Most S. pneumoniae strains gain penicillin resistance by altering the penicillin-binding proteins in their cell walls. Penicillin molecules that do not find a penicillin-binding site cannot interfere with cell wall synthesis. Several different types of penicillin-binding proteins may be involved, explaining the various levels of intermediate resistance encountered with varying strains of S. pneumoniae.
Because different penicillin-binding proteins may be involved, the oxacillin screening test cannot predict the level of penicillin resistance. Infections caused by isolates of S. pneumoniae showing penicillin resistance in the intermediate range may be successfully treated by administering high doses of antibiotics. For this reason, the level of resistance with an accurate minimum inhibitory concentration (MIC) test must be determined for all clinically significant isolates of S. pneumoniae.
