Studies of antibiotic resistance of rough and smooth Proteus mirabilis strains and influence of polymyxin E on their lipopolysaccharide composition.

The influence of type of bacterial culture media on antibiotic resistance of Proteus mirabilis R and S forms, was tested. P. mirabilis S1959 (S form), R45 and R110 strains (Re and Ra mutant, respectively) cultivated in media supplemented with 10% heat inactivated bovine serum were resistant to ampicillin, amoxicillin, nalidixic acid and nitroxoline. Proteus strains cultivated in media without serum were sensitive to these antibacterial agents. The presence of serum did not change the polymyxin E (colistin) resistance of there Proteus strains tested. The effects of the presence of colistin (1000 U/ml) in culture media on Proteus lipopolysaccharide composition was studied. The content of uronic acids and phosphate residues in lipopolysaccharides isolated from bacteria cultivated in the presence of colistin (LPS-col), were lower than in control LPSs. The contents of 4-amino-4-deoxy-L-arabinose decreases in S1959 LPS-col, increases in R110 LPS-col and remains unchanged in R45 LPS-col. These results indicate that the presence of colistin in cultivation media exerts an influence on the contents of charged components of LPSs isolated from polymyxin E-resistant Proteus R and S strains.