Mechanisms of resistance for Streptococcus pyogenes in northern Utah

The purpose of this study was to (1) determine the rates of penicillin and erythromycin resistance among Streptococcus pyogenes isolates in northern Utah, and (2) determine the genotype of the erythromycin resistant strains, thereby providing information regarding the mechanism of the resistance.

Seven hundred thirty-nine isolates of S. pyogenes were identified on 5% Sheep Blood Agar. Susceptibility to erythromycin and penicillin was performed using Muller-Hinton blood agar. All isolates resistant to erythromycin were then genotyped using PCR primers specific to one of the following: mefA gene, indicating the mechanism of resistance was an efflux pump; ermA gene, in which the mechanism was inducible methylation of the ribosomes; and ermB indicating constitutive methylation of the ribosomes.

This study was conducted at Weber State University, in the Department of Clinical Laboratory Sciences.

Samples were collected from 9 clinics ranging from North Ogden to Taylorsville, Utah. All samples were previously tested positive for S. pyogenes by the clinic from where the samples were collected.

Of the 739 S. pyogenes isolates tested, 2.4% were resistant to erythromycin with no resistance observed to penicillin. Of the strains that displayed some degree of resistance, the gene frequencies observed were as follows: 48.1% mefA, 26.0% ermA, 3.7% ermB, and 22.2% multiple genes.

The most common genotype was mefA, indicating that the efflux pump (M phenotype) is the most common mechanism in the surveyed area, followed by ermA, which produces the inducible methylating enzyme. A significant number of isolates was also observed to express both the efflux pump and the constitutive methylating enzyme.