Development of a semi-quantitative PCR assay for the detection of Francisella halioticida and its application to...

The current study describes the development and application of a TaqMan^® real-time PCR assay for the detection of the bacterium Francisella halioticida. Previously, detection of F. halioticida is relied on bacterial culture and conventional PCR; however, the real-time PCR provides many advantages because it is faster, less labour-intensive and reduces the risk of cross-contamination. DNA samples from mussels collected in April 2020 from seven sites in northern Brittany (France) were tested using the newly developed real-time PCR assay. The objective was to screen for the presence of F. halioticida during spring mortality events. The bacterium was detected in 71.4% of the samples tested and was present at all sites except for Saint-Brieuc and Mont-Saint-Michel, two sites which were not concerned by mortality at the time of sampling. Less than a month later, Saint-Brieuc was affected by unusual mortalities and F. halioticida was detected in almost all mussels (81.25%). The findings from this study provide further evidence indicating that F. halioticida may be contributing to mussel mortalities; however, a direct causal relationship has not yet been established. The real-time PCR assay developed in this study allows for rapid, specific and sensitive detection of F. halioticida which should prove useful for future studies concerning the involvement of this bacterium with shellfish mortalities.