In vitro activity of clarithromycin, cefprozil, and other common oral antimicrobial agents against gram-positive and gram-negative pathogens.

: Macrolide and beta-lactam antimicrobial agents are frequently used for the treatment of upper and lower respiratory tract infections and skin or skin structure infections. To evaluate the relative in vitro activity of these antimicrobial drugs against organisms commonly involved in these infections, we tested clarithromycin, erythromycin, cefprozil, cefuroxime, cefaclor, cephalexin, amoxicillin, amoxicillin/clavulanate, and doxycycline against 174 gram-positive and gram-negative clinical isolates, including Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pneumoniae, group A beta-hemolytic streptococci, alpha-hemolytic streptococci, Escherichia coli, and Klebsiella pneumoniae. Manual broth microdilution susceptibility testing was used with a standard inoculum of 5 x 10(4) colony-forming units/well at pH of 7.2. Clarithromycin was the most active agent against streptococci. Methicillin-susceptible S aureus exhibited resistance to both clarithromycin and erythromycin, but was susceptible to cefprozil, cefuroxime, amoxicillin/clavulanate, and doxycycline. Cefprozil was at least as active as cefuroxime, cefaclor, and cephalexin against all organisms tested, but was fourfold less active than doxycycline against E coli and 16-fold less active than clarithromycin versus S pneumoniae. The gram-negative isolates tested showed resistance to clarithromycin and erythromycin; however, cefprozil was as active as amoxicillin/clavulanate against K pneumoniae and E coli. These results demonstrate that clarithromycin provides superior in vitro activity against common streptococci, while cefprozil, cefuroxime, amoxicillin/clavulanate, and doxycycline provide greater activity against methicillin-susceptible S aureus, K pneumoniae, and E coli. Prospective clinical trials are needed to determine the clinical significance of these findings.