Insights into mucoid Acinetobacter baumannii: A review of microbiological characteristics, virulence, and pathogenic mechanisms in a threatening nosocomial pathogen

Acinetobacter baumannii (A. baumannii) is an opportunistic pathogen and leading cause of health care-associated infections. Several known pathogenic factors, including lipopolysaccharide, capsular polysaccharides, phospholipase, protein secretion systems, two component efflux pumps and biofilm formation, are associated with the bacteria. However, owing to the new biological characteristics rapidly developed under the external stress, the pathogenesis becomes complicated. The formation of mucoid phenotype is a major adaptive defense response for A. baumannii. The mucus is mainly comprised of capsular polysaccharide, which forms a barrier around the bacterial cell wall, and provides protection from environmental pressures and host immune responses. Importantly, mucoid formation has been shown to confer changes in the pathogenicity and pathogenesis of A. baumannii. Thus, this review aims to highlight the biological characteristics, virulence and underlying pathogenic mechanisms of mucoid A. baumannii. We primarily provide profound insight into the structure, function and detecting methods of the mucus in mucoid A. baumannii. Then we mainly illustrate the current studies on mucoid A. baumannii. Finally, the unsolved areas associated with mucoid A. baumannii are summarized. In summary, this review will critically offer a comprehensive understanding of mucoid A. baumannii and provide novel insight into the diagnosis and intervention strategies for the increasingly deadly human pathogen.