In Streptococcus pneumoniae (Spn), the Macrolide Genetic Assembly (Mega) provides macrolide antibiotic resistance via the efflux pump Mef(E) and the ribosomal protection protein Mel. This resistance has previously been assumed to confer traditional resistance. Heteroresistance is commonly missed during traditional clinical resistance screens, and is highly concerning as resistant sub-populations can persist despite treatment.
Spn strains containing the Mega element were screened via Etesting and Population Analysis Profiling. In addition to screening wildtype strains, strains with deletions of the 5’UTR of Mef(E) were also queried.
All wildtype Mega-containing Spn strains screened displayed heteroresistance (>eight fold range in MICs) to Mef(E)/Mel-inducing macrolides, but not to non-Mef(E)/Mel-inducing Macrolides or other classes of antibiotics. When macrolide induction uniformly increased mef(E)/mel expression, heteroresistance was eliminated. A deletion of the 5’UTR of mef(E) resulted in a mutant deficient in induction as well as heteroresistance. Only the mef(E)L leader peptide sequence of the 5’UTR is required for induction and heteroresistance.
This study finds that inducibility and heteroresistance are linked for macrolide resistance conferred by the Mega element independent of insertion class. Stochastic variation or epigenetics affecting mef(E)/mel expression inside a population of Spn are possible mechanisms for heteroresistance.