['To study the role of amoeba-associated alpha Proteobacteria as infecting agents in intensive care units (ICUs).', 'Amoeba-associated alpha Proteobacteria were isolated from water samples taken from ICU taps and general hospital reservoir tanks using an amoebal co-culture procedure. Isolates were identified by 16S rDNA gene sequence comparison, and one isolate of each species was used as an antigen in a microimmunofluorescence assay to test the sera of the patients in the ICUs and compare them with those of control subjects.', 'The four university hospitals in Marseilles, France.', 'The sera of 85 patients in the ICUs with nosocomial pneumonia were tested.', 'We isolated 64 bacterial strains that were identified as Afipia species or close relatives within the Rhizobiaceae subgroup of alpha Proteobacteria. These bacteria were assigned to 8 different species. Eleven patients were found to have an elevated antibody titer to either Afipia genospecies 1, or 3 still unnamed bacteria. No specific antibodies were detected in 100 control subjects (P < .01).'] Are patients in the intensive care unit exposed to amoeba-associated pathogens?