ABSTRACT
Lytic bacteriophage A25, which infects Streptococcus pyogenes and several related species, has been used to better understand phage-microbe interactions due to its ability to mediate high-efficiency transduction. Most of these studies, however, are decades old and were conducted prior to the advent of next-generation sequencing and bioinformatics. The aim of our study was to gain a better understanding of the mechanism of high-efficiency transduction through analysis of the A25 genome. We show here that phage A25 is related to a family of genome prophages and became a lytic phage following escape from lysogeny. A lambdoid-like residual lysogeny module consisting of an operator site with two promoters and a cro-like antirepressor gene was identified, but the genes for the cI-like repressor and integrase are missing. Additionally, the genetic organization of the A25 genome was found to be modular in nature and similar to that of many prophages of S. pyogenes as well as from other streptococcal species. A study of A25 homology to all annotated prophages within S. pyogenes revealed near identity within the remnant lysogeny module of the A25 phage genome to the corresponding regions in resident prophages of genome strains MGAS10270 (M2), MGAS315 (M3), MGAS10570 (M4), and STAB902 (M4). Host range studies of MGAS10270, MGAS315, and MGAS10750 demonstrated that these strains were resistant to A25 infection. The resistance mechanism of superinfection immunity was confirmed experimentally through complementation of the operator region and cI-like repressor from prophage MGAS10270.2 into susceptible strains SF370, CEM1Δ4 (SF370ΔSpyCIM1), and ATCC 12204, which rendered all three strains resistant to A25 infection. In silico prediction of packaging through homology analysis of the terminase large subunit from bacteriophages within the known packaging mechanism of Gram-positive bacteria as well as the evidence of terminally redundant and/or circularly permuted sequences suggested that A25 grouped with phages employing the less stringent pac-type packaging mechanisms, which likely explains the characteristic A25 high-efficiency transduction capabilities. Only a few examples of lytic phages appearing following loss of part or all of the lysogeny module have been reported previously, and the genetic mosaicism of A25 suggests that this event may not have been a recent one. However, the discovery that this lytic bacteriophage shares some of the genetic pool of S. pyogenes prophages emphasizes the importance of genetic and biological characterization of bacteriophages when selecting phages for therapeutics or disinfectants, as phage-phage and phage-microbe interactions can be complex, requiring more than just assessment of host range and carriage of toxoid or virulence genes.IMPORTANCE Bacteriophages (bacterial viruses) play an important role in the shaping of bacterial populations as well as the dissemination of bacterial genetic material to new strains, resulting in the spread of virulence factors and antibiotic resistance genes. This study identified the genetic origins of Streptococcus pyogenes phage A25 and uncovered the molecular mechanism employed to promote horizontal transfer of DNA by transduction to new strains of this bacterium as well as identified the basis for its host range.
