Development and Application of a Novel q PCR Assay for Quantifying Flavophage F p V ‐4 Against

ABSTRACT

Bacteriophage (phage) therapy is increasingly being used to control Flavobacterium psychrophilum impacts upon the health and production of salmonid aquaculture worldwide, and FpV‐4 is a phage commonly utilised for the mitigation of F. psychrophilum outbreaks. While quantitative PCR (qPCR) methods are often used for quantifying F. psychrophilum , with its high‐sensitivity and reproducibility offering a viable alternative to plate‐based microbial quantification, there are few qPCR assays currently published for quantification of F. psychrophilum phages. We have developed and optimised a highly specific qPCR assay for FpV‐4 quantification, capable of reliably detecting a minimum of seven copies. We applied the assay to intestinal content samples from FpV‐4‐fed rainbow trout ( Oncorhynchus mykiss ), showing consistent detection and quantification of the phage. This assay represents a new development for the evaluation of F. psychrophilum phage therapy, providing a fast quantification method that complements and reduces the potential uncertainty of plate‐based enumeration.