An outer membrane porin protein modulates phage susceptibility in Edwardsiella ictaluri

Bacteriophages Phi eiAU and Phi eiDWF are lytic to the catfish pathogen Edwardsiella (Edw.) ictaluri. The Edw. ictaluri host factors that modulate phage host interactions have not been described previously. This study identified eleven unique Edw. ictaluri host factors essential for phage infection by screening a transposon mutagenized library of two Edw. ictaluri strains for phage-resistant mutants. Two mutants were isolated with independent insertions in the ompLC gene that encodes a putative outer membrane porin. Phage binding and efficiency of plaguing assays with Edw. ictaluri EILO, its ompLC mutant and a complemented mutant demonstrated that OmpLC serves as a receptor for phage Phi eiAU and Phi eiDWF adsorption. Comparison of translated OmpLCs from 15 Edw. ictaluri strains with varying degrees of phage susceptibility revealed that amino acid variations were clustered on the predicted extracellular loop 8 of OmpLC. Deletion of loop 8 of OmpLC completely abolished phage infectivity in Edw. ictaluri. Site-directed mutagenesis and transfer of modified ompLC genes to complement the ompLC mutants demonstrated that changes in ompLC sequences affect the degree of phage susceptibility. Furthermore, Edw. ictaluri strain Alg-08-183 was observed to be resistant to Phi eiAU, but phage progeny could be produced if phage DNA was electroporated into this strain. A host-range mutant of Phi eiAU, Phi eiAU-183, was isolated that was capable of infecting strain Alg-08-183 by using OmpLC as a receptor for adsorption. The results of this study identified Edw. ictaluri host factors required for phage infection and indicated that OmpLC is a principal molecular determinant of phage susceptibility in this pathogen.