Simultaneous and individual quantitative estimation of Salmonella, Shigella and Listeria monocytogenes on inoculated Roma tomatoes (Lycopersicon esculentum var. Pyriforme) and Serrano peppers (Capsicum annuum) using an MPN technique.

Simultaneous and individual enumeration of Salmonella, Shigella and Listeria monocytogenes was compared on inoculated Roma tomatoes and Serrano peppers using an Most Probable Number (MPN) technique. Samples consisting of tomatoes (4 units) or peppers (8 units) were individually inoculated with a cocktail of three strains of Salmonella, Shigella or L. monocytogenes, or by simultaneous inoculation of three strains of each pathogen, at low (1.2-1.7 log CFU/sample) and high (2.2-2.7 log CFU/sample) inocula. Samples were analyzed by an MPN technique using universal pre-enrichment (UP) broth at 35 °C for 24 ±â€¯2 h. The UP tubes from each MPN series were transferred to enrichment and plating media following adequate conventional methods for isolating each pathogen. Data were analyzed using multifactorial analysis of variance (p < 0.05) and LSD multiple rang test. There were differences (p < 0.05) in recovery of simultaneous and individual bacteria inoculated (individual > simultaneous), type of bacteria (Salmonella > Shigella and L. monocytogenes), type of sample (UP broth > pepper and tomato), and inoculum level (high > low). The MPN technique was effective for Salmonella on both commodities. Shigella counts were higher on tomatoes compared to peppers, (p < 0.05), and for L. monocytogenes on peppers (p < 0.05).

Use of a novel medium, the Polymyxin Ceftazidime Oxford Medium, for isolation of Listeria monocytogenes from raw or non-pasteurized foods.

Polymyxin Ceftazidime Oxford Medium (PCOM), a novel selective and differential plating medium for Listeria monocytogenes was compared with Modified Oxford Agar (MOX) for efficacy to isolate L. monocytogenes and other Listeria spp. naturally present in non-pasteurized Mexican-style cheese (n = 50), non-pasteurized fresh squeezed orange juice (n = 50), raw beef chunks (n = 36), and fresh cabbage (n = 125). Samples were collected from retail markets and farms in Mexico and tested following the US Department of Agriculture enrichment technique. Listeria spp. were isolated from 23.4% of analyzed samples, and from those, 75.0% corresponded to raw beef chunks, 38.0% to non-pasteurized Mexican-style cheese, and 30.0% to fresh squeezed orange juice. No Listeria spp. were isolated from fresh cabbage samples. L. monocytogenes was recovered from 15.3% of food samples analyzed. Non-pasteurized Mexican-style cheese showed the highest proportion of L. monocytogenes positive samples (36.0%), followed by orange juice (26.0%) and raw beef (25.0%). The frequency of isolation of Listeria spp. and L. monocytogenes was not different (P > 0.05) between PCOM and MOX. The advantages of using PCOM when comparing to MOX, include the easier way to identify Listeria species, the lower cost per plate and the availability of its ingredients for Latin-American countries.

Quantitative distribution of Salmonella spp. and Escherichia coli on beef carcasses and raw beef at retail establishments.

Salmonella is a foodborne pathogen that commonly inhabits the gastrointestinal tract of a healthy feedlot cattle and can be transferred to the carcass surface during hide removal and evisceration procedures. Numerous investigations on Salmonella prevalence throughout different stages of the beef chain have been conducted. In contrast, limited studies are available on quantitative determinations of Salmonella at different steps in raw meat production. Quantitative data, particularly for pathogenic bacteria such as Salmonella are important for quantitative risk assessment. Salmonella spp. and Escherichia coli populations were enumerated on beef carcass samples collected at abattoirs and also in beef chunks and ground beef samples collected from butcher's shops at retail in Jalisco State, Mexico. Sponge samples from beef carcass sides (n=142) were collected immediately after final water wash and before chilling at three non-federally inspected abattoirs following USDA-FSIS sampling protocols. Beef chunks (n=84) and ground beef (n=65) samples were obtained from 86 butcher's shops. Salmonella enumeration was conducted by the Most Probable Number method and E. coli counts were determined using Petrifilm plates. Salmonella was isolated from 18% of beef carcasses, 39% of beef chunks and 71% of ground beef samples. Salmonella mean counts were 1.3±0.9 Log MPN/300 cm(2) on beef carcasses, 1.9±0.9 and 2.3±1.1 Log MPN/25 g in beef chunks and ground beef samples, respectively. Twenty-six Salmonella serotypes and 11 serogroups were identified among 432 isolates recovered. Salmonella typhimurium (14%), Salmonella sinstorf (12%) and S. Group E1 monophasic (10%) were the most frequent. Escherichia coli was present on 97, 84 and 100% of beef carcasses, beef chunks and ground beef samples, respectively. Escherichia coli mean counts were 3.2±0.7 Log CFU/300 cm(2), 3.9±1.1 and 4.5±1.2 Log CFU/25 g on beef carcasses, beef chunks and ground beef, respectively. Salmonella prevalence and mean counts found in raw beef were higher than previously reported in studies from other countries. The data collected in this study show a trend in the prevalence of Salmonella to be higher as meat processing is extended at retail. This, together with the diversity of serotypes found, indicates that raw meat is exposed to multiple contamination sources during slaughter and retail processing and highlights the necessity to implement Sanitation Standard Operating Procedures for those establishments. Finally, this study provides quantitative information for future risk assessments associated with the risk of human salmonellosis.

The Polymyxin Ceftazidime Oxford Medium as an alternative selective and differential medium for isolation of Listeria monocytogenes from raw or unpasteurized food.

The Polymyxin Ceftazidime Oxford Medium (PCOM) was developed to recover Listeria monocytogenes from raw or unpasteurized foods. It contains esculin-ferric ammonium citrate as indicator system for Listeria growth, and ceftazidime and polymyxin B as selective agents, which are available in several Latin American countries. Comparison of PCOM, Modified Oxford Medium (MOX) and Tryptic Soy agar with 0.6% yeast extract (TSAYE) indicated that both selective media were equally effective at recovering four individual strains of L. monocytogenes (Scott A, V7, California and broccoli), and a mixture of these strains (LMM) (P > 0.05). The ability of PCOM, MOX, TSAYE and TSAYE supplemented with 4% NaCl to recover heat, acid and freeze-damaged LMM was similar for all media (P > 0.05). The PCOM proved to be effective at isolating colonies of LMM from inoculated raw beef chunks, unpasteurized orange juice, cabbage, and Mexican-style cheese by direct plating and by the US Department of Agriculture's Food Safety and Inspection Service enrichment method. Differentiation of L. monocytogenes colonies was easier on PCOM than on MOX for foods with high levels of background microbiota. Based on the evaluations performed on foods naturally contaminated with L. monocytogenes, PCOM was a more economical alternative than MOX for selective and differential isolation of Listeria from raw or unpasteurized foods.

Spread of bacterial pathogens during preparation of freshly squeezed orange juice.

To study the potential of three bacterial pathogens to cross-contaminate orange juice during extraction, normal operation conditions during juice preparation at food service establishments were simulated. The spread of Salmonella enterica serovar Typhimurium, Escherichia coli O157:H7, and Listeria monocytogenes from inoculated oranges to work surfaces and to the final product was determined. The transference of these three bacterial pathogens to orange juice made from uninoculated oranges with the use of contaminated utensils was also studied. Fresh oranges were inoculated with a marker strain of rifampicin-resistant Salmonella Typhimurium, E. coli O157:H7, or L. monocytogenes. Final pathogen levels in juice were compared as a function of the use of electric or mechanical juice extractors to squeeze orange juice from inoculated oranges. Pathogen populations on different contact surfaces during orange juice extraction were determined on sulfite-phenol red-rifampicin plates for Salmonella Typhimurium and E. coli O157:H7 and on tryptic soy agar supplemented with 0.1 g of rifampicin per liter for L. monocytogenes. After inoculation, the average pathogen counts for the orange rind surface were 2.3 log10 CFU/cm2 for Salmonella Typhimurium, 3.6 log10 CFU/cm2 for E. coli O157:H7, and 4.4 log10 CFU/cm2 for L. monocytogenes. This contamination was spread over all utensils used in orange juice squeezing. Mean pathogen counts for the cutting board, the knife, and the extractor ranged from -0.3 to 2.1 log10 CFU/cm2, and the juice contained 1.0 log10 CFU of Salmonella Typhimurium per ml, 2.3 log10 CFU of E. coli O157:H7 per ml, and 2.7 log10 CFU of L. monocytogenes per ml. Contact with contaminated surfaces resulted in the presence of all pathogens in orange juice made from uninoculated oranges. These results give emphasis to the importance of fresh oranges as a source of pathogens in orange juice.