Laboratory Diagnosis of Listeria Moncytogenes Infection
The organism grows well on blood or nutrient agar and in conventional blood culture broths. On blood agar, the colonies usually are surrounded by a narrow band of B-haemolysis resembling that of B-streptococci. It can be differentiated from B-streptococci by gram-stain and by motility testing at 20-25oC and at 37oC. Listeria monocytogenes ferments glucose, producing principally lactic acid without gas. It elaborates catalase, hydrolyzes esculin, and produces acetoin (Vogt-Proskauer test). Instillation into the conjunctival sac of a rabbit produces a purulent conjunctivitis followed by a keratis (Anton test). On the basis of somatic (O) and flagellar (H) Ags, 17 serotypes have been described. Serotypes 1a, 1b, and 4b account for more than 90% of clinical isolates. Serological and phage typing can be helpful in the investigation of common source outbreaks. The methods for analysis of food are complex and time-consuming. The present FDA method requires 24 and 48 hours of enrichment, followed by a variety of other tests. Total time to identification is from 5 to 7 days.
Ampicillin or penicillin are the drugs of choice. A synergistic combination of one of these B-lactams with gentamycin is sometimes used in immunocompromized or neutropenic patients.