Susceptibility and immune responses after challenge with Flavobacterium columnare and Pseudomonas fluorescens in conventional and specific pathogen-free rare minnow (Gobiocypris rarus).

The susceptibility of fish from different culture environments to bacterial infection is not well known. The susceptibility and pathological changes of conventional (CV) and specific pathogen-free (SPF) rare minnow (Gobiocypris rarus) infected with two gram-negative bacteria, Flavobacterium columnare and Pseudomonas fluorescens are investigated. Rare minnows were intraperitoneally challenged with two bacterial species to first determine semi-lethal doses (LD50), and then with the LD50 dose, determine innate immune response. Infected rare minnows developed characteristic red bellies and then died. LD50 doses of F. columnare and P. fluorescens were 4.586 × 108 cfu/mL and 2.319 × 1010 cfu/mL for CV rare minnow, and 2.575 × 108 cfu/mL and 1.935 × 1010 cfu/mL, respectively, for SPF rare minnow. The results of RT-PCR showed that the highest levels of toll-like receptor 3 (TLR3), interleukin-6 (IL-6), interferon-2 (IFN-2) and rare minnow Z-DNA binding protein kinase (GrPKZ) mRNA were noticed at 6-48 h post-infection (hpi). In addition, TLR3, IL-6 and IFN-2 in F. columnare challenged rare minnow were more highly expressed than those in P. fluorescens challenged rare minnow, whereas as opposed in the expression of GrPKZ mRNA. Stimulation of innate immune responses is closely related to bacterial virulence. SPF rare minnow might be more susceptible to these bacteria than CV rare minnow, possibly due to their clean environment and lack of resistance. We speculate that clean environment renders rare minnow more susceptible to bacterial infections.