RB49-like Bacteriophages Recognize O Antigens as One of the Alternative Primary Receptors.

The power of most of the enterobacterial O antigen types to provide robust protection against direct recognition of the cell surface by bacteriophage receptor-recognition proteins (RBP) has been recently recognized. The bacteriophages infecting O antigen producing strains of E. coli employ various strategies to tackle this nonspecific protection. T-even related phages, including RB49-like viruses, often have wide host ranges, being considered good candidates for use in phage therapy. However, the mechanisms by which these phages overcome the O antigen barrier remain unknown. We demonstrate here that RB49 and related phages Cognac49 and Whisky49 directly use certain types of O antigen as their primary receptors recognized by the virus long tail fibers (LTF) RBP gp38, so the O antigen becomes an attractant instead of an obstacle. Simultaneously to recognize multiple O antigen types, LTFs of each of these phages can bind to additional receptors, such as OmpA protein, enabling them to infect some rough strains of E. coli. We speculate that the mechanical force of the deployment of the short tail fibers (STF) triggered by the LTF binding to the O antigen or underneath of it, allows the receptor binding domains of STF to break through the O polysaccharide layer.

Equine Intestinal O-Seroconverting Temperate Coliphage Hf4s: Genomic and Biological Characterization.

Tailed bacteriophages constitute the bulk of the intestinal viromes of vertebrate animals. However, the relationships between lytic and lysogenic lifestyles of phages in these ecosystems are not always clear and may vary between the species or even between the individuals. The human intestinal (fecal) viromes are dominated mostly by temperate phages, while in horse feces virulent phages are more prevalent. To our knowledge, all the previously reported isolates of horse fecal coliphages are virulent. Temperate coliphage Hf4s was isolated from horse feces, from the indigenous equine Escherichia coli 4s strain. It is a podovirus related to the Lederbergvirus genus (including the well-characterized Salmonella bacteriophage P22). Hf4s recognizes the host O antigen as its primary receptor and possesses a functional O antigen seroconversion cluster that renders the lysogens protected from superinfection by the same bacteriophage and also abolishes the adsorption of some indigenous equine virulent coliphages, such as DT57C, while other phages, such as G7C or phiKT, retain the ability to infect E. coli 4s (Hf4s) lysogens. IMPORTANCE The relationships between virulent and temperate bacteriophages and their impact on high-density symbiotic microbial ecosystems of animals are not always clear and may vary between species or even between individuals. The horse intestinal virome is dominated by virulent phages, and Hf4s is the first temperate equine intestinal coliphage characterized. It recognizes the host O antigen as its primary receptor and possesses a functional O antigen seroconversion cluster that renders the lysogens protected from superinfection by some indigenous equine virulent coliphages, such as DT57C, while other phages, such as G7C or phiKT, retain the ability to infect E. coli 4s (Hf4s) lysogens. These findings raise questions on the significance of bacteriophage-bacteriophage interactions within the ecology of microbial viruses in mammal intestinal ecosystems.