Comparative in-vitro activities of amoxycillin and penicillin against Streptococcus pneumoniae isolates.

Sir, In a recent issue of this Journal, Verhaegen & Verbist reported on the in-vitro activities of 21 -lactams against penicillin-susceptible and -resistant isolates of Strepto coccus pneumoniae. The results of this investigation are surprising because they suggest that amoxycillin is less active than penicillin against these strains. We have recently determined the in-vitro susceptibilities of 212 clinical isolates of S. pneumoniae by the microbroth dilution method according to recommendations of the National Committee for Clinical Laboratory Standards (NCCLS); the results are shown in the Table. The MICs of amoxycillin, in common with those of cefotaxime, were almost invariably one two-fold dilution lower than those of penicillin for strains with MICs of penicillin >0.06 mg/L. These observations in respect of pneumococci with reduced susceptibilities to penicillin are in accord with those of other investigators. In addition, Verhaegen & Verbist claimed that “Apart from penicillin, there are currently no NCCLS-approved breakpoints for oral -lactams for pneumococci.” This is incorrect. In 1995, the NCCLS recommended MIC breakpoints for both oral amoxycillin (susceptible, 0.5 mg/L; intermediate susceptibility, 1 mg/L; and resistant, 2 mg/L) and co-amoxiclav (susceptible, 0.5/ 0.25 mg/L; intermediate susceptibility, 1/0.5 mg/L; and resistant, 2/ 1 mg/L). The fact that the susceptible and intermediate susceptibility breakpoints for amoxycillin are higher than those for penicillin (susceptible, 0.06 mg/L and intermediate susceptibility, 0.12–1 mg/L) reflect the superior pharmacokinetic profile of the former drug. This and its greater in-vitro activity explain why most pneumococci with reduced susceptibilities to penicillin remain fully susceptible or of intermediate susceptibility to amoxycillin, with or without clavulanic acid.