Listeria monocytogenes: Characteristics, Virulence Factors, and Infections, Lecture notes of Biotechnology

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• (^) Listeria monocytogenes
• (^) L. monocytogenes is a small (1–3 0.5 m) Gram- positive bacillus 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.

• (^) 1. It is motile, catalase positive, and shows positive CAMP (Christie, Atkins, and Munch– Peterson) reaction and negative hippurate hydrolysis test.
• (^) 2. In CAMP reaction, L. monocytogenes produces a block type of hemolysis unlike that of arrow-head hemolysis produced by group B streptococci.

• (^) L. monocytogenes ferments sugars like glucose, salicine, and aesculin 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 histolysin.
• (^) Histolysin is an extracellular, oxygen-labile, and cholesterol- sensitive protein of molecular weight 60,000 Da. It shows antigenic cross- reactivity with pneumolysin and streptolysin O.

• (^) L. monocytogenes in adults may cause meningitis, meningoencephalitis, septicemia, endocarditis, and brain abscess. Patients receiving immunosuppressive therapy, suffering from immunosuppressive disease, are more susceptible to the infection. L. monocytogenes may cause occupation- 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 a saprophyte 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:

• (^) Laboratory diagnosis of listeriosis depends on the isolation of L. monocytogenes from clinical specimens by culture:
• (^) Cervical and vaginal secretions, meconium, blood, cerebrospinal fluid (CSF), amniotic fluid, placenta, and biopsy are frequently used specimens for diagnosis.
• (^) Gram staining of the CSF smear is useful for detection of bacteria. This is positive in nearly 50% of the cases.
• (^) Specimens are cultured on blood agar, chocolate agar, and tryptose phosphate agar and incubated for 24–72 hours at 35– 37°C.
• (^) Isolation of bacteria is much more increased by cold enrichment. In this method, the specimens are stored in thioglycolate broth or tryptose phosphate at 4°C, and subcultures are carried out at weekly intervals for 1–6 months.

• (^) L. monocytogenes is sensitive to ampicillin, gentamicin, and cotrimoxazole. Cephalosporins are not useful for infections caused by Listeria.