Screening of type Ia and Ib Streptococcus agalactiae strains with high sialic acid levels by determination of susceptibility to tetracyclines.

The type-specific capsular polysaccharide antigen of Streptococcus agalactiae is recognized to be an antiphagocytic factor in strains having large amounts of it. In the present study, it was indicated that vaginal isolates of types Ia and Ib could be classified into two groups on the basis of both their levels of the sialic acid, which occupies the terminal side chains of the polysaccharide, and their susceptibility to tetracyclines: one group comprised strains with low sialic acid levels (less than 9 micrograms/mg of cell dry weight) as well as with susceptibility to tetracyclines (MIC, less than or equal to 0.5 micrograms/ml), and the other comprised strains with higher sialic acid levels (greater than or equal to 9 micrograms/mg) and resistance to tetracyclines (MIC, greater than or equal to 8 micrograms/ml). A few isolates were found to have low levels of sialic acid and to be resistant to tetracyclines, but no isolates that were both relatively high in sialic acid and susceptible to tetracyclines were ever detected. Among strains of those serotypes, the MICs of tetracyclines were not in proportion to the sialic acid levels and were not affected when the sialic acid levels of each strain were altered by using Todd-Hewitt broth with various concentrations of Na2HPO4 and glucose. It was, therefore, apparent that the correlation of sialic acid levels with susceptibility to tetracyclines was not related directly to the sialic acid content or to the amount of the capsular polysaccharide. Since no plasmid DNAs were detected among representative strains that were tetracycline resistant, it was apparent that at least for the strains tested, resistance was chromosomal gene associated. In strains of S. agalactiae of types of Ia and Ib, the determination of susceptibility to tetracyclines was considered to be useful for screening strains with higher sialic acid levels.