cattle case farms (24.4%) was similar to that in control farms (20.2%), small-ruminant (goat and sheep) farms showed a significantly (P < 0.0001) higher prevalence in case farms (32.9%) than in control farms (5.9%). EcoRI ribotyping of clinical (n = 17) and farm (n = 414) isolates differentiated 51 ribotypes. L. monocytogenes ribotypes isolated from clinical cases and fecal samples were more frequent in environmental than in feed samples, indicating that infected animals may contribute to L. monocytogenes dispersal into the farm environment. Ribotype DUP-1038B was significantly (P < 0.05) associated with fecal samples compared with farm environment and animal feedstuff samples. Ribotype DUP-1045A was significantly (P < 0.05) associated with soil compared to feces and with control farms compared to case farms. Our data indicate that (i) the epidemiology and transmission of L. monocytogenes differ between small-ruminant and cattle farms; (ii) cattle contribute to amplification and dispersal of L. monocytogenes into the farm environment, (iii) the bovine farm ecosystem maintains a high prevalence of L. monocytogenes, including subtypes linked to human listeriosis cases and outbreaks, and (iv) L. monocytogenes subtypes may differ in their abilities to infect animals and to survive in farm environments.

Natural atypical Listeria innocua strains with Listeria monocytogenes pathogenicity island 1 genes

Identification of bona fide Listeria isolates into the six species of the genus normally requires only a few tests. Aberrant isolates do occur, but even then only one or two extra confirmatory tests are generally needed for identification to species level. We have discovered a hemolytic-positive, rhamnose and xylose fermentation-negative Listeria strain with surprising recalcitrance to identification to the species level due to contradictory results in standard confirmatory tests. The issue had to be resolved by using total DNA-DNA hybridization testing and then confirmed by further specific PCR-based tests including a Listeria microarray assay. The results show that this isolate is indeed a novel one. Its discovery provides the first fully documented instance of a hemolytic Listeria innocua strain. This species, by definition, is typically nonhemolytic. The L. innocua isolate contains all the members of the PrfA-regulated virulence gene cluster (Listeria pathogenicity island 1) of L. monocytogenes. It is avirulent in the mouse pathogenicity test. Avirulence is likely at least partly due to the absence of the L. monocytogenes-specific allele of iap, as well as the absence of inlA, inlB, inlC, and daaA. At least two of the virulence cluster genes, hly and plcA, which encode the L. monocytogenes hemolysin (listeriolysin O) and inositol-specific phospholipase C, respectively, are phenotypically expressed in this L. innocua strain. The detection by PCR assays of specific L. innocua genes (lin0198, lin0372, lin0419, lin0558, lin1068, lin1073, lin1074, lin2454, and lin2693) and noncoding intergenic regions (lin0454-lin0455 and nadA-lin2134) in the strain is consistent with its L. innocua DNA-DNA hybridization identity. Additional distinctly different hemolytic L. innocua strains were also studied.

Impact of intervention strategies on Listeria contamination patterns in crawfish processing plants: a longitudinal study

Two ready-to-eat crawfish processing plants were monitored for 2 years to study the impact of Listeria control strategies, including employee training and targeted sanitation procedures, on Listeria contamination. Environmental, raw material, and finished product samples were collected weekly during the main processing months (April to June) and tested for Listeria spp. and Listeria monocytogenes. Before implementation of control strategies (year 1), the two processing plants showed Listeria spp. prevalences of 29.5% (n = 78) in raw, whole crawfish, 5.2% (n = 155) in the processing plant environment, and 0% (n = 78) in finished products. In year 2, after plant-specific Listeria control strategies were implemented, Listeria spp. prevalence increased in raw crawfish (57.5%, n = 101), in the processing plant environment (10.8%, n = 204), and in the finished product (1.0%, n = 102). Statistical analysis showed a significant increase in Listeria spp. prevalence (P < 0.0001) and a borderline nonsignificant increase in L. monocytogenes prevalence (P = 0.097) on raw material in year 2. Borderline nonsignificant increases were also observed for Listeria spp. prevalence in environmental samples (P = 0.082). Our data showed that Listeria spp. prevalence in raw crawfish can vary significantly among seasons. However, the increased contamination prevalence for raw materials only resulted in a limited Listeria prevalence increase for the processing plant environment with extremely low levels of finished product contamination. Heat treatment of raw materials combined with Listeria control strategies to prevent cross-contamination thus appears to be effective in achieving low levels of finished product contamination, even with Listeria spp. prevalences for raw crawfish of more than 50%.

Prevalence and growth of Listeria on naturally contaminated smoked salmon over 28 days of storage at 4 degrees C

Only limited data are available on the growth characteristics of Listeria in naturally contaminated ready-to-eat foods. To evaluate Listeria contamination patterns and growth in smoked salmon, 72 smoked salmon product samples from two processing plants were tested for Listeria spp. and L. monocytogenes. Samples were divided into four approximately equal portions: one portion was tested on receipt, and the other three were vacuum sealed and stored at 4 degrees C for 7, 14, and 28 days. Listeria testing was performed using both an enrichment procedure and direct plating to enumerate Listeria in samples that contained >2 to 10 CFU/g. Five samples were positive for Listeria spp., including one sample that was positive for L. monocytogenes. Most samples yielded only sporadic positive results among the portions tested on days 0, 7, 14, and 28. Only one sample contained Listeria spp. in numbers above the detection limit for enumeration. For this sample, the portions tested on days 7 and 28 contained 46 and 52 CFU/g, respectively, whereas the portion tested on day 14 was negative. Overall, our data indicate that there is considerable heterogeneity in Listeria spp. distribution within a single positive smoked fish sample. Even with refrigerated storage for 28 days, none of the naturally contaminated samples reached Listeria spp. numbers >100 CFU/g, which indicates that Listeria growth was limited within a 4-week storage period. However, because of the apparent heterogeneity of Listeria distribution within samples, the interpretation of growth data collected on naturally contaminated samples is difficult.

Tracking of Listeria monocytogenes in smoked fish processing plants

Four smoked fish processing plants were used as a model system to characterize Listeria monocytogenes contamination patterns in ready-to-eat food production environments. Each of the four plants was sampled monthly for approximately 1 year. At each sampling, four to six raw fish and four to six finished product samples were collected from corresponding lots. In addition, 12 to 14 environmental sponge samples were collected several hours after the start of production at sites selected as being likely contamination sources. A total of 234 raw fish, 233 finished products, and 553 environmental samples were tested. Presumptive Listeria spp. were isolated from 16.7% of the raw fish samples, 9.0% of the finished product samples, and 27.3% of the environmental samples. L. monocytogenes was isolated from 3.8% of the raw fish samples (0 to 10%, depending on the plant), 1.3% of the finished product samples (0 to 3.3%), and 12.8% of the environmental samples (0 to 29.8%). Among the environmental samples, L. monocytogenes was found in 23.7% of the samples taken from drains, 4.8% of the samples taken from food contact surfaces, 10.4% of the samples taken from employee contact surfaces (aprons, hands, and door handles), and 12.3% of the samples taken from other nonfood contact surfaces. Listeria spp. were isolated from environmental samples in each of the four plants, whereas L. monocytogenes was not found in any of the environmental samples from one plant. Overall, the L. monocytogenes prevalence in the plant environment showed a statistically significant (P < 0.0001) positive relationship with the prevalence of this organism in finished product samples. Automated EcoRI ribotyping differentiated 15 ribotypes among the 83 L. monocytogenes isolates. For each of the three plants with L. monocytogenes-positive environmental samples, one or two ribotypes seemed to persist in the plant environment during the study period. In one plant, a specific L. monocytogenes ribotype represented 44% of the L. monocytogenes-positive environmental samples and was also responsible for one of the two finished product positives found in this plant. In another plant, a specific L. monocytogenes ribotype persisted in the raw fish handling area. However, this ribotype was never isolated from the finished product area in this plant, indicating that this operation has achieved effective separation of raw and finished product areas. Molecular subtyping methods can help identify plant-specific L. monocytogenes contamination routes and thus provide the knowledge needed to implement improved L. monocytogenes control strategies.

Listeria monocytogenes: diagnostic problems

The first isolation methods for the detection of Listeria spp. were generally based on the direct culture of samples on simple agar media, but isolation of the pathogenic Listeria monocytogenes was difficult. In time, new techniques were developed, based on a variety of selective and elective agents in isolation and enrichment media, which gained better and quicker results. Current reference methods allow the recovery of L. monocytogenes from a variety of foods with relative ease. However, more comparative studies are needed to select one horizontal method. It is suggested that the procedure of the International Organization for Standardization is a good base for such comparisons.

Evaluation and interlaboratory validation of a selective agar for phosphatidylinositol-specific phospholipase C activity using a chromogenic substrate to detect Listeria monocytogenes from foods

Phosphatidylinositol-specific phospholipase C (PI-PLC) activity is a potential virulence factor and is exhibited only by the Listeria species Listeria monocytogenes and Listeria ivanovii. A chromogenic substrate for the direct detection of PI-PLC activity is available in a new medium (BCM L. monocytogenes plating agar). The use of a chromogenic substrate offers a mechanism with which to directly screen for L. monocytogenes and L. ivanovii other than the esculin used in Oxford (OXF) and Palcam (PAL) agars, which screen for all Listeria species. The specificity levels of BCM plating agar and of BCM confirmation and rhamnose agars were evaluated with 107 Listeria and 10 Bacillus species isolates. In addition, BCM L. monocytogenes plating agar was compared with standard Listeria selective agars (OXF and PAL agars) with regard to the recovery of L. monocytogenes from 2,000 food and environmental samples obtained from eight participating laboratories. A Listeria species was isolated from at least one of the agars in 209 analyses, and L. monocytogenes was isolated in 135 of these analyses. In 27 of the analyses in which L. monocytogenes was isolated, one or more of the selective differential agars used failed to isolate L. monocytogenes, and therefore the results of these analyses were discrepant. Relative to a reference method involving the use of all three agars (OXF, PAL, and BCM agars), the OXF-BCM, PAL-BCM, and OXF-PAL combinations had sensitivities of 99.3, 99.2, and 90.2%, respectively. In statistical analyses of the different combinations of agars, the OXF-BCM and BCM-PAL combinations were found to be superior to the OXF-PAL combination for the detection of L. monocytogenes.

Serotyping of Listeria monocytogenes by enzyme-linked immunosorbent assay and identification of mixed-serotype cultures by colony immunoblotting

Routine analysis of Listeria monocytogenes by serotyping using traditional agglutination methods is limited in use because of the expense and limited availability of commercially prepared antisera and intra- and interlaboratory discrepancies arising from differences in antiserum preparation and visual determination of agglutination. We have adapted a commercially available set of L. monocytogenes antisera to an enzyme-linked immunosorbent assay (ELISA) format for high-throughput, low-cost serotype determination. Rather than subjective visualization of agglutination, positive antigen and antiserum reactions were scored by a quantitative, colorimetric reaction. ELISA serotyping of 89 of 101 L. monocytogenes isolates agreed with slide agglutination serotyping data, and 100 previously uncharacterized isolates were serotyped unambiguously by the ELISA method. In addition, mixed-serotype cultures of L. monocytogenes were identified by a colony immunoblot procedure, in which serogroup 1/2 and serogroup 4 colonies were discriminated by differential staining.

Listeria monocytogenes contamination patterns for the smoked fish processing environment and for raw fish

Reliable data on the sources of Listeria monocytogenes contamination in cold-smoked fish processing are crucial in designing effective intervention strategies. Environmental samples (n = 512) and raw fish samples (n = 315) from two smoked fish processing facilities were screened for L. monocytogenes, and all isolates were subtyped by automated ribotyping to examine the relationship between L. monocytogenes contamination from raw materials and that from environmental sites. Samples were collected over two 8-week periods in early spring and summer. The five types of raw fish tested included lake whitefish, sablefish, farm-raised Norwegian salmon, farm-raised Chilean salmon, and feral (wild-caught) salmon from the U.S. West Coast. One hundred fifteen environmental samples and 46 raw fish samples tested positive for L. monocytogenes. Prevalence values for environmental samples varied significantly (P < 0.0001) between the two plants; plant A had a prevalence value of 43.8% (112 of 256 samples), and plant B had a value of 1.2% (3 of 256 samples). For plant A, 62.5% of drain samples tested positive for L. monocytogenes, compared with 32.3% of samples collected from other environmental sites and 3.1% of samples collected from food contact surfaces. Ribotyping identified 11 subtypes present in the plant environments. Multiple subtypes, including four subtypes not found on any raw fish, were found to persist in plant A throughout the study. Contamination prevalence values for raw fish varied from 3.6% (sablefish) to 29.5% (U.S. West Coast salmon), with an average overall prevalence of 14.6%. Sixteen separate L. monocytogenes subtypes were present on raw fish, including nine that were not found in the plant environment. Our results indicate a disparity between the subtypes found on raw fish and those found in the processing environment. We thus conclude that environmental contamination is largely separate from that of incoming raw materials and includes strains persisting, possibly for years, within the plant. Operational and sanitation procedures appear to have a significant impact on environmental contamination, with both plants having similar prevalence values for raw materials but disparate contamination prevalence values for the environmental sites. We also conclude that regular L. monocyrogenes testing of drains, combined with molecular subtyping of the isolates obtained, allows for efficient monitoring of persistent L. monocytogenes contamination in a processing plant.

Detection of Listeria in crawfish processing plants and in raw, whole crawfish and processed crawfish (Procambarus spp.)

The foodborne pathogen Listeria monocytogenes represents a major concern to the food industry and particularly to producers of ready-to-eat (RTE) foods because of the severity of human listeriosis infections and because of the ubiquitous nature of this organism. Although several studies on the prevalence and sources of L monocytogenes in various RTE seafoods have been conducted, limited information is available on the presence and potential sources of this organism in RTE crawfish products. We thus monitored the presence of L monocytogenes and other Listeria spp. in the processing environment, in raw, whole crawfish, and in cooked crawfish meat from two processing plants. Samples were collected from the two plants throughout one crawfish season (April to June 2001) at 5 and 8 separate visits, respectively. At each visit, 6 raw, whole crawfish, 6 finished product samples (crawfish meat), and 14 mid- or end-of-processing environmental sponge samples were collected and tested for L. monocytogenes and Listeria spp. Of the 337 samples tested, 31 contained Listeria spp. Although Listeria innocua was the predominant Listeria spp. found (20 samples), four samples were positive for L monocytogenes. L. monocytogenes was detected in three raw material samples and in one environmental sample. Listeria spp. were found in 29.5% of raw, whole crawfish (n = 78) and in 4.4% of environmental samples (n = 181) but in none of the finished product samples. Among the environmental samples, Listeria spp. were found in 15.4% of the drains (n = 39) and in 5.1% of the employee contact surfaces (gloves and aprons) (n = 39) but in none of the samples from food contact surfaces. Even though a high prevalence of Listeria spp. was detected on raw materials, it appears that the heat treatment during the processing of crawfish and the practices preventing postprocessing recontamination can significantly reduce Listeria contamination of RTE crawfish meat.

Comparison of growth kinetics for healthy and heat-injured Listeria monocytogenes in eight enrichment broths

Detection of Listeria in food products is often limited by performance of enrichment media used to support growth of Listeria to detectable levels. In this study, growth curves were generated using healthy and heat-injured Listeria monocytogenes strain F5069 in three nonselective and five selective enrichment broths. Nonselective enrichment media included the current Food and Drug Administration Bacteriological Analytical Manual Listeria enrichment broth base (BAM), Listeria repair broth (LRB), and Trypticase soy broth. Selective enrichment media included BAM with selective agents and LRB with selective agents, BCM L. monocytogenes preenrichment broth, Fraser broth, and UVM-modified Listeria enrichment broth. The Gompertz equation was used to model the growth of L. monocytogenes. Gompertz parameters were used to calculate exponential growth rate, lag-phase duration (LPD), generation time, maximum population density (MPD), and time required for repair of injured cells. Statistical differences (P < 0.05) in broth performance were noted for LPD and MPD when healthy and injured cells were inoculated into the broths. With the exception of Fraser broth, there were no significant differences in the time required for the repair of injured cells. Results indicate that the distinction between selective and nonselective broths in their ability to grow healthy Listeria and to repair sublethally injured cells is not solely an elementary issue of presence or absence of selective agents.

Modelling the competitive growth of Listeria monocytogenes and Listeria innocua in enrichment broths

The overgrowth of Listeria innocua in enrichment broths designed for the isolation of Listeria monocytogenes is believed to result from two factors: a selective growth advantage of L. innocua, and/or an inhibitory interspecies interaction. The generation times of 13 isolates of L. innocua and L. monocytogenes were determined in Brain Heart Infusion (BHI) and a variety of enrichment media. No significant differences were found in growth characteristics between either species in the various media, suggesting that the growth advantage of L. innocua in enrichment media was not as significant as previously described. Kinetic analysis of mixed cultures of L. monocytogenes and isolates of L. innocua producing a variety of inhibitory activities demonstrated the possibility of an inhibitory interaction between these two species resulting in the overgrowth of the enrichment culture with L. innocua. Modelling the evolution of the ratio between two populations in an enrichment process was used to analyze the impact of a selective growth advantage in L. innocua in an enrichment process for growth of L. monocytogenes. These findings support the widely held view that an overgrowth of L. innocua in the enrichment process can result from both a selective growth advantage as well as the production of inhibitory compounds. From a practical perspective, these interactions can result in an increase in false negatives.

Comparative evaluation of culture- and BAX polymerase chain reaction-based detection methods for Listeria spp. and Listeria monocytogenes in environmental and raw fish samples

Two commercial polymerase chain reaction (PCR)-based Listeria detection systems, the BAX for Screening/Listeria monocytogenes and the BAX for Screening/Genus Listeria, and a culture-based detection system, the Biosynth L. monocytogenes Detection System (LMDS), were evaluated for their ability to detect L. monocytogenes and Listeria spp. in raw ingredients and the processing environment. For detection of L. monocytogenes from raw fish, enrichment was performed in Listeria enrichment broth (LEB), followed by plating on both Oxford agar and LMDS L. monocytogenes plating medium (LMPM). Detection of Listeria and L. monocytogenes from environmental samples was performed using LMDS enrichment medium, followed by plating on both Oxford agar and LMPM. A total of 512 environmental samples and 315 raw fish were taken from two smoked fish processing facilities and screened using these molecular and cultural Listeria detection methods. The BAX for Screening/L monocytogenes was used to screen raw fish and was 84.8% sensitive and 100% specific. The BAX for Screening/Genus Listeria was evaluated on environmental samples and had 94.7% sensitivity and 97.4% specificity. In conjunction with enrichment in LEB, LMPM had a sensitivity and specificity for detection of L. monocytogenes from raw fish of 97.8 and 100%, respectively. Use of LMDS enrichment medium followed by plating on LMPM allowed for sensitivity and specificity rates of 94.8 and 100%, respectively, for detection of L. monocytogenes from environmental samples. We conclude that both the BAX systems and the use of LMPM allow for reliable and rapid detection of Listeria spp. and L. monocytogenes. While the BAX systems provide screening results in about 3 days, the use of LMPM allows for L. monocytogenes isolation in 4 to 5 days.

Incidence and characterization of Listeria spp. from foods available in Korea

A total of 410 domestic Korean food samples were analyzed for the presence of Listeria spp. by the conventional U.S. Department of Agriculture protocol, and presumptive strains were identified by morphological, cultural and biochemical tests according to Bergey's manual and confirmed by API-Listeria kit. Among the total 410 food samples, 46 samples (11.2%) were found to be contaminated with Listeria species. Among the 46 strains of Listeria spp. isolates, 8 strains (17.42%) for Listeria monocytogenes, 3 strains (6.5%) for Listeria seeligeri, 33 strains (71.7%) for Listeria innocua, and 2 strains (4.4%) for Listeria welshimeri were identified, respectively. Also, only beef, chicken, pork, frozen foods, and sausage were contaminated with L. monocytogenes, and the other products were free of L. monocytogenes. Of 46 Listeria spp. isolates, L. innocua (71.7%) was the most predominantly isolated in a variety of foods compared to other Listeria spp. An in vitro virulence assay for Listeria spp. using myeloma and hybridoma cells from murine and human sources was performed. The result showed that only L. monocytogenes killed approximately 95 to 100% hybridoma cells after 6 h and the other Listeria species, such as L. innocua, L. seeligeri, and L. welshimeri strains had about 0 to 10% lethal effect on hybridoma cells. Also, an antibiotic susceptibility test showed that Listeria spp. isolates were very susceptible to the antibiotics tested, except for nalidixic acid. Also, serotyping results showed 75% of L. monocytogenes isolates from beef, chicken, and frozen pizza belonged to serotype 1 and 25% from sausage were type 4.

Synthesis of fluorogenic substrates for continuous assay of phosphatidylinositol-specific phospholipase C

An improved synthesis of fluorogenic substrate analogues for phosphatidylinositol-specific phospholipase C (PI-PLC) is described. The water-soluble substrates, which are derived from fluorescein, are not fluorescent until cleaved by the enzyme, and provide a convenient means to continuously monitor PI-PLC activity. The improvement in the synthesis lies in the method used to protect the hydroxyl groups of the inositol portion of the substrate molecule and allows a milder deprotection procedure to be used. The result is a much more reproducible synthesis of the substrate. The improved procedure has been employed to synthesize a series of fluorogenic substrates, which differ in the length of the aliphatic tail attached to the fluorescein portion of the molecule. The length of the tail was found to have a significant effect on the rate of cleavage of these substrates.

New developments in chromogenic and fluorogenic culture media

This review describes some recent developments in chromogenic and fluorogenic culture media in microbiological diagnostic. The detection of beta-D-glucuronidase (GUD) activity for enumeration of Escherichia coli is well known. E. coli O157:H7 strains are usually GUD-negative and do not ferment sorbitol. These characteristics are used in selective media for these organisms and new chromogenic media are available. Some of the new chromogenic media make the Salmonella diagnostic easier and faster. The use of chromogenic and fluorogenic substrates for detection of beta-D-glucosidase (beta-GLU) activity to differentiate enterococci has received considerable attention and new media are described. Rapid detection of Clostridium perfringens, Listeria monocytogenes, Bacillus cereus and Staphylococcus aureus are other application of enzyme detection methods in food and water microbiology.

Application of a chromogenic medium and the PCR method for the rapid confirmation of L. monocytogenes in foodstuffs

Detection of Listeria monocytogenes in foodstuffs by conventional cultivation methods carried out according to EN ISO guidelines is rather time-consuming. Therefore, two alternative methods were applied for rapid confirmation of L. monocytogenes in foodstuffs. Inoculum from liquid selective broth was plated on PALCAM and OXFORD agar and on chromogenic agar medium RAPID L. mono. Suspect colonies from PALCAM were confirmed according to EN ISO standards and by the multiplex PCR method. In total, 990 samples of foodstuffs were investigated and 63 strains of L. monocytogenes were isolated. The chromogenic medium RAPID L. mono provided results comparable to PCR, it is easier to handle and provides considerable financial savings.