Antibiotic-resistant pneumococci--facts and fiction

Area of Science:

• Microbiology
• Clinical Laboratory Science
• Infectious Diseases

Background:

• Effective treatment of penicillin-resistant pneumococci relies on rapid and accurate bacterial susceptibility assessment.
• Established methods include minimum inhibitory concentration (MIC) determination, disk diffusion, and the E-test.

Purpose of the Study:

• To evaluate current laboratory methods for pneumococcal susceptibility testing.
• To discuss proposed changes to National Committee for Clinical Laboratory Standards (NCCLS) guidelines.
• To assess the impact of susceptibility patterns on antibiotic selection for pneumococcal infections.

Main Methods:

• Review of established and emerging laboratory techniques for pneumococcal susceptibility testing.
• Analysis of evidence regarding minimum inhibitory concentration (MIC) values and disk diffusion zone diameters.
• Discussion of current and proposed breakpoint recommendations for penicillin and cephalosporin susceptibility.

Main Results:

• The E-test provides a faster and easier method for pneumococcal susceptibility testing.
• Evidence suggests MIC values of 0.06 mg/L for penicillin should indicate intermediate susceptibility.
• Many intermediate strains show zone diameters below susceptible breakpoints in oxacillin/methicillin disk tests.

Conclusions:

• Revising susceptibility breakpoints is necessary for accurate classification of pneumococcal strains.
• Older antibiotics like amoxycillin and piperacillin demonstrate low MICs against pneumococci.
• These antibiotics may be effective against penicillin-intermediate and resistant pneumococcal strains.