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Group A streptococci: evaluation of in vitro resistance to two macrolides
1Department of Clinical and Biological Sciences, University of Torino, San Luigi Hospital, Italy.
Abstract:
In recent years an increase in severe group A streptococcal infections has been observed. The possible relation between the failure of therapy and an increase of resistance to antibiotics, which are often used for streptococcal infections (clarithromycin and erythromycin), has been assessed in vitro. Streptococcus pyogenes strains tested for susceptibility were isolated in different years from pharyngotonsillar swabs of symptomatic children and typed; another nine strains came from the American Type Culture Collection. The evaluation of antimicrobial activity demonstrated that the percentage of resistance of these bacteria to the two macrolides was 4, 4.4 and 15.5%, respectively, for strains isolated in 1990, 1991 and 1994. Clarithromycin showed a better antistreptococcal, above all bactericidal, activity. The presence of M protein in streptococci does not seem to modify the kinetic activity of the two drugs, while a slower bactericidal effect was observed against capsulated strains. The resurgence of severe group A Streptococcus infections may be due to an increase in the circulation of strains with a capsule expression, which is critical also for resistance to phagocytic killing.
Insights
Severe group A Streptococcus infections are rising, possibly due to antibiotic resistance. Clarithromycin showed better activity against Streptococcus pyogenes, but capsule expression may hinder treatment effectiveness.
Area of Science:
- Microbiology
- Infectious Diseases
- Pharmacology
Background:
- An increase in severe group A streptococcal infections has been observed recently.
- Antibiotic resistance to commonly used drugs like clarithromycin and erythromycin may be linked to therapy failures.
Purpose of the Study:
- To assess the in vitro relationship between increasing antibiotic resistance and the failure of therapies for streptococcal infections.
- To evaluate the antimicrobial activity of clarithromycin and erythromycin against Streptococcus pyogenes strains.
Main Methods:
- Streptococcus pyogenes strains were isolated from children's pharyngotonsillar swabs across different years and from the American Type Culture Collection.
- In vitro susceptibility testing was performed to evaluate antimicrobial activity against these strains.
- The impact of M protein presence and capsule expression on drug activity was assessed.
Main Results:
- Resistance to macrolide antibiotics (clarithromycin and erythromycin) in Streptococcus pyogenes increased from 4% in 1990 to 15.5% in 1994.
- Clarithromycin demonstrated superior antistreptococcal and bactericidal activity compared to erythromycin.
- Capsule expression in strains correlated with a slower bactericidal effect, while M protein presence did not significantly alter drug kinetics.
Conclusions:
- The resurgence of severe group A Streptococcus infections might be associated with increased circulation of encapsulated strains.
- Capsule expression appears critical for resistance to phagocytic killing, potentially contributing to treatment challenges.
- Clarithromycin exhibits promising activity, but understanding resistance mechanisms, including capsule's role, is crucial for effective treatment.