L. monocytogenes is a small (1–330.5mm) Gram-positivebacillus with a tendency to occur in chains. Typically, the bacteria show a slow tumbling type of motility by means of peritrichous flagella at 22–25°C, but are nonmotile at 37°C. The bacterium is aerobic or microaerophilic. It grows between 35 and 37°C and in the presence of reduced oxygen tension and 5–10% CO2. It is nonsporing, noncapsulated, and non– acid fast. L. monocytogenes grows on ordinary medium, but grows better on blood agar and tryptose phosphate agar. The bacteria on blood agar after 24 hours incubation produce smooth, round, translucent, easily emulsifiable, and nonpig-mented colonies. On blood agar, they produce a narrow zone of weak beta hemolysis. Due to this hemolysis, the colonies of L. monocytogenes may be confused with those of group B streptococci. L. monocytogenes differs from group B strepto-cocci by following tests:
1. It is motile, catalase positive, and shows positive CAMP (Christie, Atkins, and Munch–Peterson) reaction and nega-tive hippurate hydrolysis test.
2. In CAMP reaction, L. monocytogenes produces a block type of hemolysis unlike that of arrow-head hemolysis pro-duced by group B streptococci.
monocytogenes ferments sugars like glucose, salicine, andaesculin with production of acid. It is MR (methyl red) and VP (Voges–Proskauer) test positive but oxidase, urease, indole, and H2S negative. Hemolysin, a protein, is an important virulence factor of L. monocytogenes. This is also known as his-tolysin. Histolysin is an extracellular, oxygen-labile, and choles-terol-sensitive protein of molecular weight 60,000 Da. It shows antigenic cross-reactivity with pneumolysin and streptolysin O.
L. monocytogenes produces a variety of infections in preg-nant women, neonates, adults, and elderly people. The bac-teria in nonpregnant ladies may cause asymptomatic or mild febrile illness. However, infection in pregnant ladies may cause abortion or stillbirth of the child. In nonpregnant females, asymptomatic infections of the genital tract may lead to infertility. L. monocytogenes in adults may cause men-ingitis, meningoencephalitis, septicemia, endocarditis, and brain abscess. Patients receiving immunosuppressive ther-apy, suffering from immunosuppressive disease, are more susceptible to the infection. L. monocytogenesmay cause occu-pation-related infections in poultry workers, butchers, and veterinarians. Due to direct contact with infected animals or birds or other products, these people may develop cutaneous infections.
L. monocytogenes is distributed worldwide. It occurs as asaprophyte in soil, water, and sewage. It is also widely prevalent in different mammals or birds, fish, ticks, and crustaceans. The bacteria have been isolated from milk, cheese, and other milk products. The infection is transmitted by:
1. ingestion of milk, milk products, and other food prepara-tions contaminated with the bacteria;
2. direct contact with infected animal or animal products; and
3. inhalation of contaminated dust.
Many outbreaks of listeriosis caused by food contaminated with Listeria have been documented in Europe and North America.Nosocomial outbreaks of Listeria have also been documented in hospitals.
L. monocytogenes is sensitive to ampicillin, gentamicin, andcotrimoxazole. Cephalosporins are not useful for infections caused byListeria.