A new group of cosmopolitan bacteriophages induce a carrier state in the pandemic strain of Vibrio parahaemolyticus

A clonal population of pathogenic Vibrio parahaemolyticus O3 : K6 serovar has spread in coastal waters, causing outbreaks worldwide since 1996. Bacteriophage infection is one of the main factors affecting bacterial strain concentration in the ocean. We studied the occurrence and properties of phages infecting this V. parahaemolyticus pandemic strain in coastal waters. Analysing 143 samples, phages were found in 13. All isolates clustered in a closely related group of podophages with at least 90% nucleotide sequence identity in three essential genes, despite distant geographical origins. These bacteriophages were able to multiply on the V. parahaemolyticus pandemic strain, but the impact on host concentration and subsequent growth was negligible. Infected bacteria continued producing the phage but were not lysogenized. The phage genome of prototype strain VP93 is 43 931 nucleotides and contains 337 bp direct terminal repeats at both ends. VP93 is the first non-Pseudomonas phage related to the Phi KMV-like subgroup of the T7 supergroup. The lack of a major effect on host growth suggests that these phages exert little control on the propagation of the pandemic strain in the environment. This form of phage growth can be modelled if phage-sensitive and -resistant cells that convert to each other with a high frequency are present in clonal cultures of pandemic V. parahaemolyticus.