TESTING OF STREPTOCOCCUS-PNEUMONIAE FOR RESISTANCE TO PENICILLIN

The increasing prevalence of penicillin-resistant Streptococcus pneumo niae requires antibiotic susceptibility tests that can be done with gr eater ease and reliability. We measured the MIC of penicillin for pneu mococci by the tube macrodilution method with Mueller-Hinton broth (MH B), Haemophilus Test Medium (HTM), Todd-Hewitt broth with 0.5% yeast e xtract (THY), and MHB with 3% lysed horse blood (LHB). Eight (19%) and 6 (14%) of 42 pneumococcal isolates failed to generate turbid growth in MHB and HTM, respectively, whereas all pneumococcal isolates did so in THY and LHB. For those strains that replicated to turbidity, the m ean MICs of penicillin were lower in MHB and HTM than in THY and LHB, with differences being significant (P < 0.05) for comparisons with LHB . Four isolates appeared to be penicillin susceptible in HTM but were actually moderately resistant in THY and LHB, and two isolates appeare d to be moderately resistant but were resistant. A similar failure to detect resistance was seen with MHB. S. pneumoniae ATCC 4%19, a modera tely penicillin-resistant strain that has been proposed for quality co ntrol testing, gave variable results in MHB or THM and appeared to be susceptible to penicillin in some assays, whereas the MICs for S. pneu moniae ATCC 4%19 in THY or LHB fell within a twofold dilution range, w ith geometric means of 0.16 and 0.18 mug/ml, respectively. Pneumococca l isolates thus may appear falsely susceptible to penicillin when test ed in MHB or HTM. LHB remains the standard medium; however, because TH Y is an easily prepared clear medium that can be used in automated sys tems and appears to yield results similar to those obtained with LHB, THY deserves consideration for routine use.