Aerobic, nonmotile, gram negative cocci. Usually strikes children under 2. Diagnosis requires spinal tap - centrifuge CSF and gram stain. There are also latex agglutination tests using antibodies to this organism and cultures may be done as well. Usually a mild disease, but bacteria are shed in feces indefinitely.
Listeria grow inside phagocytes and proliferate within macrophages.
Mainly affects adults who are immunosuppressed, pregnant, or have cancer. Growth in CNS results in a meningitis. In pregnant women, infects placenta and leads to stillbirth or spontaneous abortion. Serious infection if newborn survives. Human outbreaks are often food borne and associated with dairy products - something complicated by the fact that it can grow at refrigerator temps.