Code blue: Acinetobacter baumannii, a nosocomial pathogen with a role in the oral cavity

Abstract

Actinetobacter baumannii is an important nosocomial pathogen that can cause a wide range of serious conditions including pneumonia, meningitis, necrotizing fasciitis and sepsis. It is also a major cause of wound infections in military personnel injured during the conflicts in Afghanistan and Iraq, leading to its popular nickname of 'Iraqibacter'. Contributing to its success in clinical settings is resistance to environmental stresses such as desiccation and disinfectants. Moreover, in recent years there has been a dramatic increase in the number of A. baumannii strains with resistance to multiple antibiotic classes. Acinetobacter baumannii is an inhabitant of oral biofilms, which can act as a reservoir for pneumonia and chronic obstructive pulmonary disease. Subgingival colonization by A. baumannii increases the risk of refractory periodontitis. Pathogenesis of the organism involves adherence, biofilm formation and iron acquisition. In addition, A. baumannii can induce apoptotic cell death in epithelial cells and kill hyphal forms of Candida albicans. Virulence factors that have been identified include pili, the outer membrane protein OmpA, phospholipases and extracellular polysaccharide. Acinetobacter baumannii can sense blue light through a blue-light sensing using flavin (BLUF) domain protein, BlsA. The resulting conformational change in BlsA leads to changes in gene expression, including virulence genes.

Keywords: Acinetobacter; infection models; oral microbiology; periodontal disease; respiratory tract microbiology.

Figure 1

Virulence mechanisms of Acinetobacter baumannii. The organism is capable of forming biofilms on biotic and abiotic surfaces by attaching via its type IV pili. Subsequently Bap is secreted to help biofilm maturation and adherence to eukaryotic cells. Contact with host cells leads to secretion of OmpA, which induces apoptosis in the host by causing cytochrome c release from the mitochondria. This in turn stimulates to Apoptosis Inducing Factor localization in the nucleus. Capsulated A. baumannii are protected from detection by the host due to the inability of antibodies and complement to bind to the bacterial surface and diminished recognition by Toll-like receptors. To acquire the iron needed for survival, A. baumannii secretes Acinetobactin, a siderophore, which sequesters iron from the host.

Figure 2

Structure of BlsA. The Blue-light sensing protein of Acine-tobacter baumannii. The protein consists of two α helices that bind flavin adenine dinucleotide upon excitation by blue light at a wavelength of 470 nm. From Brust et al. (2014).