Comparison of Infectious Dose of Listeria monocytogenes F5817 as Determined for Normal Versus Compromised C57B1/6J Mice

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Abstract
The infectious dose of Listeria monocytogenes F5817, a serotype 4b human patient isolate, was determined following oral challenge in normal and compromised C57B1/6J mice. In an attempt to mimic human populations previously shown to be at risk to ingestion of L. monocytogenes, groups of mice used in this study consisted of the following: mice pretreated with hydrocortisone acetate or cimetidine; pregnant mice (12–14 d gestation); or beige mutants of C57B1/6J mice (deficient in lysosome production within monocytes and granulocytes). Mice were gavaged with varying levels of L. monocytogenes suspended in sterile 11% non-fat milk solids (NFMS). Upon expiration, the spleen, liver, lung, and brain were aseptically removed from mice. Organs were plated on LPM agar, and colonies were enumerated and biochemically confirmed as L. monocytogenes. Mice were considered infected if L. monocytogenes was recovered from at least one of the examined organs. In normal resistant C57B1/6J mice, the infectious dose 50 (ID50) ranged from 3.24–4.55 log10 CFU. In comparison, the ID50 for mice treated with 2.5 mg hydrocortisone acetate/day for 3d prior to infection decreased to 0.41 log10 CFU (range −1.91–2.74 log10 CFU). Administration of 0.25 mg hydrocortisone acetate/day for 3d prior to infection resulted in an ID50 similar to that calculated for normal mice. The ID50 calculated for pregnant mice was 2.48 log10 CFU, a value not significantly different from that of normal control mice. The response of beige mutants and cimetidine treated mice was comparable to that of normal controls, with ID50 values of 4.00 and 3.30 log10 CFU, respectively.