Resumo

Chitosan is an acetylglucosamine polymer obtained from chitin, the second most abundant polysaccharide in nature. This non toxic natural biopolymer has demonstrated an important antimicrobial activity. This study was performed to investigate the in vitro antimicrobial activity of that polysaccharide on growth rate and biofilm formation of two Staphylococcus aureus strains isolated from bovine subclinical mastitis: strains V329 and VM07. The minimal inhibitory (MIC) and bactericidal (MIB) concentrations were estimated as well. The addition of 100, 250, and 500 g/ml of chitosan to culture medium exhibited effect on the growth rate of the strains varying in concentration and time-dependent manner (p<0.05). The cell viable number of both the tested strains was reduced 30 minutes after treatment at various concentrations of chitosan, whereas the viable number of bacteria in the control group increased continuously with time. The estimation of MIC and MIB revealed that inhibition of V329 required higher chitosan concentration (625 g/ml), whereas VM07 was inhibited at concentrations of 19.53 and 39.06 g/ml, respectively. The activities of different concentrations of chitosan against biofilm formation were assayed quantitatively by the tissue culture plate method and crystal violet staining. The bacterial population of both the strains treated with chitosan demonstrated a significant (p<0.05) rate of biofilm reduction mainly the strain VM07 which showed 36, 71, and 93% of reduction at 100, 250, and 500 µg/mL of chitosan, respectively. There was statistical difference (p<0.05) among that rates and that one from control group. As observed in all experiments of this study the strain V329 demonstrated a kind of resistance to low concentrations of chitosan with a significant (p<0.05) rate of reduction only at 500 µg/mL of chitosan. The use of chitosan as an antibacterial compound opens new therapeutic perspectives for its use in Staphylococcus aureus mastitis control strategies. However, a therapeutic concentration should be established after evaluation of a large number and variety of wild strain, since different strains may display different degrees of sensitivity to chitosan as showed by this study.