Development of a quantitative PCR assay for monitoring Streptococcus agalactiae colonization and tissue tropism in experimentally infected tilapia

• Published in: Journal of fish diseases 2016-02, Vol.39 (2), p.229-238
• Main Authors: Su, Y‐L, Feng, J, Li, Y‐W, Bai, J‐S, Li, A‐X
• Format: Article
• Language: English
• Subjects: Animals; annealing; aquaculture; bacterial colonization; Bacterial Load; brain; detection limit; DNA, Ribosomal Spacer - genetics; eyes; financial economics; fish; Fish Diseases - diagnosis; Fish Diseases - microbiology; fish farms; heart; intergenic DNA; intestines; kidneys; liver; microbial load; monitoring; muscles; Oreochromis niloticus; pathogens; quantitative polymerase chain reaction; quantitative real-time PCR; Real-Time Polymerase Chain Reaction - standards; Real-Time Polymerase Chain Reaction - veterinary; ribosomal RNA; Sensitivity and Specificity; spleen; Streptococcal Infections - diagnosis; Streptococcal Infections - microbiology; Streptococcal Infections - veterinary; Streptococcus agalactiae; Streptococcus agalactiae - genetics; Streptococcus agalactiae - metabolism; Temperature; Tilapia; tissue tropism; Viral Tropism; virulence
• ISSN: 0140-7775; 1365-2761
• DOI: 10.1111/jfd.12358

Streptococcus agalactiae has become one of the most important emerging pathogens in the aquaculture industry and has resulted in large economic losses for tilapia farms in China. In this study, three pairs of specific primers were designed and tested for their specificities and sensitivities in quantitative real‐time polymerase chain reactions (qPCRs) after optimization of the annealing temperature. The primer pair IGS‐s/IGS‐a, which targets the 16S‐23S rRNA intergenic spacer region, was finally chosen, having a detection limit of 8.6 copies of S. agalactiae DNA in a 20 μL reaction mixture. Bacterial tissue tropism was demonstrated by qPCR in Oreochromis niloticus 5 days post‐injection with a virulent S. agalactiae strain. Bacterial loads were detected at the highest level in brain, followed by moderately high levels in kidney, heart, spleen, intestines, and eye. Significantly lower bacterial loads were observed in muscle, gill and liver. In addition, significantly lower bacterial loads were observed in the brain of convalescent O. niloticus 14 days post‐injection with several different S. agalactiae strains. The qPCR for the detection of S. agalactiae developed in this study provides a quantitative tool for investigating bacterial tissue tropism in infected fish, as well as for monitoring bacterial colonization in convalescent fish.