Legionella - Detection

Detection

Legionella is traditionally detected by culture on buffered charcoal yeast extract (BCYE) agar. Legionella requires the presence of cysteine and iron to grow and therefore does not grow on common blood agar media used for laboratory based total viable counts or on site dipslides. Common laboratory procedures for the detection of Legionella in water concentrate the bacteria (by centrifugation and/or filtration through 0.2 micrometre filters) before inoculation onto a charcoal yeast extract agar containing antibiotics (e.g. glycine vancomycim polymixin cyclohexamide, GVPC) to suppress other flora in the sample. Heat or acid treatment are also used to reduce interference from other microbes in the sample.

After incubation for up to 10 days, suspect colonies are confirmed as Legionella if they grow on BCYE containing cysteine, but not on agar without cysteine added. Immunological techniques are then commonly used to establish the species and/or serogroups of bacteria present in the sample.

Many hospitals use the Legionella Urinary Antigen test for initial detection when Legionella pneumonia is suspected. Some of the advantages offered by this test is that the results can be obtained in a matter of hours rather than the five days required for culture, and that a urine specimen is generally more easily obtained than a sputum specimen. One disadvantage is that the urine antigen test only detects anti-bodies towards Legionella pneumophila; only a culture will detect infection by the other Legionella species.

New techniques for the rapid detection of Legionella in water samples are emerging including the use of polymerase chain reaction (PCR) and rapid immunological assays. These technologies can typically provide much faster results.