Coping with low water activities and osmotic stress in Acinetobacter baumannii: significance, current status and perspectives

Abstract

Summary Multidrug resistant (MDR) pathogens are one of the most pressing challenges of contemporary health care. Acinetobacter baumannii takes a predominant position, emphasized in 2017 by the World Health Organization. The increasing emergence of MDR strains strengthens the demand for new antimicrobials. Possible targets for such compounds might be proteins involved in resistance against low water activity environments, since A. baumannii is known for its pronounced resistance against desiccation stress. Despite the importance of desiccation resistance for persistence of this pathogen in hospitals, comparable studies and precise data on this topic are rare and the mechanisms involved are largely unknown. This review aims to give an overview of the studies performed so far and the current knowledge on genes and proteins important for desiccation survival. ‘Osmotic stress’ is not identical to ‘desiccation stress’, but the two share the response of bacteria to low water activities. Osmotic stress resistance is in general studied much better, and in recent years it turned out that accumulation of compatible solutes in A. baumannii comprises some special features such as the bifunctional enzyme MtlD synthesizing the unusual solute mannitol. Furthermore, the regulatory pathways, as understood today, will be discussed.

Identification of osmo‐dependent and osmo‐independent betaine‐choline‐carnitine transporters in Acinetobacter baumannii: role in osmostress protection and metabolic adaptation

“…The adaptation of the Gram-negative A. baumannii to extreme dry and salty environments is based on the outstanding feature to cope with low water activities and osmotic stress (Breisch et al, 2018, Jawad et al, 1996, Wendt et al, 1997, Zeidler et al, 2017, Zeidler and Müller, 2019. These features also allow A. baumannii to persist on dry surfaces in clinical environments for weeks or even month.…”

Unusual deprivation of compatible solutes in Acinetobacter baumannii

“…However, it is unquestioned that the pronounced desiccation resistance of A . baumannii is of relevance, which enables the bacterium to persist for long periods of time in hospitals (Jawad et al ., ; Zeidler and Müller, ). Furthermore, adaptation to and persistence in the human host is of vital importance for pathogenic bacteria, which requires resistance mechanisms against several stress factors (Chowdhury et al ., ).…”

“…The response of A . baumannii to low water activities (osmotic stress and desiccation) has been reviewed recently (Zeidler and Müller, ).…”

“…Several factors contributing to the multifactorial virulence of the pathogen have already been identified (for a recent review, see Wong et al (2017)), but nevertheless, researchers are far from overlooking the crucial traits enabling its success. However, it is unquestioned that the pronounced desiccation resistance of A. baumannii is of relevance, which enables the bacterium to persist for long periods of time in hospitals (Jawad et al, 1998;Zeidler and Müller, 2019a). Furthermore, adaptation to and persistence in the human host is of vital importance for pathogenic bacteria, which requires resistance mechanisms against several stress factors (Chowdhury et al, 1996).…”

“…Furthermore, otsB is required for full virulence in a Galleria mellonella infection model (Gebhardt et al, 2015), highlighting the link between resistance against osmotic stress and pathogenicity. The response of A. baumannii to low water activities (osmotic stress and desiccation) has been reviewed recently (Zeidler and Müller, 2019a).…”