Capsule depolymerase activity of phages infecting the Acinetobacter baumannii-calcoaceticus complex

Abstract

To be able to enter and replicate in exopolysaccharide (EPS) slime or capsule surrounded bacteria, bacteriophages (phages) have evolved the ability to overcome the EPS structure by producing virion-associated proteins with polysaccharide depolymerization activities. We have studied phages infecting the Acinetobacter baumannii-Acinetobacter calcoaceticus (ACB) complex, which groups A. baumannii, A. calcoaceticus, A. pittii, A. nosocomialis and A. seifertii species. It is known that about 100 different capsule polysaccharide (CPS) synthetic loci are found in A. baumannii genomes alone. This situation is even more complex, with some strains of A. baumannii having nearly identical CPS synthetic loci to strains of A. nosocomialis or A. pittii, and supposedly producing identical CPS. We have isolated and characterized 21 phages infecting the ACB complex and demonstrate that they have specialized depolymerases that degrade polymers (e.g. capsular and structural polysaccharides) to facilitate their access to the hosts. To further characterize the phage-host interactions, we have sequenced bacterial genomes and mutated the CPS synthetic loci to create CPS-deficient mutants, to prove that the ACB phages recognize the CPS as the primary receptor. We further demonstrate that recombinantly expressed depolymerases are active and key components in the tail specificity apparatus of Podoviridae viruses. We could conclude that phages infecting the ACB complex represent a source of enzymes that degrade a complex variety of polymeric substances that can be further exploited as a serotyping scheme currently inexistent for Acinetobacter species.