Thirty food and clinical isolates of Listeria were compared quantitatively in regard to lethality in immunocompromised mice, hemolytic activity for sheep erythrocytes, invasiveness towards Hep-2 epithelial cells, and cytotoxicity to CHO cells. All Listeria monocytogenes isolates were hemolytic, invasive, weakly cytotoxic, and lethal to immunocompromised mice. Listeria ivanovii isolates expressed the first three properties but were non-virulent. There was little quantitative correlation among the virulence markers, suggesting that there may be additional virulence related factors that may influence the pathogenicity of L. monocytogenes isolates. No systematic differences between the clinical and food isolates were apparent. Electron and light microscopy of infected Hep-2 cells revealed L. monocytogenes and L. ivanovii encapsulated within cell processes containing an actin matrix.

The effectiveness of Modified Vogel Johnson agar and lithium chloride-phenylethanol-moxalactam agar for detection of Listeria spp. in foods was compared by using the media to analyze retail-level meat, poultry, and seafood both by direct plating and in conjunction with a three-tube most-probable-number enrichment. The most-probable-number protocol detected Listeria species, including Listeria monocytogenes, in a substantial portion of the fresh meat and seafood samples. In most instances the Listeria levels were less than 2 CFU/g, which precluded detection by direct plating. Modified Vogel Johnson agar performed as well as did lithium chloride-phenylethanol-moxalactam agar and was considerably easier to use because of its ability to differentiate Listeria spp. from other microorganisms.

During a recent survey of retail fresh foods of animal origin (fish and seafood, raw milk, poultry, and red meats) for organisms of the Aeromonas hydrophila group, we isolated representative strains from the various foods. In this study, we sought to characterize these isolates for biochemical properties and virulence-associated factors and to compare the food isolates with clinical isolates. We identified all food and clinical isolates as A. hydrophila and found that all isolates were typical in their biochemical reactions. Examination of the isolates for various virulence-associated factors indicated that most food and clinical isolates were serum resistant, beta-hemolytic, cytotoxin positive (against Y1 adrenal cells), hemagglutinin positive, Congo red positive, elastase positive, and staphylolysin positive. Mouse 50% lethal doses were loglo 8 to 9 CFU for most isolates. All isolates had biotypes identical to those of enterotoxin-positive strains. The public health significance of these organisms in foods is not known at present, although their widespread occurrence and ability to grow competitively in foods kept at 5°C represents a potential hazard.

Motile Aeromonas spp. virulent for fish were studied with regard to their adhesion profile. Electron microscopy demonstrated the presence of fimbriae (pili) on Aeromonas cells regardless of virulence potential. The results show no significant correlations between ability to haemagglutinatc, yeast cell co-agglutination and virulence. All strains expressed mannose-sensitive haemagglutinin activity against guinea pig erythrocytes. Using four types of red blood cells no characteristic haemagglutinin pattern related to virulence could be discerned. Expression of surface haemagglutin(s) on Aeromonas hydrophila appears to be medium dependent; strains grown in liquid media demonstrated enhanced haemaggiutination activity. Both virulent and avirulent strains had in vitro epithelial cell adhesive capabilities. Cell surface characteristics measured by agglutination in acriflavine and stability after boiling indicated that most virulent strains agglutinated in the presence of acriflavine, but not all sedimcnted after boiling. The ability of 10 selected strains oi A. hydrophila to grow in normal pooled catfish serum was determined. Only 17% of the virulence variations can be explained by their sensitivity to serum.

Sixty ready-to-cook broiler carcasses obtained from several local supermarkets were tested for the presence of Yersinia enterocolitica and other Yersinia species. In the present study, the authors used two enrichment broths, yeast-extract/rosebengal-bile oxalate sorbose (YER-BOS) and phosphate-buffered saline with a postenrichment KOH treatment (PBS-KOH), and two plating media, cefsulodin-irgasan-novobiocin (CIN) and pectin agar. Yersinia organisms were found on 34 of 60 carcasses (56.7%) and Y. enterocolitica, on 16 of 60 carcasses (26.7%). There was no significant difference between CIN and pectin agar; however, PBS-KOH yielded a significantly higher (P less than or equal to .05) detection rate than YER-BOS, regardless of the plating media used. In addition to Y. enterocolitica, Y. frederiksenii and Y. intermedia were also isolated from the market broilers. None of the Y. enterocolitica isolates were found to be presumptively virulent.