Characterisation of and polysaccharide production by amoxycillin-resistant streptococci

Small numbers of bacteria capable of growing on agar supplemented with amoxycillin 40 mg/L were isolated from the saliva of 9 out of 20 adult volunteers in a previous study. All the bacteria were identified as Streptococcus sanguis although no strains produced dextran in conventional tests. However, using a specific assay, all the antibiotic-resistant strains were found to secrete glucosyltransferases (GTF), the enzymes that synthesise these extracellular polysaccharides; the production of GTF-S, the enzyme that synthesizes dextran, was 22-43% less than that of an antibiotic-sensitive control strain. Enzyme production by both antibiotic-resistant and sensitive bacteria was markedly inhibited by dextran primer. The amoxycillin-resistant bacteria were resistant to other penicillins; their resistance to erythromycin was variable but they were uniformly sensitive to cephalothin and clindamycin. As dextran production has been proposed as a key factor in the colonisation of damaged heart valves by bacteria such as S. sanguis, these highly resistant bacteria may not pose a threat to the susceptible individual.