Influence of relative humidity and suspending menstrua on survival of Acinetobacter spp. on dry surfaces

Acinetobacter spp. are being reported with increasing frequency as a cause of nosocomial infection and have been isolated from the skin of healthy individuals, patients, hospital staff, dry nonbiotic objects, and different pieces of medical equipment. Factors affecting the survival of Acinetobacter spp. under conditions closely similar to those found in the hospital environment were investigated in the present study to help us understand the epidemiology of nosocomial Acinetobacter infection. Bacterial cells were suspended in distilled water or bovine serum albumin and were dried onto glass coverslips and kept at different relative humidities. Cells washed from coverslips were used to determined viable counts. Freshly isolated strains of Acinetobacter spp. belonging to the clinically important Acinetobacter calcoaceticus-Acinetobacter baumannii complex were found to be more resistant to drying conditions (e.g., 30 days for A. baumannii 16/49) than American Type Culture Collection strains (e.g., 2 days for A. baumannii ATCC 9955). The majority of strains belonging to the Acb complex had survival times similar to those observed for the gram-positive organism Staphylococcus aureus tested in the experiment. Survival times were prolonged for almost all the strains tested when they were suspended in bovine serum albumin (e.g., 60 days for A. baumannii R 447) compared with those for strains suspended in distilled water (11 days for R 447). The survival times for strains at higher relative humidity (31 or 93%) were longer than those for strains of Acinetobacter kept at a relative humidity of 10% (11 days at 31% relative humidity and 4 days at 10% relative humidity for R447). These findings are consistent with the observed tendency of Acinetobacter spp. to survive on dry surfaces, and they can be transferred not only by moist vectors but also under dry conditions in a hospital environment during nosocomial infection outbreaks. The results obtained in the experiment support the previously suggested airborne spread of Acinetobacter spp. in hospital wards and repeated outbreaks after incomplete disinfection of contaminated dry surfaces.