640. Does In-Vitro Resistance of Streptococcus Pyogenes to Erythromycin Produce Clindamycin Resistance?
Session: Poster Abstract Session: Pharmacokinetics and Adverse Drug Reactions
Friday, October 21, 2011
Room: Poster Hall B1
Background: Throughout the last decade there has been a trend of increasing macrolide resistance to Streptococcus pyogenes, which is responsible for severe, life-threatening infections including toxic shock syndrome and necrotizing fasciitis. A 2010 pilot study at Scripps Mercy Hospital in San Diego, California showed that 20% of S. pyogenes isolates were resistant to macrolides. This pilot prompted further investigation. Can observed macrolide resistance lead to cross-resistance in clindamycin and through what mechanism? Our primary objective was to determine the prevalence of erythromycin and clindamycin resistance among clinical isolates of S. pyogenes in a five hospital system in the San Diego area. Our secondary goal was to determine the main mechanism of resistance: drug inactivation, efflux pumps or methylation.  

Methods: 146 consecutive isolates of S. pyogenes were analyzed from five Scripps hospitals. The antimicrobial susceptibilities of the isolates were determined using double-disk diffusion testing (D-test).  Analysis was performed using a two-sample test of proportions (2-tailed). A p<0.05 will be considered significant for all statistical analysis.  

Results: 146 strains of S. pyogenes were analyzed for both erythromycin and clindamycin susceptibility. There was no statistically significant difference between the resistance patterns of erythromycin or clindamycin to S. pyogenes (19.2% vs 18.5%, p=0.88). Overall, there were 27 positive D-tests out of 29 isolates resistant to erythromycin, demonstrating the main mechanism of resistance was through ribosomal methylation.

Conclusion: S. pyogenes was resistant to both erythromycin and clindamycin in ~20% of strains in the San Diego area. Therefore, if a patient is resistant to erythromycin there is a high likelihood it will also be resistant to clindamycin.  Consequently, empiric use of clindamycin might not be advisable. With the increasing trend of macrolide resistance in S. pyogenes strains, the penicillin family still remains the drug of choice in these infections and macrolide use should be reserved for patients with true beta lactam allergies.

Subject Category: A. Antimicrobial agents and Resistance

Bridgen Christine, Pharm.D.1, Daniel Keays, C.L.S.2 and Harminder Sikand, PharmD2, (1)Pharmacy, Scripps Mercy Hospital, San Diego, CA, (2)Scripps Mercy Hospital, San Diego, CA


B. Christine, None

D. Keays, None

H. Sikand, None

Findings in the abstracts are embargoed until 12:01 a.m. EST Thursday, Oct. 20 with the exception of research findings presented at IDSA press conferences.