Feeding Trials ofListeria monocytogeneswith a NonhumanPrimate Model

Oneofthemajorunanswered questions regarding thepresenceofListeria monocytogenes infoods ishow many cells mustbeingested inorder tocauseillness. Toanswerthis question, studies wereundertaken byusing Macacafascicularis (cynomolgus monkey) asan animal model. Healthy nonhumanprimates were dosed with various concentrations ofL.monocytogenes suspended insterile wholemilk.Final concentrations of105,107, and109total cells oftheorganism wereused; a control was also included. Bloodsamples, aswell asfecal and nasal specimens, weretaken atvarious timeintervals. Onlyanimalsthat received 109cells ofL.monocytogenes becamenoticeably ill, withsymptomsofsepticemia, irritability, lossofappetite, andoccasional diarrhea. Monkeysthatreceived 107and109cells shedL.monocytogenes inthefeces forapproximately 21days. In monkeys thatreceived thedoseof109cells, severelymphopenia andneutrophilia occurred within 48h.Ina separate trial, monkeys received Maaloxtoreduce thegastric acidity ofthestomach. However, no substantial differences were observed between Maalox-treated andcontrol monkeys. Listeria monocytogenes isafoodborne pathogen which hascaused manyproblems forbothfoodindustries and regulatory agencies worldwide. ThenumberofL.monocytogenes cells required tocausedisease inbothnormal and debilitated individuals isunknown. Thishascreated many problems fromaregulatory point ofview, because insome countries (the United States), ithasbeennecessary torecall all foods containing theorganism, regardless ofthenumber ofcells present.