Laboratory Diagnosis of Listeria Moncytogenes Infection



Laboratory Diagnosis


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.