Ans. is 'c' i.e., 4b {Ref: Harrison 18th/e p. 1194]"The vast majority ' of cases of human listerial disease can be traced to serotype l/2a, l/2b and 4b". -- HarrisonListeria monocytogenes (Review)o Gram positiveo Coccobacillus with tendency to occur in chains.o Exhibits characteristic, slow, tumbling motility when grown at 2degC but at 37degC is non motile. This is because peritrichous flagella are produced by the bacillus optimally at 20-3CPC but only scantily or not at all at 37degC.o Grows on ondinary media with in a temprature range of 1deg to 45degC.o Most cases of human disease are caused by serotypes l/2a, l/2b and 4b.o The organism can be found as a part of the gastrointestinal flora in healthy individuals.o Human disease due to L. monocytogenes generally occurs in the setting of pregnancy or immunosuppression.o Mode of transmission;Food borne Most common (most cases or due to serotype 4b)Nosocomial -In late onset neonatal infection.o Foods implicated in infection include contaminated colesaw, pasteurized milk. soft cheese, pate, ready to eat turkey and pork productes, hot dogs, butter, prepared salads, undercooked chicken,o Listerosis in human may present in several form :