Inducible Mega-Mediated Macrolide Resistance Confers Heteroresistance in Streptococcus pneumoniae.

In Streptococcus pneumoniae (), the 5.4 to 5.5 kb Macrolide Genetic Assembly (Mega) encodes an efflux pump (Mef[E]) and a ribosomal protection protein (Mel) conferring antibiotic resistance to commonly used macrolides in clinical isolates. We found the macrolide-inducible Mega operon provides heteroresistance (more than 8-fold range in MICs) to 14- and 15-membered ring macrolides. Heteroresistance is commonly missed during traditional clinical resistance screens but is highly concerning as resistant subpopulations can persist despite treatment. strains containing the Mega element were screened via Etesting and population analysis profiling (PAP). All Mega-containing strains screened displayed heteroresistance by PAP. The heteroresistance phenotype was linked to the mRNA expression of the (E)/ operon of the Mega element. Macrolide induction uniformly increased Mega operon mRNA expression across the population, and heteroresistance was eliminated. A deletion of the 5' regulatory region of the Mega operon results in a mutant deficient in induction as well as in heteroresistance. The (E) leader peptide sequence of the 5' regulatory region was required for induction and heteroresistance. Treatment with a noninducing 16-membered ring macrolide antibiotic did not induce the (E)/ operon or eliminate the heteroresistance phenotype. Thus, inducibility of the Mega element by 14- and 15-membered macrolides and heteroresistance are linked in The stochastic variation in (E)/ expression in a population containing Mega provides the basis for heteroresistance.