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In vitro selection of resistance to four beta-lactams and azithromycin in Streptococcus pneumoniae
G A Pankuch1, S A Jueneman, T A Davies
1Department of Pathology (Clinical Microbiology), Hershey Medical Center, Hershey, Pennsylvania 17033, USA.
Abstract:
Selection of resistance to amoxicillin (with or without clavulanate), cefaclor, cefuroxime, and azithromycin among six penicillin G- and azithromycin-susceptible pneumococcal strains and among four strains with intermediate penicillin sensitivities (azithromycin MICs, 0.125 to 4 microg/ml) was studied by performing 50 sequential subcultures in medium with sub-MICs of these antimicrobial agents. For only one of the six penicillin-susceptible strains did subculturing in medium with amoxicillin (with or without clavulanate) lead to an increased MIC, with the MIC rising from 0.008 to 0.125 microg/ml. Five of the six penicillin-susceptible strains showed increased azithromycin MICs (0.5 to >256.0 microg/ml) after 17 to 45 subcultures. Subculturing in medium with cefaclor did not affect the cefaclor MICs of three strains but and led to increased cefaclor MICs (from 0.5 to 2.0 to 4.0 microg/ml) for three of the six strains, with MICs of other beta-lactams rising 1 to 3 twofold dilutions. Subculturing in cefuroxime led to increased cefuroxime MICs (from 0.03 to 0.06 microg/ml to 0.125 to 0.5 microg/ml) for all six strains without significantly altering the MICs of other beta-lactams, except for one strain, which developed an increased cefaclor MIC. Subculturing in azithromycin did not affect beta-lactam MICs. Subculturing of the four strains with decreased penicillin susceptibility in amoxicillin (with or without clavulanate) or cefuroxime did not select for beta-lactam resistance. Subculturing of one strain in cefaclor led to an increase in MIC from 0.5 to 2.0 microg/ml after 19 passages. In contrast to strains that were initially azithromycin susceptible, which required >10 subcultures for resistance selection, three of four strains with azithromycin MICs of 0.125 to 4.0 microg/ml showed increased MICs after 7 to 13 passages, with the MICs increasing to 16 to 32 microg/ml. All azithromycin-resistant strains were clarithromycin resistant. With the exception of strains that contained mefE at the onset, no strains that developed resistance to azithromycin contained ermB or mefE, genes that have been found in macrolide-resistant pneumococci obtained from clinic patients.
Insights
Pneumococcal strains developed resistance to azithromycin and cefuroxime after sequential subculturing. Beta-lactam resistance was rarely observed, and genetic mechanisms for azithromycin resistance differed from clinical isolates.
Area of Science:
- Microbiology
- Antimicrobial Resistance
- Molecular Biology
Background:
- Streptococcus pneumoniae is a major cause of bacterial infections.
- Antimicrobial resistance in S. pneumoniae poses a significant public health threat.
- Understanding resistance selection mechanisms is crucial for effective treatment.
Purpose of the Study:
- To investigate the selection of antimicrobial resistance in S. pneumoniae.
- To determine the development of resistance to amoxicillin, cefaclor, cefuroxime, and azithromycin.
- To compare resistance development in susceptible and intermediately resistant pneumococcal strains.
Main Methods:
- Sequential subculturing of pneumococcal strains in sub-inhibitory concentrations of antibiotics.
- Monitoring of Minimum Inhibitory Concentrations (MICs) for various antibiotics.
- Genetic analysis of resistance mechanisms (ermB, mefE).
Main Results:
- Five of six susceptible strains developed high-level azithromycin resistance (>256.0 microg/ml) after 17-45 subcultures.
- All six strains showed increased cefuroxime MICs (0.125 to 0.5 microg/ml).
- Three of four intermediately resistant strains showed increased azithromycin MICs (16 to 32 microg/ml) after 7-13 passages.
Conclusions:
- S. pneumoniae can rapidly develop resistance to azithromycin and cefuroxime under sub-MIC selection pressure.
- Beta-lactam resistance was infrequently selected, and resistance mechanisms differed from clinical isolates.
- The genetic basis of experimentally selected azithromycin resistance may differ from that in clinical settings.