A total of 245 strains of Listeria monocytogenes were investigated. These strains were isolated from human and animal cases of listeriosis as well as from different kinds of raw and processed foods. Thirty-three electrophoretic types (ETs) were identified among the 245 strains. The strains investigated included all human clinical strains isolated in Denmark during 1989 and 1990. Seventy-three percent of the strains isolated in this period were assigned to one of only two ETs (ET 1 and ET 4). ET 1, which was found to be the most frequently occurring ET among strains isolated from human clinical cases, was also found to occur rather frequently in animal clinical cases. ET 1 was, however, found only sporadically among strains isolated from foods and food factories. The data indicate that there might be something distinctive about the physiology or ecology of the ET 1 clone which makes it more likely to bring about disease in human beings either because of high pathogenicity or because of a special ability to multiply to infectious doses in processed foods. Another type, designated ET 4, was found to be the next most frequently occurring ET, after ET 1, among human clinical isolates. This could be explained by the fact that ET 4 was found to be the most frequently occurring ET within food isolates.
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Thermal inactivation of a mixture of five strains of Listeria monocytogenes, four strains of Escherichia coli O157:H7 and eight serotypes of Salmonella were compared with that of indigenous microflora in 75% lean ground beef.
Inoculated meat was packaged in bags that were completely immersed in a circulating water bath and held at 55, 57.5 and 60 degrees C for predetermined lengths of time. The surviving cell population was enumerated by spiral plating heat-treated samples onto tryptic soya agar supplemented with 0.6% yeast extract and 1% sodium pyruvate. D-values, determined by linear regression, in beef were 77.49, 21.9, and 10.66 min at 55, 57.5, and 60 degrees C, respectively, for indigenous microflora (z = 5.81 degrees C). When either of the three pathogens were heated in beef, their D-values calculated were significantly lower (P
To investigate the bacteriological quality, and the occurrence of selected pathogenic bacteria from organically grown Iceberg lettuce fertilized with bovine manure in the form of compost, firm manure and slurry in a 2-year field trial.
Samples of soil, fertilizer, fertilized soil, seedlings and lettuce were analysed for aerobic plate counts (APC), thermotolerant coliform bacteria (TCB), Escherichia coli, E. coli O157:H7, Salmonella spp. and Listeria monocytogenes. No difference in bacteriological quality could be shown in lettuce at harvest, however, APC varied significantly from year to year in the study. The various treatments gave significantly different APC and numbers of TCB isolated from fertilized soil. Escherichia coli O157:H7 was isolated from firm manure and slurry, and soils fertilized with the respective fertilizers the second year, but were not recovered from the lettuce.
No difference in bacteriological quality could be detected in lettuce at harvest after application of various types of manure-based fertilizers grown under Norwegian conditions.
The results may indicate that the use of manure does not have considerable influence on the bacteriological quality of organic lettuce. However, others have suggested that there is a risk by using manure. There is a need for more research in the field.
A total of 800 meat and poultry products were purchased from the retail marketplace in Edmonton, Alberta, Canada. The products consisted of raw ground beef, chicken legs, pork chops, and ready-to-eat fermented sausage, roast beef, processed turkey breast, chicken wieners, and beef wieners. The samples were analyzed to determine the prevalence of Shiga toxin-producing Escherichia coli, Salmonella, Campylobacter spp., and Listeria monocytogenes. Shiga toxin-producing E. coli 022: H8 was found in one raw ground beef sample. Salmonella and Campylobacter were found in 30 and 62% of raw chicken legs, respectively. L. monocytogenes was found in 52% of raw ground beef, 34% of raw chicken legs, 24% of raw pork chops, 4% of fermented sausages, 3% of processed turkey breast, 5% of beef wieners, and 3% of chicken wieners. The occurrence of pathogens in this study is similar to that in retail products in many other international locales.
The molecular epidemiology of Listeria monocytogenes was investigated in a longitudinal study of three Finnish dairy farms during 2013 to 2016. A total of 186 bulk tank milk (BTM), 224 milk filter sock (MFS), and 1,702 barn environment samples were analyzed, and isolates of L. monocytogenes were genotyped using pulsed-field gel electrophoresis. L. monocytogenes occurred throughout the year in all sample types, and the prevalence in MFS increased significantly during the indoor season. L. monocytogenes was more prevalent in MFS (29%) than in BTM (13%) samples. However, the prevalence of L. monocytogenes varied more between farms in samples of MFS (13 to 48%) than in BTM (10 to 16%). For each farm, the L. monocytogenes genotypes detected were classified by persistence (defined as persistent if isolated from =3 samples during =6 months) and predominance (defined as predominant if >5% prevalence on at least one farm visit). The prevalence of sporadic genotypes was 4 to 5% on all three farms. In contrast, the prevalence of persistent predominant genotypes varied between farms by 4% to 16%. The highest prevalence of persistent predominant genotypes was observed on the farm with the poorest production hygiene. Persistent predominant genotypes were most prevalent on feeding surfaces, water troughs, and floors. Genotypes isolated from the milking system or from cow udders had a greater relative risk of occurring in BTM and MFS than genotypes that only occurred elsewhere in the farm, supporting the hypothesis that L. monocytogenes is transmitted to milk from contamination on the udder surface or in the milking equipment.IMPORTANCEListeria monocytogenes is a ubiquitous environmental bacterium and the causative agent of a serious foodborne illness, listeriosis. Dairy products are common vehicles of listeriosis, and dairy cattle farms harbor L. monocytogenes genotypes associated with human listeriosis outbreaks. Indeed, dairy cattle farms act as a reservoir of L. monocytogenes, and the organism is frequently detected in bulk tank milk (BTM) and in the feces of clinically healthy cows. The ecology of L. monocytogenes in the farm environment is complex and poorly understood. Isolates of the same L. monocytogenes genotype can occur in the farm for years, but the factors contributing to the persistence of genotypes on dairy farms are unknown. Knowledge of the persistence patterns and contamination routes of L. monocytogenes on dairy farms can improve management of the contamination pressure in the farm environment and aid in the development of focused control strategies to reduce BTM contamination.
The aim of this study was to survey the presence of Staphylococcus aureus and Listeria monocytogenes during the cheese making process in small-scale raw milk cheese production in Norway. The prevalence of S. aureus in bovine and caprine raw milk samples was 47.3% and 98.8%, respectively. An increase in contamination during the first 2-3 h resulted in a 73.6% prevalence of contamination in the bovine curd, and 23 out of 38 S. aureus-negative bovine milk samples gave rise to S. aureus-positive curds. The highest contamination levels of S. aureus were reached in both caprine and bovine cheese after 5-6 h (after the first pressing). There was no contamination of L. monocytogenes in caprine cheeses and only one (1.4%) contaminated bovine cheese. This work has increased our knowledge about S. aureus and L. monocytogenes contamination during the process of raw milk cheese production and gives an account of the hygiene status during the manufacture of Norwegian raw milk cheeses.
In Sweden, many Listeria monocytogenes strains belonging to serovar 4b and isolated during the last five years from different sources share the same phagovar--2389:2425:3274:2671:47:108:340. The object of the present study was to investigate if 31 L. monocytogenes serovar 4b strains belonging to this particular phagovar could be differentiated by use of a simple restriction endonuclease analysis (REA). Among the enzymes tested, Xho I was found to be the most useful, since this enzyme could divide the 31 strains into five groups. The profiles of all human clinical isolates were indistinguishable from each other, which indicates that these strains may represent a single clone. The food isolates and the strains of human origin did not share the same profile. This further characterization may be of epidemiological importance as this phagovar of L. monocytogenes has been associated with at least two outbreaks of human listeriosis in Europe.