Heteroresistance to penicillin in Streptococcus pneumoniae

Heteroresistance to β-lactam antibiotics has been mainly described for staphylococci, for which it complicates diagnostic procedures and therapeutic success. This study investigated whether heteroresistance to penicillin exists in Streptococcus pneumoniae. Population analysis profile (PAP) showed the presence of subpopulations with higher penicillin resistance in four of nine clinical pneumococcal strains obtained from a local surveillance program (representing the multiresistant clones ST179, ST276, and ST344) and in seven of 16 reference strains (representing the international clones Spain23F-1, Spain9V-3, Spain14-5, Hungary19A-6, South Africa19A-13, Taiwan23F-15, and Finland6B-12). Heteroresistant strains had penicillin minimal inhibitory concentrations (MICs) (for the majority of cells) in the intermediate- to high-level range (0.19–2.0 μg/ml). PAP curves suggested the presence of subpopulations also for the highly penicillin-resistant strains Taiwan19F-14, Poland23F-16, CSR19A-11, and CSR14-10. PAP of bacterial subpopulations with higher penicillin resistance showed a shift toward higher penicillin-resistance levels, which reverted upon multiple passages on antibiotic-free media. Convergence to a homotypic resistance phenotype did not occur. Comparison of two strains of clone ST179 showed a correlation between the heteroresistant phenotype and a higher-penicillin MIC and a greater number of altered penicillin-binding proteins (PBP1a, -2b, and -2x), respectively. Therefore, heteroresistance to penicillin occurs in international multiresistant clones of S. pneumoniae. Pneumococci may use heteroresistance to penicillin as a tool during their evolution to high penicillin resistance, because it gives the bacteria an opportunity to explore growth in the presence of antibiotics before acquisition of resistance genes