TAXONOMY: Bacteria -> Terrabacteria group -> Firmicutes -> Bacilli -> Bacillales -> Listeriaceae -> Listeria
Regular, short rods, 0.4–0.5 × 1–2 µm with parallel sides and blunt ends. Usually occur singly or in short chains. In older or rough cultures, filaments of ≥6 µm in length may develop. Gram-positive with even staining, but some cells, especially in older cultures, lose their ability to retain the Gram strain. Not acid-fast. Capsules not formed. Do not form spores. All species motile with peritrichous flagella when cultured <30°C. Aerobic and facultative anaerobic. Colonies (24–48 h) are 0.5–1.5 mm in diameter, round, translucent, low convex with a smooth surface and entire margin, non-pigmented with a crystalline central appearance. May be sticky when removed from agar surfaces, usually emulsify easily, and may leave a slight impression on the agar surface after removal. Older cultures (3–7 d) are larger, 3–5 mm in diameter, have a more opaque appearance, sometimes with a sunken center: rough colonial forms may develop. In 0.25% (w/v) agar, 8% (w/v) gelatin and 1.0% (w/v) glucose semi-solid medium, growth along the stab after 24 h at 37°C is followed by irregular, cloudy extensions into the medium. Growth spreads slowly through the entire medium. An umbrella-like zone of maximal growth occurs 3–5 mm below the surface. Temperature limits of growth <0 to 45°C; optimal growth at 30–37°C. Do not survive heating at 60°C for 30 min. Growth occurs between pH 6 and pH 9. Growth occurs in nutrient broth supplemented with up to 10% (w/v) NaCl. Catalase-positive, oxidase-negative. Cytochromes produced. Homofermentative anaerobic catabolism of glucose results in production of l(+)-lactic acid, acetic acid plus other end products. Acid but no gas produced from other sugars. Methyl-red-positive, Voges–Proskauer-positive. Exogenous citrate not utilized. Organic growth factors are required. Indole is not produced. Esculin and sodium hippurate are hydrolyzed. Urea is not hydrolyzed. Gelatin, casein, and milk are not hydrolyzed.
DNA G+C content (mol%): 36–42.5 (Tm).
Type species: Listeria monocytogenes