Legionella pneumophila

It causes atypical pneumonia seen in people with reduced immunity due to old age, cigarette smoking, AIDS, cancer chemotherapy, alcoholism, corticosteroid use etc. who are exposed to the bacteria in aerosols from air conditioners , humidifiers, respiratory therapy instruments, showers and cooling towers. It is a Gram negative bacillus that is not stained easily by the Gram or H and E stain and needs special stains such as Dieterle silver impregnation technique and Gimenez stain. It is difficult to culture. Charcoal yeast extract medium with a high concentration of iron and cysteine is used for the culture of Legionella.

The disease (also called Pontiac fever) presents with mild to severe influenza like pneumonia with cough and scanty sputum developing hemoptysis in late stages, confusion, stupor, headache, myalgia with GI symptoms such as abdominal pain and diarrhea. Hyponatremia is a characteristic finding and helps to make a diagnosis in the setting of typical clinical features. Interestingly, although it causes pneumonia, due to the fastidious nature of the organism, identification is preferably done by detection of antigens in urine. Direct fluorescent antibody staining of respiratory specimens, culture on special media, PCR and antibody detection by indirect immunofluorescence assay are some of the other tests that can be employed.