Is Streptococcus pyogenes resistant or susceptible to trimethoprim-sulfamethoxazole?

Area of Science:

• Microbiology
• Antimicrobial Susceptibility Testing
• Clinical Infectious Diseases

Background:

• Streptococcus pyogenes is often considered resistant to trimethoprim-sulfamethoxazole (SXT).
• This perceived resistance is linked to thymidine content in culture media, affecting folate metabolism inhibition.
• Standardized Mueller-Hinton agar (MHA) now has regulated thymidine concentrations.

Purpose of the Study:

• To investigate Streptococcus pyogenes susceptibility to SXT across various standardized media.
• To re-evaluate the in vitro efficacy of SXT against S. pyogenes.
• To provide evidence supporting or refuting SXT use in skin and soft tissue infections (SSTIs).

Main Methods:

• Two sets of 100 clinical S. pyogenes isolates were tested.
• Susceptibility was determined using SXT Etests on MHA with defibrinated horse blood and β-NAD (MHF), MHA with sheep blood (MHS), MHA alone, MHA with horse blood (MHBA), and MHA with lysed horse blood (MHLHBA).
• European Committee on Antibacterial Susceptibility Testing (EUCAST) breakpoints were used for interpretation.

Main Results:

• In study 1, 99% of isolates were susceptible to SXT on MHA, MHBA, and MHLHBA (geometric mean MICs: 0.04–0.05 mg/liter).
• In study 2, 100% of isolates were susceptible on MHF, MHS, MHA, and MHLHBA (geometric mean MICs: 0.07–0.16 mg/liter).
• All tested media demonstrated high in vitro susceptibility of S. pyogenes to SXT.

Conclusions:

• This study confirms the in vitro susceptibility of Streptococcus pyogenes to trimethoprim-sulfamethoxazole.
• The findings challenge the long-held belief of SXT resistance in S. pyogenes.
• Results support the potential clinical use of SXT for skin and soft tissue infections caused by S. pyogenes.