To determine the incidence of Listeria monocytogenes in raw milk, an isolation method was evaluated and used to analyze milk from three areas of the United States. The incidence varied by area from 0% in California to 7% in Massachusetts, with an overall incidence of 4.2%. The highest incidence found in any area during a single sampling period was 12% in Massachusetts in March 1985. During that same sampling, the incidence for all Listeria species was 26%. Of the 27 L. monocytogenes strains isolated during the survey, 25 were pathogenic in adult mice. One of three Listeria ivanovii isolated was pathogenic. No other isolates demonstrated pathogenicity.

A total of 100 fresh eviscerated whole market chickens, purchased one per week over a 5-week period from each of 20 different food stores in the Ontario and Ohio regions, were examined for the presence of Campylobacter jejuni. The microorganism was recovered from 62 and 54% of the chickens in Ontario and Ohio, respectively.

After an outbreak of listeriosis in Massachusetts in 1983, the ability of Listeria monocytogenes to survive in raw and pasteurized milk was investigated. An enrichment broth (EB) containing acriflavine, nalidixic acid, and cycloheximide was used to eliminate overgrowth of the culture by competing organisms, and a modification of McBride's agar (MMA) was used as the isolation medium. The culture was incubated 24 h at 30°C. To isolate Listeria from soft cheese, the incubation period was lengthened to 1 week, and the EB culture was streaked to MMA at 1 and 7 days. Physical and biochemical patterns, the CAMP test, serological tests, and mouse pathogenicity studies were helpful in determining the identity of L. monocytogenes

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The lethality of Listeria isolates was determined with normal adult mice and mice that were immunocompromised by treatment with 20 mg of carrageenan per kg. The mean 50% lethal doses (LD50s) of the pathogenic isolates were significantly lower (a = 0.05) in' the immunocompromised mice than in the untreated mice, with an average reduction of 5.8 1glo units. Ini contrast, the mean LD50s of the nonpathogenic isolates were lowèr in the immunocompromised mice by an average of only 0.4 loglo unit, a differehce that was not significant (a = 0.05). Whén immunocompromised mice were used, the LD50s of pathogenic Listeria monocytogenes isolates were lower than those of nonpathogenic L. innocua and L. seeligeri isolates by '6 loglo units and lower than those of nonpathogenic L. ivanovii isolates by .-41glo units. Pathogenic L. monocytogenes isolates could be distinguished from nonpathogenic isolates by their ability to cause deaths in immunocompromised mice in 3 days at a dose of-104 CFU per mouse. An alternative procedure using iron-overloaded mice failed to effectively differentiate pathogenic Listeria isolates.

Campylobacter jejuni was isolated from raw milk by a method that can routinely detect i1 organism per ml. This procedure was used in a survey of 195 separate farms and showed a 1.5% incidence of C. jejuni in milk from bulk tanks.

Three of 36 raw milk isolates of Yersinia enterocolitica produced enterotoxin in milk at 25 degrees C, but not at 4 degrees C. No strain tested could survive pasteurization.