[Growth rates of two virulence plasmids carrying Yersinia enterocolitica after contamination of heated milk, raw minced pork and vegetables].

The psychrotrophic organism Y. enterocolitica (Y.e.) is able to grow in foodstuffs thus rendering it dangerous for the consumer. Among others, the preparation of food in the kitchen might be an important way of transmission. Not only contaminated raw meat but aswell latently infected humans and pet animals (cats, dogs) may serve as a reservoir for Y.e. contamination. After artificial contamination of UHT milk and vanilla sauce with subsequent storage at 4 degrees C, two Y.e. strains of serovars 0:3 and 0:9 carrying the virulence plasmid multiplied during the first 6 d (9 d) at a rate of 1 log/d (Fig. 1). In pasteurized milk the multiplication rate was 0,65 log/d (Fig.2). Maximum bacterial counts in UHT milk after 15 d were in the order of 10(9) CFU/ml (Fig.4). In pasteurized milk, even minimal inoculation doses of less than 5 CFU/100ml (calculated) produced multiplication of Y.e. to 1,7-27 X 10(4) CFU/ml after 7 d (Tab.1). In this case the milk has been contaminated by an appropriately diluted suspension of faeces from infected mice. When selected colonies were tested on MOX-Agar, a loss of the virulence plasmid could not be observed. In minced fresh pork meat with a bacterial count of 4,2 X 10(6) CFU/g--most of them i.e. 2,6 X 10(6) belonging to Pseudomonas sp.--added Y.e. did not multiply, but viable organisms could be recovered after contamination with only 2,9-6,6 X 10(2) CFU Y.e./g and storage at 4 degrees C for 72 h (Tab.2). Similarly, after contamination of quartered paprika husks and halves of carrots, Y.e. strains could be re-isolated after 15 d storage at 4 degrees C or room temperature if the number of bacteria used for inoculation was in the range of 10(6) (Tab.3). Feeding contaminated carrots to NMRI-mice a calculated number of 5 X 10(3) CFU Y.e./mouse were sufficient to cause infection of part of the animals together with shedding of the organisms through the faeces (Tab.4). These results support the assumption that milk and home made milk products like vanilla sauce create a risk for human health when they are stored for a few days after contamination with Y.e. carrying the virulence plasmid. The same may be true for raw minced pork and fresh vegetables but only if the initial contamination was high. This might occur during food processing procedures.(ABSTRACT TRUNCATED AT 400 WORDS)