Listeria monocytogenes utilizes internalin A (InlA; encoded by inlA) to cross the intestinal barrier to establish a systemic infection. Multiple naturally occurring mutations leading to a premature stop codon (PMSC) in inlA have been reported worldwide, and these mutations are causally associated with attenuated virulence. Five inlA PMSC mutations recently discovered among isolates from France and the United States were included as additional markers in our previously described inlA single-nucleotide polymorphism (SNP) genotyping assay. This assay was used to screen >1,000 L. monocytogenes isolates from ready-to-eat (RTE) foods (n ‫؍‬ 502) and human listeriosis cases (n ‫؍‬ 507) for 18 inlA PMSC mutations. A significantly (P < 0.0001) greater proportion of RTE food isolates (45.0%) carried a PMSC mutation in inlA compared to human clinical isolates (5.1%). The proportion of L. monocytogenes with or without PMSC mutations in inlA was similar among isolates from different RTE food categories except for deli meats, which included a marginally higher proportion (P ‫؍‬ 0.12) of isolates carrying a PMSC in inlA. We also analyzed the distribution of epidemic clone (EC) strains, which have been linked to the majority of listeriosis outbreaks worldwide and are overrepresented among sporadic cases in the United States. We observed a significant (P < 0.05) overrepresentation of EC strains in deli and seafood salads and a significant (P < 0.05) underrepresentation of EC strains in smoked seafood. These results provide important data to predict the human health risk of exposure to L. monocytogenes strains that differ in pathogenic potential through consumption of contaminated RTE foods.Listeria monocytogenes is the etiological agent of listeriosis, a potentially life-threatening food-borne disease that primarily affects individuals with underlying immune-compromising circumstances. Listeriosis is associated with an exceptionally high hospitalization rate of 85 to 90% and 20 to 30% of cases are fatal. L. monocytogenes infections account for nearly 30% of all fatalities attributed to known food-borne pathogens each year in the United States (18). Although L. monocytogenes is readily inactivated by cooking and pasteurization, subsequent crosscontamination of ready-to-eat (RTE) food products exposed to the food processing plant environment following a lethality treatment represents the route through which RTE foods become contaminated (31). L. monocytogenes is a hardy pathogen; it is relatively resistant to acid, is able to grow in high salt concentrations, and is capable of growing at refrigeration temperatures (2, 4). Because L. monocytogenes tolerates intrinsic and extrinsic properties of food typically used to control the growth of pathogens, consumption of RTE foods contaminated by L. monocytogenes represents a significant health risk for immunocompromised individuals. The combined foodborne route of L. monocytogenes transmission and severity of listeriosis prompted establishment of strict regulations regarding the presence...

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SUMMARYFollowing oral inoculation of newly-hatched or three-week-old chickens, Salmonella organisms persisted longest in the caeca and they were also present for a shorter time in the crop. S. cholerae-suis, E. coli K12 and Saccharomyces cerevisiae did not colonise the alimentary tract when inoculated orally, but they persisted longer in the caeca than in other regions of the gut. This suggested that preferential localisation of Salmonella in the caeca may be the result of non-specific or host factors. This may include the slow rate of flow of contents through this organ which would allow greater microbial multiplication and persistence. Some Salmonella strains, particularly S. typhimurium, inoculated orally into three-week-old chickens were isolated in greater numbers from homogenates of the crop wall and, much less frequently, from the caecal wall, than from the luminal contents of these organs. None of the strains associated with the crop wall to the same extent as did lactobacilli.After oral inoculation of food-poisoning Salmonella serotypes the number of chickens excreting Salmonellae in their faeces gradually declined over a period of at least 4 weeks. In contrast, host adapted serotypes such as S. cholerae-suis and S. abortus-ovis were not excreted for longer than a few days. By testing mutants of S. typhimurium and S. infantis which were devoid of easily recognised characteristics it was concluded that neither flagellar (H) antigens, somatic (O) antigens, mannose-sensitive haemagglutinins nor the possession of the virulenceassociated 85 kilobase plasmid of S. typhimurium, were essential for colonisation (persistent faecal excretion). However, a mutant of S. infantis which was created by nitroso-guanidine treatment still possessed flagellar and somatic antigens and haemagglutinins and yet did not colonise the alimentary tract, indicating that an unidentified microbial factor was essential for colonisation.

Oral administration of strains of food poisoning salmonellas to day-old chickens produced a profound inhibition in the subsequent colonization of the caeca by a strain of Salmonella typhimurium given one day later. Closely related genera were unable to produce a similar inhibition. The inhibition was not the result of bacteriophages produced by the first strain. Neither was it the result of an immunological response by the host induced by the first strain. In additional experiments in day-old chickens, inhibition of an Escherichia coli Nalr strain and of a Citrobacter sp. Nalr strain was produced by the antibiotic-sensitive forms of the homologous strains while strains from other genera did not produce any inhibition. When an avirulent mutant of S. typhimurium was used for pre-treatment a statistically significant reduction in the excretion of the super-infecting S. typhimurium Nalr strain over several weeks was produced. A genus specific inhibition was reproduced in vitro by mixed culture experiments. Live cultures were necessary for in vitro inhibition. Killed cells or a culture supernatant produced no inhibition.

Four strains of Salmonella gallinarum isolated from independent cases of fowl typhoid all possessed both an 85-kilobase and a 2.5-kilobase plasmid. Each plasmid was eliminated in turn from one of the strains by transposon labeling and curing at 42 degrees C. Elimination of the small plasmid had no effect on the high virulence of the strain for newly hatched and 2-week-old chickens. Whereas oral inoculation of 2-week-old chickens with the parent strain produced 90% mortality with characteristic signs of fowl typhoid, inoculation of the large-plasmid-minus strain produced 0% mortality. A corresponding increase in the 50% lethal dose from log10 1.1 to greater than log10 7.3 was seen with the large-plasmid-minus strain after intramuscular inoculation. Reintroduction of the large plasmid completely restored virulence. A role for the plasmid-linked virulence genes in both invasion and growth in the reticuloendothelial system is suggested by the failure of the large-plasmid-minus strain to penetrate to the liver and spleen after oral inoculation and by its increased clearance from the reticuloendothelial system after intravenous inoculation. These results clearly demonstrate that the large plasmid of S. gallinarum contributes toward virulence in fowl typhoid of chickens.