A new sequence type (ZST-1) of infectious Streptococcus agalactiae from Chinese cyprinid fish, Schizopygopsis pylzovi

Streptococcus agalactiae (group B streptococcus, GBS) infection poses a serious threat to fish farming, particularly in warm-water locations. Recently, an infectious disease of unknown etiology with accumulative mortality rates of 25% to 40% was observed in cyprinid fish, Schizopygopsis pylzovi, of the Sichuan Province, China during autumn 2015. The clinical and pathological features of the disease were similar to those caused by streptococcosis infection in other fish species. Two Gram-positive, chain-forming coccus strains (ZYW151011 and ZYW151012) isolated from dying S. pylzovi were considered as the main pathogenic agents according to the experimental infections, and the LD50 of both isolates were 1.61×106 and 2.07×106cfu per fish respectively. According to morphology, physiological and biochemical, 16S rRNA gene and cfb gene specific sequence analyses, the two isolates were identified as S. agalactiae. The results of serotypes and Multiple Locus Sequence Typing (MLST) indicated that the isolates were Ia. and a new sequence typing (ZST-1) was identified, which is a close relative to ST-261. The sensitivity results showed that both isolates were sensitive to ceftriaxone, enrofloxacin and ofloxacin but resistant to doxycycline, florfenicol and amoxicillin. To our knowledge, this is the first report on S. agalactiae natural infection in S. pylzovi, and the isolate ZST-1 is a new sequence type for the variation of glnA. •First report of occurrence of the bacterium S. agalactiae in farmed Schizopygopsis pylzovi in China•Histopathology and PCR-based molecular diagnosis confirm the occurrence caused by S. agalactiae.•A new sequence typing (ZST-1) associated with the variation of glnA was identified.