The susceptibility of anaerobic bacteria to rosamicin, erythromycin and clindamycin and factors affecting the activity of rosamicin

The in vitro susceptibility to rosamicin, erythromycin and clindamycin of 208 isolates of clinically significant anaerobic bacteria was assessed by agar dilution method. Generally, rosamicin showed better in vitro activity against most of the anaerobes than did erythromycin, but less activity than did clindamycin. At achievable peak serum levels of the 3 antibiotics, (0.5 μ/ml for rosamicin, 2 μ/ml for erythromycin, 8 μ/ml for clindamycin), 69% of the annerobes were inhibited by rosamicin, 65% by erythromycin and 94% by clindamycin.Rosamicin was slightly more active at pH 8.2 than at pH 7.2. At pH 9.2 its activity showed no further improvement for some anaerobes and was the same as at pH 72 against the other anaerobes. Young cultures were more resistant to rosamicin than old cultures and cultures tested in an anaerobic chamber were somewhat more resistant than those tested on the bench.