Acinetobacter baumannii is a well-known pathogen responsible for recurring outbreaks of serious hospital or nosocomial infections across the world. It causes all kinds of infections, ranging from pneumonia to blood infections in people admitted to hospital and above all in those in intensive care units. The Department of Immunology, Microbiology and Parasitology of the UPV/EHU’s Faculty of Science and Technology ran a piece of research to explore and assess "the capacity of this bacterium to persist in various conditions that may be similar to those found in the hospital environment, the survival strategies it uses and its capacity to resist disinfectants", explained Maite Orruño-Beltrán, PhD-holder in Biology and one of the lead authors of this study. The results of the study have been published recently by the Journal of Hospital Infection.

For their research they used “four strains of A. baumannii obtained from standard culture collection as well as from clinical isolates from the Hospital de Valdecilla”, said Orruño. “To conduct the survival studies, we incubated the strains for 30 days without nutrients at various temperatures (20 and 37 degrees) and in various media (in liquid and on different solid surfaces). We also checked the effect exerted by various disinfectants routinely used in hospitals. From time to time we gathered samples to determine how many remained cultivable and/or viable”.

Surviving for periods of up to 90 day

Leaving disinfectants on one side, they found that the survival of these bacteria was affected by temperature and by the surface on which they had been placed. As the researcher stressed, “above all, we found that those kept at 20 degrees on solid surfaces resisted 30 days in a cultivable state without any apparent changes. We even extended some experiments to 90 days and the population still remained cultivable”.

The bacteria that remained at 37 degrees displayed reduced survival. “That temperature is the optimum one for growth, but not for survival when they are deprived of nutrients. In these conditions bacteria in general withstand lower temperatures better,” explained Dr Orruño. As they were able to confirm in the research, at that temperature the surviving bacteria adopted two survival strategies: “Some moved to a state known as viable but nonculturable. In that state although the bacteria are unable to grow, they remain in a dormant state from which they could recover if conditions become favourable again. This state could be of considerable importance, because many techniques for detecting microorganisms are based on their cultivability and, therefore, do not detect the presence of these bacteria, which can however retain their pathogenicity,” said the researcher.

The second strategy that some strains developed was that many of the cells lose viability, they may be dead cells, “but a more or less significant fraction of the population remains cultivable by using the nutrients from the cells that have died off”.

With respect to the effect of disinfectants they found that even though the vast majority of the cells stopped being cultivable, Acinetobacter baumannii was not eliminated from the surfaces studied and a considerable fraction retained viability (they were not dead cells). This phenomenon could entail risks since we could regard the disinfection procedure as having been sufficient, but some bacteria remain on that surface as potential spreaders of diseases.