Recovery Of Listeria Monocytogenes Research Articles

Impact of food residue on recovery of Listeria monocytogenes and Salmonella Typhimurium from stainless steel surface

AbstractThis study evaluated the impact of different food residues on the recovery of Listeria monocytogenes and Salmonella Typhimurium from stainless steel surfaces. Food residues tested include lettuce rinsate, blended lettuce, low‐fat milk, and whole milk for Listeria monocytogenes, and powdered infant formula, all‐purpose flour, and whole milk dairy powder for Salmonella Typhimurium. Bacterial suspensions were inoculated on stainless steel surfaces with or without food residues and held for different time periods. Significant differences (P < 0.0001) in Salmonella Typhimurium recovery were observed between the no food residue control and all food residues over 24 hours. For Listeria monocytogenes, minimal variability in recovery was observed among food residue types, with significant differences from the no food residue control (P < 0.05) observed after 24 hours. The study also found that surface sampling can spread Listeria monocytogenes and Salmonella Typhimurium on stainless steel surfaces, suggesting that food residue type may affect microbial recovery during environmental monitoring.

Effect of Time, Temperature, and Transport Media on the Recovery of Listeria monocytogenes from Environmental Swabs

Environmental monitoring for Listeria monocytogenes in food processing environments is key for ensuring the safety of ready-to-eat foods. For sampling, swabs are often hydrated with a wetting or transport medium that may contain neutralizers and other ingredients. After swabbing the environment, the swabs may then be transported or shipped cold to an off-site laboratory for testing, ideally within 48 h. Extended shipping times may subject the pathogen to increased temperatures in the presence of the wetting medium, organics, and other chemicals from the processing facility that could confound detection. This study evaluated growth and detection of L. monocytogenes on stainless steel exposed to either buffer or sodium hypochlorite before drying. Swabs were rehydrated with Butterfield's phosphate buffer, neutralizing buffer, Letheen broth, or Dey-Engley neutralizing broth before swabbing. Swabs were stored in the presence of no added food, cheese whey, or ice cream under both optimal (4°C) and suboptimal (15°C) temperatures for up to 72 h. Overall, there was no growth of L. monocytogenes at 4°C through 72 h of storage, although enrichment from these swabs was dependent on the presence and type of food matrix. Pathogen growth during storage at 15°C was more variable and depended on both the food matrix and transport media used, with Dey-Engley and Letheen broths allowing for the highest population increases. Overall, more enrichments resulting in L. monocytogenes detections were observed when using Letheen broth and neutralizing buffer than Dey-Engley broth, which resulted in fewer detections at 15°C. Logistic regression and Cochran-Mantel-Haenszel analyses determined that storage temperature, transport media, and food matrix all significantly affected detection of L. monocytogenes, whereas storage time did not have a clear effect on recovery from swabs.

High-pressure processing-induced transcriptome response during recovery of Listeria monocytogenes

BackgroundHigh-pressure processing (HPP) is a commonly used technique in the food industry to inactivate pathogens, including L. monocytogenes. It has been shown that L. monocytogenes is able to recover from HPP injuries and can start to grow again during long-term cold storage. To date, the gene expression profiling of L. monocytogenes during HPP damage recovery at cooling temperature has not been studied. In order identify key genes that play a role in recovery of the damage caused by HPP treatment, we performed RNA-sequencing (RNA-seq) for two L. monocytogenes strains (barotolerant RO15 and barosensitive ScottA) at nine selected time points (up to 48 h) after treatment with two pressure levels (200 and 400 MPa).ResultsThe results showed that a general stress response was activated by SigB after HPP treatment. In addition, the phosphotransferase system (PTS; mostly fructose-, mannose-, galactitol-, cellobiose-, and ascorbate-specific PTS systems), protein folding, and cobalamin biosynthesis were the most upregulated genes during HPP damage recovery. We observed that cell-division-related genes (divIC, dicIVA, ftsE, and ftsX) were downregulated. By contrast, peptidoglycan-synthesis genes (murG, murC, and pbp2A) were upregulated. This indicates that cell-wall repair occurs as a part of HPP damage recovery. We also observed that prophage genes, including anti-CRISPR genes, were induced by HPP. Interestingly, a large amount of RNA-seq data (up to 85%) was mapped to Rli47, which is a non-coding RNA that is upregulated after HPP. Thus, we predicted that Rli47 plays a role in HPP damage recovery in L. monocytogenes. Moreover, gene-deletion experiments showed that amongst peptidoglycan biosynthesis genes, pbp2A mutants are more sensitive to HPP.ConclusionsWe identified several genes and mechanisms that may play a role in recovery from HPP damage of L. monocytogenes. Our study contributes to new information on pathogen inactivation by HPP.

Sublethal injury and recovery of Listeria monocytogenes and Escherichia coli O157:H7 after exposure to slightly acidic electrolyzed water

Foodborne pathogens can survive as the state of sublethal injury in adverse environments and recover physiological function and virulence under suitable conditions. Slightly acidic electrolyzed water (SAEW) is a non-thermal treatment to decontaminate in food industry and can induce sublethal injury. However, sublethal injury induced by SAEW and recoveries of different pathogens have not been fully investigated. In this study, it was observed that sublethally injured L. monocytogenes and E. coli O157:H7 cells widely persisted after exposure to different SAEW treatments (both of two pathogens: volumes of SAEW 1–10 ml, concentrations of SAEW 25%–100%; L. monocytogenes: treatment time 30–180 s, E. coli O157:H7: treatment time 15–90 s). And the highest recovery ratio of injured L. monocytogenes or E. coli O157:H7 was observed in TSB-YE or TSB and these cells were completely recovered within 60 min or 2.5 h, respectively. The recovery ratio of SAEW-injured L. monocytogenes and E. coli O157:H7 increased as recovery temperature increasing (4–37 °C). Mg, Ca and Zn cations significantly improved the recovery ratios of both two pathogens, but Fe and Mn cations had negative effects on recovery. Moreover, the concentrations of cations (0.5–8 mM) affected repair. The most appropriate concentration of Mg, Ca or Zn cations improving the repair of injured L. monocytogenes was 1 mM, 8 mM or 0.5 mM, respectively. The recovery ratio of injured E. coli O157:H7 increased with the concentrations of Mg, Ca and Zn cations increasing. The understanding of SAEW-injured and resuscitated conditions of L. monocytogenes and E. coli O157:H7 can avoid the occurrence of injured cells in food processing and develop a medium for detecting injured pathogens.

Evaluation of TA10 Broth for Recovery of Listeria monocytogenes from Ground Beef.

In 2009, the enrichment broth TA10 was released for simultaneous recovery of Salmonella spp., Listeria monocytogenes, and Escherichia coli O157:H7. This medium was compared with other Salmonella enrichment broths [lactose (LAC) broth, buffered peptone water (BPW), and universal pre-enrichment (UP) broth] for the recovery of heat- and freeze-injured Salmonella spp. in beef by the conventional culture method. There was a significant difference between TA10 and LAC enrichment broths for detecting injured Salmonella spp. In this study, the International Organization for Standardization Listeria pre-enrichment broth (Half-Fraser/Fraser) was compared with TA10 broth for the recovery of L. monocytogenes from ground beef. Ground beef samples were contaminated with single Listeria serovars at levels of 0.096 to 0.001 most probable number/g. Twenty 25 g test portions of the contaminated ground beef were pre-enriched in each broth, and the ISO-11290-1 Listeria official isolation protocol was used thereafter. There was a significant difference between TA10 broth (48 h) and Half-Fraser/Fraser broth (72 h) in the recovery of L. monocytogenes. In addition, the incubation time for TA10 broth was shorter than for Half-Fraser/Fraser broth. The results indicate that TA10 broth should be used instead of Half-Fraser/Fraser broth for analysis of beef that may be contaminated with very low levels of L. monocytogenes.

Enrichment dynamics of Listeria monocytogenes and the associated microbiome from naturally contaminated ice cream linked to a listeriosis outbreak.

BackgroundMicrobiota that co-enrich during efforts to recover pathogens from foodborne outbreaks interfere with efficient detection and recovery. Here, dynamics of co-enriching microbiota during recovery of Listeria monocytogenes from naturally contaminated ice cream samples linked to an outbreak are described for three different initial enrichment formulations used by the Food and Drug Administration (FDA), the International Organization of Standardization (ISO), and the United States Department of Agriculture (USDA). Enrichment cultures were analyzed using DNA extraction and sequencing from samples taken every 4 h throughout 48 h of enrichment. Resphera Insight and CosmosID analysis tools were employed for high-resolution profiling of 16S rRNA amplicons and whole genome shotgun data, respectively.ResultsDuring enrichment, other bacterial taxa were identified, including Anoxybacillus, Geobacillus, Serratia, Pseudomonas, Erwinia, and Streptococcus spp. Surprisingly, incidence of L. monocytogenes was proportionally greater at hour 0 than when tested 4, 8, and 12 h later with all three enrichment schemes. The corresponding increase in Anoxybacillus and Geobacillus spp.indicated these taxa co-enriched in competition with L. monocytogenes during early enrichment hours. L. monocytogenes became dominant after 24 h in all three enrichments. DNA sequences obtained from shotgun metagenomic data of Listeria monocytogenes at 48 h were assembled to produce a consensus draft genome which appeared to have a similar tracking utility to pure culture isolates of L. monocytogenes.ConclusionsAll three methods performed equally well for enrichment of Listeria monocytogenes. The observation of potential competitive exclusion of L. mono by Anoxybacillus and Geobacillus in early enrichment hours provided novel information that may be used to further optimize enrichment formulations. Application of Resphera Insight for high-resolution analysis of 16S amplicon sequences accurately identified L. monocytogenes. Both shotgun and 16S rRNA data supported the presence of three slightly variable genomes of L. monocytogenes. Moreover, the draft assembly of a consensus genome of L. monocytogenes from shotgun metagenomic data demonstrated the potential utility of this approach to expedite trace-back of outbreak-associated strains, although further validation will be needed to confirm this utility.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-016-0894-1) contains supplementary material, which is available to authorized users.

Effects of Different Nitrite Concentrations from a Vegetable Source with and without High Hydrostatic Pressure on the Recovery of Listeria monocytogenes on Ready-to-Eat Restructured Ham

Sodium nitrite exerts an inhibitory effect on the growth of Listeria monocytogenes. The objective of this study was to investigate the effects of various nitrite concentrations from a vegetable source with and without high hydrostatic pressure (HHP) on the recovery and growth of L. monocytogenes on ready-to-eat restructured ham. A preconverted celery powder was used as the vegetable source of nitrite. Targeted concentrations of natural nitrite investigated were 0, 50, and 100 mg/kg. HHP treatments evaluated were 400 MPa for 4 min and 600 MPa for 1 or 4 min at 12 ± 2°C (initial temperature of the pressurization fluid). Viable L. monocytogenes populations were monitored on modified Oxford medium and thin agar layer medium through 98 days of storage at 4 ±1°C. Populations on both media did not differ. The HHP treatment at 600 MPa for 4 min resulted in L. monocytogenes populations below the detection limit of our sampling protocols throughout the storage period regardless of the natural nitrite concentration. The combination of HHP at 400 MPa for 4 min or 600 MPa for 1 min with natural nitrite resulted in initial inhibition of viable L. monocytogenes. Ham formulations that did not contain natural nitrite allowed faster growth of L. monocytogenes than did those with nitrite, regardless of whether they were treated with HHP. The results indicate that nitrite from a vegetable source at the concentrations used in this study resulted in slower growth of this microorganism. HHP treatments enhanced the inhibitory effects of natural nitrite on L. monocytogenes growth. Thus, the combination of natural nitrite plus HHP appears to have a synergistic inhibitory effect on L. monocytogenes growth.

Influence of prior growth conditions, pressure treatment parameters, and recovery conditions on the inactivation and recovery of Listeria monocytogenes, Escherichia coli, and Salmonella Typhimurium in turkey meat

The relatively high prevalence of Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella enterica serovar Typhimurium in various food products is of great concern to the food industry. The objective of this study was to determine the pressure-inactivation of the pathogens in a representative food model as affected by prior growth temperature, physiological age of the culture, pressure level and treatment temperature. The effect of post-treatment conditions (incubation temperature and gas atmosphere) on the bacterial recovery was also determined. The pathogens being studied were inoculated into sterile turkey breast meat to a final level of ca. 3 logCFU/g and then grown to two stages, the early stage (representative of exponential phase) and late stage (representative of stationary phase), at 15, 25, 35, and 40 °C. Turkey meat samples were pressure-treated at 400 and 600 MPa for 2 min at initial sample temperatures of 4, 20 and 40 °C. Following treatment, bacterial counts in the samples were determined aerobically or anaerobically at incubation temperatures of 15, 25, 35, and 40 °C. Pressure inactivation of the bacterial pathogens increased as a function of the pressure levels and treatment temperatures. Generally speaking, early stage cells were more resistant than late stage cells (P<0.05). The incubation gas atmosphere did not affect bacterial recovery. Bacteria grown at 15-35 °C underwent higher population reductions than those grown at 40 °C. With regard to recovery temperatures, low temperatures promoted greater recovery of injured early and late stage cells than higher temperatures (P<0.05). This study indicates the importance of environmental conditions to which bacteria are exposed prior to pressure treatment and recovery conditions of the bacteria after pressure treatment when considering the adequacy of pressure treatments to enhance the microbiological safety of foods.

Inactivation and recovery of Listeria monocytogenes, Salmonella enterica and Staphylococcus aureus after high hydrostatic pressure treatments up to 900 MPa.

High hydrostatic pressure (HP) processing is used in the food industry to enhance the safety and extend the shelf-life of food. Although a drastic decrease in microbial viability is achieved immediately after the application of HP treatments, under favorable conditions the injured bacteria can recover. The present study evaluated the inactivation and recovery of five strains of Listeria monocytogenes, Salmonella enterica and Staphylococcus aureus subjected to pressures of 400, 600, and 900 MPa under stressing and non-stressing conditions in a complex medium. Treatments at 400 and 600 MPa were found to greatly affect the viability of L. monocytogenes and S. enterica, but only a treatment of 5 min at 900 MPa decreased the levels of the three pathogens to below the detection limit (8-9 log units reduction). After HP treatment, not only the baroresistant S. aureus but also several replicates of L. monocytogenes and S. enterica strains recovered during subsequent incubation under favorable conditions. However, when HP was combined with low pH and nitrite but not with NaCl or lactate, the viability of pressurized S. aureus cells progressively decreased. As pathogenic bacteria can recover even after the application of very high pressure levels, the combination of HP with other hurdles for microbial growth, either intrinsically present in the food product or extrinsically applied, may be needed to guarantee the efficacy of technologies aimed at pathogen reduction and shelf-life extension.

Impact of Nitrite on Detection of Listeria monocytogenes in Selected Ready‐to‐Eat (RTE) Meat and Seafood Products

The impact of sodium nitrite (NaNO2) on detection and recovery of Listeria monocytogenes from select ready-to-eat (RTE) foods including smoked salmon, smoked ham, beef frankfurters, and beef bologna was assessed. Nitrite-containing (NC; 100 to 200 ppm NaNO2) or nitrite-free (NF) foods were inoculated with a 5-strain cocktail of L. monocytogenes by immersion into Butterfield's buffer solution containing 5.4 to 7.4 x 10(3) L. monocytogenes per milliliter. Inoculated products were vacuum-packaged and stored at 5 degrees C. A weekly comparative analysis was performed for presence of L. monocytogenes using 5 detection methods on products held at 5 degrees C for up to 8 wk. L. monocytogenes initially present at <100 CFU/g during the first 2 wk of storage increased throughout the study, attaining final populations of approximately 1 x 10(4) to 1 x 10(5) CFU/g. Lactic acid bacteria predominated throughout the study in all products. Exposure to NaNO2 (100 to 200 ppm) resulted in 83% to 99% injury to the L. monocytogenes strains tested. The genetic-based BAX System (DuPont Qualicon, Wilmington, Del., U.S.A.) and modified USDA/FSIS methods detected 98% to 100% of Listeria-positive food samples and were consistently superior to and significantly different (P < 0.05) from conventional cultural methods in recovering Listeria from NC samples. Data show that nitrite-induced injury adversely affects detection and recovery of L. monocytogenes from NC food, confirming earlier findings that nitrite-induced injury masks L. monocytogenes detection in NC RTE food products. Nitrite-injured Listeria can subsequently repair upon nitrite depletion and grow to high levels over extended refrigerated storage.

Evaluation of Quantitative Recovery Methods for Listeria monocytogenes Applied to Stainless Steel

The ability of Listeria monocytogenes to attach to various food contact surfaces, such as stainless steel, polypropylene, and rubber compounds, is well documented. The retention of these or other pathogenic bacteria on food contact surfaces increases the risk of transmission to food products. The objective of this study was to compare several methods for quantitative recovery of Listeria monocytogenes from stainless steel surfaces. A cocktail of 4 serotypes of Listeria monocytogenes mixed in equivalent concentrations was inoculated onto type 304 stainless steel coupons in a 2 x 2 cm area. After 1 h exposure, coupons were sampled by one of the following methods: (1) swabbing with a premoistened Dacron swab; (2) rinsing with phosphate-buffered saline; (3) direct contact onto tryptic soy agar containing 0.6% yeast extract (TSA + YE) plates for 10 s; (4) sonication in an ultrasonic water bath (40 kHz); (5) contact with the bristles of a sonicating brush head for 1 min; and (6) indirect contact (2-4 mm distance) with a sonicating brush head for 1 min. The 3 sonication methods yielded higher recovery than the other 3 methods (P < 0.05). Brushing the coupons with the sonicating brush head (contact or noncontact) yielded a recovery level of about 60%. The lowest cell recovery (about 20%) was observed with the swab and direct agar contact methods. After a 12 h exposure, recoveries ranged from 17.4 (brush contact method) to 2% (swab method).

Effect of prior growth temperature, type of enrichment medium, and temperature and time of storage on recovery of Listeria monocytogenes following high pressure processing of milk

A five-isolate cocktail of Listeria monocytogenes (10 3 cfu/ml in skim or whole raw milk) was subjected to 450 MPa for 900 s or 600 MPa for 90 s. The effects of prior growth temperature, type of milk (skim vs. whole), type of recovery-enrichment media (optimized Penn State University [oPSU] broth, Listeria Enrichment Broth [LEB], Buffered LEB [BLEB], Modified BLEB [MBLEB], and milk), storage temperature and storage time on the recovery of L. monocytogenes were examined. Optimized PSU broth significantly increased the recovery of L. monocytogenes following high pressure processing (HPP), and was 63 times more likely to recover L. monocytogenes following HPP, compared to LEB, BLEB and MBLEB broths ( p<0.05; Odds Ratio=63.09, C.I. 23.70–167.96). There was a significant main effect for prior growth temperature ( p<0.05). However, this relationship could not be interpreted given the significant interaction effects between temperature and both pressure and milk type. HPP-injured L. monocytogenes could be recovered using both LEB and oPSU broths after storage of milk at 4, 15 and 30 °C, with recovery being maximal after 24 to 72 h of storage; however, recovery yield dropped to 0% after prolonged storage of milk at 4 and 30 °C. In contrast, storage of milk at 15 °C yielded the most rapid rate of recovery and the highest recovery yield (100%), which remained high throughout the 14 days of storage at 15 °C. The above factors need to be taken into consideration when designing challenge studies to insure complete inactivation of L. monocytogenes and possibly other foodborne pathogens during high pressure processing of foods.

Mechanistic study of membrane concentration and recovery ofListeria monocytogenes

Detection of the foodborne pathogen Listeria monocytogenes requires that food samples be processed to remove proteins and lipids, concentrate microorganisms to a detectable concentration, and recover the concentrated cells in a small volume compatible with micron-scale biochips. Mechanistic considerations addressed in this research include the roles of membrane structure, pore size, and detergents in maximizing recovery of cells from a complex biological fluid. The fluid in this case was a food sample (hotdog extract) inoculated with L. monocytogenes. This study showed how membrane filtration using a syringe filter is able to concentrate L. monocytogenes by 95x with up to 95% recovery of living microorganisms by concentrating 50 mL of food sample into a volume of 500 microL. Tween 20 was added to the sample to prevent irreversible adsorption of the microorganism to the membrane and thereby help to ensure high recovery. Comparison of polycarbonate, mixed cellulose, nylon, and PVDF membranes with 0.2 to 0.45 microm pores showed the 0.2 microm polycarbonate membrane with straight through, mono-radial pores gives the highest recovery of living microorganisms. The mixed cellulose, nylon, and PVDF membranes have a fibrous structure whose characteristic openings are much larger than their effective pore size cut-offs of 0.22 or 0.45 microm. We define conditions for rapid membrane-based cell concentration and recovery that has the potential to supplant enrichment steps that require a day or more. This approach has the added benefit of facilitating examination of a large amount of fluid volume by reducing its volume to a range that is compatible with the microliter scales of biochip or other biosensor detection systems.

Survival and Recovery of Listeria monocytogenes on Ready-to-Eat Meats Inoculated with a Desiccated and Nutritionally Depleted Dustlike Vector

Dust from construction was theorized to serve as a vector for L. monocytogenes transmission to ready-to-eat (RTE) meats after heat processing but before packaging. A five-strain Listeria monocytogenes culture including serotype 4b was continually stressed on a sand vector under four sets of nutritionally depleted and dry conditions to simulate postprocessing contamination by dustlike particulates. The stresses included that associated with sand stored at different temperatures (10 and 22°C) and levels of humidity (40% relative humidity [RH], 88% RH, or complete desiccation). Irradiated RTE meats, including frankfurters, bologna, chopped ham, and deli-style roast beef, were inoculated with the L. monocytogenes–contaminated sand every 2 to 3 days over a period of 1½ months. After inoculation, the RTE meats were vacuum packed and stored at 4°C for 24 h. Populations of L. monocytogenes were enumerated by surface plating on nonselective and selective media to recover cells on the basis of the different stresses presented (osmotic or antibiotic). L. monocytogenes was demonstrated to be capable of surviving on the sand vector for >151 days at 10°C and 88% RH, 136 days at 10°C and 0% RH, 73 days at 22°C and 40% RH, and 82 days at 22°C and 0% RH. These results show that under the most conservative scenario, the 73-day-old L. monocytogenes–contaminated sand was able to attach to and be recovered from the RTE meats. This study illustrated that dust contaminated with L. monocytogenes, once in contact with meat surfaces, can survive and grow, posing a health hazard to consumers.

Recovery of Listeria monocytogenes on selective agar media in a collaborative study using reference samples

Sixteen laboratories compared counts of Listeria monocytogenes in reference samples using Blood agar, Palcam(y) agar and Oxford agar. Significant differences were found between laboratories. The mean counts on Blood agar were significantly higher than on Palcam(y) or Oxford agar. The mean counts on Palcamy agar were somewhat higher than on Oxford agar (only after 48 h incubation), but no significant difference was found. Addition of egg yolk to Palcam agar seems to be benificial for the recovery of sublethally injured cells. Recovery of L. monocytogenes was higher after 48 h incubation for all media tested.

Selection, differentiation and counting of haemolytic Listeria spp. on PALCAM medium

An agar overlay containing sheep erythrocytes and the supernatant fluid of a blood culture of Staphylococcus aureus, poured onto inoculated PALCAM agar plates, allows a reliable visual reading of haemolysis. The method gave up to 95% recovery of Listeria monocytogenes from raw food samples.

Effect of Enrichment Media and Sampling Protocol on Recovery of Listeria monocytogenes

These studies examined the differences in recovery of Listeria monocytogenes from pure culture and in the populations of mixed aerobic microflora from chicken and Brie cheese incubated in University of Vermont (UVM) and Listeria enrichment broth (LEB) enrichment broths for different times and conditions. No significant differences were observed in levels of L. monocytogenes from pure cultures in UVM or LEB on any sampling day. No differences were observed in the levels of mixed microflora from Brie cheese in either UVM or LEB, but from chicken rinse the level of mixed flora competitors was significantly higher on all sampling days in LEB as compared to UVM. No differences were observed between a single enrichment in UVM or LEB for 2 d and a transfer to a secondary enrichment tube after 1 d. Overall, the level of mixed microflora capable of growing in enrichment broths was greater from chicken rinse than from Brie cheese. The ratio of L. monocytogenes to mixed microflora which survived the selective enrichments was most favorable for recovery of L. monocytogenes after 2 d of enrichment.

Recovery and Serotype Distribution of Listeria monocytogenes from Broiler Chickens in the Southeastern United States

A total of ninety broiler carcasses from three processing plants were obtained from retail stores in the southeastern United States. The optimum plating medium was determined and carcasses were rinse sampled and the recovery of Listeria monocytogenes and other Listeria species determined. Presumptive L. monocytogenes isolates were serotyped. Listeria were recovered from 34 of 90 (38%) of the carcasses sampled, while L. monocytogenes were recovered from 21 of 90 (23%) of the carcasses sampled. Of the 35 L. monocytogenes isolates serotyped, 21 (64%) were confirmed to be serotype 1/2 b and 6 (18%) were confirmed to be serotype 1/2 c. Although there is no epidemiological evidence to suggest a relationship between consumption of chicken and listeriosis, the presence of L. monocytogenes on 23% of sampled broilers emphasizes the importance of maintaining hygienic practices in production, processing and preparation of fresh broilers.

Improved Listeria monocytogenes selective agar.

By increasing the LiCl concentration to 5 g/liter and adding 20 mg of moxalactam per liter to modified McBride agar base, it was possible to inhibit the growth of many bacteria which interfered with the recovery of Listeria monocytogenes from beef.