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Clindamycin resistance among erythromycin-resistant Streptococcus pneumoniae
R N Jones1, M G Cormican, A Wanger
1Department of Pathology, University of Iowa College of Medicine, Iowa City 52242, USA.
Abstract:
The increasing proportion of Streptococcus pneumoniae isolates with reduced susceptibility to penicillin has created an urgent need for therapeutic alternatives to some beta-lactam agents. Clindamycin is an antimicrobial agent with excellent bioavailability after oral administration which has been considered for the therapy of community-acquired pneumococcal otitis media. Using the Etest methodology, we have studied the in vitro susceptibility of 59 erythromycin-resistant strains of S. pneumoniae to clindamycin, penicillin, trimethoprim-sulfamethoxazole, and rifampin. The study also addressed the impact of the susceptibility test medium [Mueller-Hinton (MH) vs IsoSensitest (Iso), both 5% blood supplement] on the results. A total of 20 isolates (37%) displayed constitutive clindamycin resistance on Iso blood agar, compared with only 11 (22%) on MH blood agar. The remaining nine strains found to be clindamycin susceptible on MH manifested resistance only with erythromycin induction. Resistance to penicillin, rifampin, and trimethoprim-sulfamethoxazole in erythromycin-resistant isolates was 83%, 2%, and 85%-89% (medium dependent), respectively. These results indicate that the choice of susceptibility test medium affects the expression (constitutive or inducible) of macrolide-lincosamide-streptogramin (MLS) resistance in S. pneumoniae. In addition, the common assumption that erythromycin resistance in S. pneumoniae implies clindamycin resistance may need to be reconsidered and routine susceptibility tests (including induction if MH medium is used) should be considered for MLS-class drugs.
Insights
Testing susceptibility medium impacts clindamycin resistance detection in Streptococcus pneumoniae. Routine testing, including induction, is crucial for macrolide-lincosamide-streptogramin (MLS) drugs.
Area of Science:
- Microbiology
- Antimicrobial Resistance
- Clinical Pharmacy
Background:
- Increasing penicillin resistance in Streptococcus pneumoniae necessitates alternative therapies.
- Clindamycin is a potential oral treatment for pneumococcal otitis media.
Purpose of the Study:
- To evaluate in vitro clindamycin susceptibility in erythromycin-resistant S. pneumoniae strains.
- To determine the influence of susceptibility testing media on MLS resistance expression.
Main Methods:
- Etest methodology used for in vitro susceptibility testing.
- Tested 59 erythromycin-resistant S. pneumoniae strains against clindamycin, penicillin, trimethoprim-sulfamethoxazole, and rifampin.
- Compared Mueller-Hinton (MH) and IsoSensitest (Iso) blood agar media.
Main Results:
- Constitutive clindamycin resistance observed in 37% of isolates on Iso vs. 22% on MH agar.
- Nine strains showed inducible clindamycin resistance only on MH agar.
- High resistance rates to penicillin (83%) and trimethoprim-sulfamethoxazole (85%-89%) were noted.
Conclusions:
- Susceptibility testing medium significantly affects the detection of constitutive and inducible MLS resistance in S. pneumoniae.
- The assumption of cross-resistance between erythromycin and clindamycin may not always hold true.
- Routine susceptibility testing, including induction methods when using MH agar, is recommended for MLS drugs.