Antimicrobial Susceptibility of Streptococci and Enterococci Isolated from Infectious Oral Lesions

Abstract

Antimicrobial susceptibility and β-lactamase production was examined for streptococci and enterococci isolated from infectious oral lesions.
Morphological changes in CEX-susceptible and CEX-nonsusceptible enterococci were observed by transmission electron microscopy to ascertain the origin in these differences in susceptibility.
Seventeen strains of S. milleri, 10 of S. sanguis, 10 of S. morbillorum, 5 of S. mitior, 7 of S. sp., and 3 of S. faecalis were isolated. Streptococci were generally highly susceptible to EM, CLDM, JM, ABPC and PIPC. Enterococci indicated moderate susceptibility to ABPC, AMPC and EM, although virtually none of the strains were susceptible to other antibiotics. No strains producing β-lactamase were found in the streptococci or enterococci when nitrocefin was used.
Elongated cells were observed in the strain of CEX-nonsusceptible enterococci exposed to 1/2 MIC of CEX, while the cells of the CEX-susceptible strain collapsed without morphological changes. It is known that differences in the affinity for β-lactam antibiotics in the penicillin-binding proteins in the two strains of bacteria induce different morphological changes. Therefore it was thought that the differences in the MIC for the two enterococci resulted from differences in the affinity for CEX in the penicillin-binding proteins.
It was concluded that ABPC and CLDM were appropriate for oral infections caused by streptococci, enterococci, and other anaerobic bacteria. The lack of susceptibility of Enterococcus was most likely caused by a decrease in the affinity for CEX in the penicillin-binding protein.