1. Listeria monocytogenes
It is a small,
coccoid, gram positive bacillus, measuring 1-3 mm
in length and 0.5 mm width, with a tendency to occur in chains. They are
often angled at the point of contact and may resemble diphtheroids or
diplococci. Rough forms may be seen as long filaments. It exhibits a
characteristic tumbling motility when grown at 25°C,
but at 37°C is non-motile.This is
because peritrichous flagella are produced at 20-30°C but not at 37°C. They
are aerobic, facultatively anaerobic or microaerophilic. Growth can be improved
at reduced O2 tension and with 5-10% CO2. It grow on
ordinary media at 40-42°C, growth
improved by the addition of glucose, blood. After 24 hours incubation at 35-370C
Colonies are hemolytic. It is catalase positive.
