Antibiotic susceptibility of streptococcal strains associated with infective endocarditis

Seventy-six strains of various species of streptococci isolated from patients with infective endocarditis were tested for their susceptibility to 13 antibiotics by an agar dilution method. The antibiotics tested were: benzyl-penicillin, ampicillin, cefotaxime, vancomycin, erythromycin, rifampicin, pristinamycin, gentamicin, netilmicin, tobramycin, amikacin, dibekacin and streptomycin. Excluding enterococci, 91% of strains were sensitive to benzylpenicillin. Resistance to benzylpenicillin was only found in some strains of S. sanguis I, S. sanguis II and S. mitis. Enterococci were more sensitive to ampicillin. Cefotaxime was highly active against all strains, except enterococci. Vancomycin was active against all strains. Resistance to erythromycin was found in 16% of isolates. Rifampicin and pristinamycin were highly active against all strains, except some enterococci. Gentamicin and netilmicin were the most active of the six aminoglycosides tested. High level resistance to streptomycin was seen in six strains. Overall, S. agalactiae was more resistant to the aminoglycosides than the other species. Among the non-groupable streptococci, strains of S. mitis, S. sanguis I and S. sanguis II were the least sensitive to many antibiotics. Benzylpenicillin remains the antibiotic of choice for the treatment of IE caused by streptococci. If the MIC exceeds 0.1 mg l-1, an aminoglycoside (netilmicin or gentamicin) should be added and the duration of treatment increased from 4 to 6 weeks.