Penicillin susceptibility of Streptococcus pneumoniae in 1978. Screening for resistance by disk testing

Recently, strains of Streptococcus pneumoniae with greatly increased resistance to penicillin (minimal inhibitory concentrations or MICs, 1--8 microgram/ml) were recovered in cultures of blood from patients in South Africa and from one in Minnesota who had serious pneumococcal infections. The authors undertook a study to determine whether these resistant strains have become prevalent in their locale. Between January and July 1978, the laboratories of hospitals serving the greater Madison, Wisconsin, area (population 200,000) contributed 243 pneumococcal isolates for susceptibility testing by an agar dilution technic. Strains with greatly increased resistance (MICs greater than or equal to 1.0 microgram/ml) were not found; only six strains (2.4%) had relative resistance to penicillin (MICs .125--.50 microgram/ml), a range of susceptibility that has been associated with inconsistent clinical responses to treatment with penicillin. Overall, the susceptibility patterns of these 243 isolates are similar to those reported from other centers in North America over the past 30 years. Routine susceptibility testing of pneumococci by hospital laboratories in our area does not appear to be necessary now, but laboratories are advised to screen blood and spinal fluid isolates by a disk-diffusion method. Studies of these 243 isolates and seven South African multiply-resistant strains using a modified Kirby-Bauer technic, showed that a zone of inhibition less than 35 mm around a 10-unit penicillin disk, or better, less than 17 mm around a 1-microgram oxacillin disk, correlates strongly (P less than .001) with resistance to penicillin (MIC greater than or equal to .1 microgram/ml).