Unusual resistance patterns in macrolide-resistant Streptococcus pyogenes harbouring erm(A)

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Streptococcus pyogenes can develop resistance to macrolide antibiotics like erythromycin, azithromycin, and clindamycin.
• Three novel resistance phenotypes were observed in erm(A)-harbouring S. pyogenes, including low-level erythromycin resistance with high azithromycin resistance, and high-level constitutive MLS resistance.

Purpose of the Study:

• To investigate the genetic basis of the novel antibiotic resistance phenotypes in erm(A)-harbouring Streptococcus pyogenes.
• To identify specific mutations within the erm(A) gene and their regulatory regions responsible for these resistance patterns.

Main Methods:

• Confirmed erm(A) presence and absence of other resistance genes using PCR.
• Sequenced erm(A), 23S rRNA, L4, and L22 genes.
• Cloned mutant erm(A) genes into Escherichia coli for functional analysis and performed in silico studies on mutation effects.

Main Results:

• Identified point mutations (G98A, A137C, C140T, G205A) in the erm(A) regulatory region of all eight S. pyogenes isolates.
• Transforming E. coli with erm(A) containing G98A, A137C, or C140T mutations did not confer high-level resistance.
• High-level constitutive MLS resistance was observed in E. coli transformed with erm(A) carrying both A137C and G205A mutations, which abolished a key hairpin structure.

Conclusions:

• Reported point mutations in the erm(A) regulatory region contribute to constitutive methylase expression.
• Unidentified mechanisms likely mediate high-level azithromycin and clindamycin resistance in some erm(A)-harboring S. pyogenes strains.