Collective behavior and virulence arsenal of Piscirickettsia salmonis biofilms. Transcriptomes of the fish pathogen piscirickettsia salmonis in biofilm and planktonic states

Piscirickettsiosis is a fish disease caused by the Gram-negative bacterium Piscirickettsia salmonis. It has been described as a multifactorial disease with high socio-economic impact on the Chilean salmonid aquaculture. Bacterial biofilms are among risk factors to pathogenesis of infectious diseases through different mechanisms, but the implications of biofilm microbiology for waterborne veterinary diseases have seldom been evaluated, such as piscirickettsiosis. This study reports a comprehensive analysis about in vitro biofilm behavior of P. salmonis LF-89T and CA5 by using a multi-method approach to know the virulence arsenal of biofilms and its potential relationship with the pathogenesis of piscirickettsiosis. P. salmonis exhibited a kinetics of biofilm formation that followed a multi-step and highly strain-dependent process. There were not major differences in enzymatic profiles and cytotoxicity between biofilm-derived and planktonic bacteria. Whole-transcriptome analysis of P. salmonis LF-89T revealed gene expression profiles in biofilms that were not enriched enough with upregulated virulence-related genes regarding planktonic bacteria in order to recognize the early biofilm development as a hot spot of virulence. Thus, P. salmonis would form biofilms that act more suitably as a tolerance mechanism to stressful conditions instead of seedbanks of virulence factors for piscirickettsiosis.