A fluoroquinolone induces a novel mitogen-encoding bacteriophage in Streptococcus canis

This study investigated whether the recently recognized emergence of canine streptococcal toxic shock syndrome (STSS) and necrotizing fasciitis (NF) might be partly attributed to the use of fluoroquinolones to treat Streptococcus canis infections in dogs. Both mitomycin and the fluoroquinolone enrofloxacin caused bacteriophage-induced lysis of S. canis strain 34, an isolate from a case of canine STSS and NF. Fluoroquinolone-evoked, bacteriophage-induced lysis occurred over a range of concentrations similar to those that would occur after treatment of dogs with these agents. To search for a possible bacteriophage-encoded streptococcal superantigen gene(s), a library of the 36.5 (+/-1.1)-kb bacteriophage, designated Phisc1, was made by ligating 3- to 7-kb Tsp5091-digested Phisc1 fragments into an EcoRI-digested lambdaZapII vector. Recombinants were screened for mitogenic activity by using canine peripheral blood lymphocytes. Of 800 recombinants, screened, 11 recombinants with mitogenic effects were identified, and their inserts were sequenced. The highest homology of 11.6 kb of sequenced Phisc1 DNA was to the completely sequenced Streptococcus pneumoniae bacteriophage MM1. Seven of the 11 Phisc1 sequenced inserts contained a 552-bp open reading frame, sent, with 27% amino acid similarity to pokeweed (Phytolacca americana) mitogen. PCR showed this gene to be present in 22 of 23 S. canis isolates tested. Quantitative reverse transcription-PCR showed that bacteriophage induction was associated with a 58-fold enhancement of expression of this gene relative to that in a noninduced culture of a similar age. The presence of this gene on a fluoroquinolone-induced bacteriophage may explain tween fluoroquinolone use in dogs and the development of canine STTS and NF.