Environmental signals presiding to biofilm formation and extracellular polysaccharide (EPS) production in Acinetobacter baumannii

Acinetobacter baumannii, an environmental Gram-negative bacterium, is emerging as an important nosocomial pathogen. We tested the ability of an A. baumannii clinical isolate (named SMAL) to form biofilm and to produce extracellular polysaccharides (EPS), factors likely to contribute to its persistence in hospital settings. We find that, similar to many other environmental bacteria, biofilm production by A. baumannii is higher at sub-optimal growth temperatures. Production of adhesion factors is strongly affected by growth media: in the presence of glucose, biofilm formation is totally inhibited by treatment with cellulase, thus suggesting that cellulose, or another cellulase-sensitive EPS, is an important biofilm determinant. In contrast, biofilm formation in peptone-based medium is insensitive to cellulase treatment. Biofilm formation by A. baumannii SMAL clone is increased by exposure to subinhibitory concentrations of the antibiotic imipenem, which also stimulates production of iron uptake proteins. Consistent with this observation, biofilm formation is increased in iron-supplemented medium, suggesting that biofilm stimulation by imipenem might be mediated by increased iron uptake. Biofilm formation in A. baumannii SMAL is finely regulated at gene expression level: glucose can induce transcription of A1S_2485, a gene encoding a putative glycosyl-transferase. In contrast, imipenem activates expression of another glycosyl-transferase, A1S_0939, responsible for poly-N-acetylglucosamine production. Consistent with this result, biofilm stimulation by imipenem can be prevented by treatment with Dispersin B, which specifically degrades poly-N-acetylglucosamine. Our results suggest that environmental signals can trigger biofilm formation in A. baumannii SMAL by induction of EPS biosynthetic genes.