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Detection and prevalence of pneumococci with increased resistance to penicillin
Abstract:
Susceptibility to penicillin was determined for 6000 strains of pneumococci isolated during 1974--76 from patients in Alberta and the adjacent region of the Northwest Territories. Strains were considered to be relatively resistant if the minimum inhibitory concentration (MIC) of penicillin was 0.16 microgram (0.26 U)/mL or more, which is eight or more times greater than the MIC for fully susceptible strains. Resistance was detected in 143 strains (2.4%) isolated from 122 patients and belonging to four capsular types. The MIC of the most resistant strains was 0.32 microgram (0.53 U/mL. Penicillin-resistant strains were highly resistant to oxacillin, the MIC being at least 30 times greater than that for penicillin-susceptible strains. Pneumococci resistant to penicillin may readily be detected by the narrowness or absence of a zone of inhibition around a 1-microgram oxacillin disc in susceptibility tests on blood agar. The degree of resistance reported here is relative and does not necessarily preclude successful treatment with full therapeutic doses of penicillin G, but penicillin preparations that give low blood concentrations may not be suitable for treating infections caused by these strains.
Insights
Penicillin resistance in pneumococci (Streptococcus pneumoniae) was found in 2.4% of strains. While not precluding treatment with full penicillin doses, this resistance impacts choices for low-concentration penicillin therapies.
Area of Science:
- Microbiology
- Infectious Diseases
- Antimicrobial Resistance
Background:
- Penicillin resistance in Streptococcus pneumoniae (pneumococci) is a growing public health concern.
- Understanding regional resistance patterns is crucial for effective treatment strategies.
- Previous studies have documented varying levels of penicillin resistance globally.
Purpose of the Study:
- To determine the prevalence and characteristics of penicillin-resistant pneumococci strains.
- To assess the susceptibility of resistant strains to oxacillin.
- To identify a reliable method for detecting penicillin resistance in clinical settings.
Main Methods:
- Isolated 6000 pneumococcal strains from patients in Alberta and Northwest Territories between 1974-1976.
- Determined penicillin susceptibility using minimum inhibitory concentration (MIC) breakpoints.
- Evaluated oxacillin resistance and disc diffusion test effectiveness.
Main Results:
- 143 strains (2.4%) exhibited relative penicillin resistance (MIC ≥ 0.16 μg/mL).
- Resistant strains belonged to four distinct capsular types.
- Penicillin-resistant strains showed high resistance to oxacillin (MIC ≥ 30x susceptible strains).
- Absence of an oxacillin inhibition zone reliably indicated penicillin resistance.
Conclusions:
- A low but significant prevalence of penicillin-resistant pneumococci was identified in the study region.
- Oxacillin disc diffusion tests offer a practical method for detecting these resistant strains.
- Treatment guidelines should consider the potential ineffectiveness of low-dose penicillin therapies against these isolates.