Antimicrobial susceptibility patterns of Streptococcus pneumoniae

Fifty clinical isolates of Streptococcus pneumoniae received by the Streptococcus Laboratory of the Center for Disease Control from August 1976 through March 1977 and 50 pneumococcal strains retrieved from 13- to 16-year storage (originally isolated October 1961 through December 1964) were tested for susceptibility to 10 antimicrobial agents by disk-agar diffusion and agar dilution tests. No appreciable differences in susceptibility patterns were apparent between the two groups, and, except for one isolate, all were highly susceptible to every drug tested except gentamicin. This single isolate required higher drug concentrations to inhibit macroscopic growth and had corresponding decrements in zones of disk inhibition with penicillin, ampicillin, and cephalothin. An additional 43 pneumococci recently received from various areas of the United States and Canada were screened by a disk agar diffusion method for penicillin resistance. Four of these isolates had penicillin zone diameters <30 mm, and subsequent agar dilution test results showed that the penicillin minimum inhibitory concentrations were elevated with these organisms. Antimicrobial susceptibility patterns of pneumococci to antimicrobials other than penicillin and its analogs have not changed substantially in over a decade. However, due to the emergence of strains with decreased susceptibility to penicillin, the screening test for penicillin resistance in pneumococci, especially in isolates from spinal fluid and blood, could be clinically useful as an aid in selecting optimal therapy.