Analysis of Legionella pneumophila strains associated with nosocomial pneumonia in a neonatal intensive care unit.

A premature child received continuous mechanical ventilation in a neonatal intensive care unit. On day 10 of his life he developed pneumonia due to Legionella pneumophila serogroup 1, monoclonal subtype Bellingham. The strain was cultured from a tracheal secretion taken on day 10 and detected by immunofluorescence using monoclonal antibodies on days 10, 12 and 17. Legionella pneumophila serogroups 1 and 6 (10(2)-4 x 10(4) cfu/l) were cultured from both central and peripheral hot water systems. Monoclonal antibody testing, macrorestriction analysis of the genomic DNA using pulse-field electrophoresis, and electrophoretic alloenzyme typing showed the isolate from the child to be identical to the serogroup 1 strains from the hot water system. Four unrelated Legionella strains of the same monoclonal subgroup Bellingham were studied for comparison. Legionellae were also isolated from two other incubators, but no clinical or microbiological indications of legionellosis were found in the neonates hospitalised there. Serogroup 1 strains isolated from the patient and from the hot water system and serogroup 6 isolates from the hot water supply were able to multiply in cultured Acanthamoeba castellanii cells and in guinea pigs. The serogroup 6 strain, although prevalent in the incubators, was not found in any of the clinical specimens by either culture of immunofluorescence.