Norfloxacin Sub-Inhibitory Concentration Affects Streptococcus suis Biofilm Formation and Virulence Gene Expression

Streptococcus suis (S. suis) is a major pathogen causing economic losses to the swine industry. Norfloxacins are usually used at subMIC (Minimum Inhibitory Concentration) doses to prevent S. suis infection. This study demonstrates the effect of norfloxacin sub-MIC on biofilm formation and virulence gene expression in S. suisit was found that 1/4 MIC of norfloxacin increased biofilm formation in S. suis, the biofilms formed contained a higher number of viable bacteria. Additionally, bacterial growth rates were inhibited at 1/2 MIC of norfloxacin. Furthermore, the mRNA level of S. suis virulence gene cps, ef, sly, fpbs, gdh and gapdh increased by real-time PCR, while the virulence gene mrp decreased at 1/4 MIC. In conclusion, Norfloxacin sub-MICs affects biofilm formation and virulence gene expression in S. suis. These findings suggest that investigating the effect of the administration of antibiotics sub-MICs on bacterial biofilms and infection may lead to the development of future antibiotic treatments modalities.