Mechanisms of Acinetobacter baumannii Capsular Polysaccharide Cleavage by Phage Depolymerases

Aerobic gram-negative bacterium Acinetobacter baumannii has recently become one of the most relevant pathogens associated with hospital-acquired infections worldwide. A. baumannii produces a capsule around the cell, which represents a thick viscous layer of structurally variable capsular polysaccharide (CPS). The capsule protects the bacteria against unfavorable environmental factors and biological systems, including bacteriophages and host immune system. Many A. baumannii phages have structural depolymerases (tailspikes) that specifically recognize and digest bacterial CPS. In this work, we studied the interaction of tailspike proteins of four lytic depolymerase-carrying phages with A. baumannii CPS. Depolymerases of three bacteriophages (Fri1, AS12, and BS46) were identified as specific glycosidases that cleave the CPS of A. baumannii strains 28, 1432, and B05, respectively, by the hydrolytic mechanism. The gp54 depolymerase from bacteriophage AP22 was characterized as a polysaccharide lyase that cleaves the CPS of A. baumannii strain 1053 by β-elimination at hexuronic acid (ManNAcA) residues.