Dose-response model for Listeria monocytogenes-induced stillbirths in nonhuman primates.

A dose-response model using rhesus monkeys as a surrogate for pregnant women indicates that oral exposure to 10(7) CFU of Listeria monocytogenes results in about 50% stillbirths. Ten of 33 pregnant rhesus monkeys exposed orally to a single dose of 10(2) to 10(10) CFU of L. monocytogenes had stillbirths. A log-logistic model predicts a dose affecting 50% of animals at 10(7) CFU, comparable to an estimated 10(6) CFU based on an outbreak among pregnant women but much less than the extrapolated estimate (10(13) CFU) from the FDA-U.S. Department of Agriculture-CDC risk assessment using an exponential curve based on mouse data. Exposure and etiology of the disease are the same in humans and primates but not in mice. This information will aid in risk assessment, assist policy makers, and provide a model for mechanistic studies of L. monocytogenes-induced stillbirths.

Nonhuman primate model for Listeria monocytogenes-induced stillbirths.

Listeria monocytogenes, isolated from outbreaks in either human or nonhuman primate populations, was administered orally at doses ranging from 10(6) to 10(10) CFU. Four of 10 treated animals delivered stillborn infants. L. monocytogenes was isolated from fetal tissue, and the pathology was consistent with L. monocytogenes infection as the cause of pregnancy loss. For all pregnancies resulting in stillbirths, L. monocytogenes was isolated from maternal feces, indicating that L. monocytogenes had survived and had probably colonized the gastrointestinal tract. Antibodies and antigen-specific lymphocyte proliferation against Listeria increased in animals that had stillbirths.

Association of Listeria spp. Contamination in the Dairy Processing Plant Environment with the Presence of Staphylococci.

Monitoring the dairy processing plant environment for Listeria spp. has become common practice for determining sanitation effectiveness. Improved effectiveness and efficiency of sanitation programs could be achieved through the use of an indicator test. The objective of this research was to select a suitable indicator for the presence of Listeria spp. in the dairy processing plant and to develop criteria to guide its use. Microbial groups selected for study included: total aerobes, anaerobes, acid producers, salt tolerant aerobes, Enterobacteriaceae , lactobacilli, staphylococci, enterococci, gram-negative bacteria, and yeast and mold. Environmental swabs from 15 dairy plants were analyzed for these microbial groups and the presence of Listeria spp. Staphylococci were determined to be the best indicator group by using discriminant analysis selection. A discriminant model based on staphylococcus data from 409 environmental swabs grouped 85% of the 47 Listeria spp.-containing swabs correctly using 6,600 CFU staphylococci as the break point. Of the 362 swabs not contaminated with Listeria spp. 63% were correctly grouped. Of those swabs classified as negative using the 6,600 CFU criterion, 97% were confirmed negative. Of those swabs classified as positive, 23% were confirmed positive. Swabs classified as positive by the indicator test had a 7.7 times greater risk of listeria contamination than swabs classified as negative.