Highly stable bacteriophages PIN1 and PIN2 have hallmarks of flagellotropic phages but infect immotile bacteria

Bacteriophages (phages) are viruses that kill bacteria, with potential as antibacterial agents in industrial settings, agriculture, and human health. Here, we identified two phages, PIN1 and PIN2, that can kill clinical isolates of the human pathogen Klebsiella pneumoniae. The phages are highly stable; PIN2 in particular resisted multiple freeze-thaw cycles over 12 months without loss of activity. PIN1 and PIN2 are related to flagellotropic phages, an idiosyncratic group of viruses that bind to bacterial flagellae, but K. pneumoniae is an immotile pathogen that does not have flagellae. Genetic mosaicism is observed, wherein the long, flexible tail fiber of the flagellotropic phages has been substituted by a more compact tail fiber that binds the Klebsiella host through cell-surface capsular polysaccharide and lipopolysaccharide. PIN1 and PIN2 belong to the Yonseivirus group of phages, with initial analyses across the group suggesting further recent diversification in the tail-fiber cassette in the Yonseivirus genomes.