Pneumococcal empyema: Resistance patterns, fitness cost and serotype distribution.

Background: Streptococcus pneumoniae is a recognized etiology of invasive infections including parapneumonic empyema, and its resistance to antibiotics is evolving worldwide, raising concerns of encountering untreatable strains. This study measured the serotype distribution, antimicrobial susceptibility and biological cost incurred by resistance of pneumococci from pleural samples.

Methods: The serotype profiles, susceptibility results and growth rates were phenotypically determined for a panel of clinical strains of S. pneumoniae from cases of empyema between 2011 and 2019.

Results: Of 24 empyema cases, the isolated strains belonged to seven serotypes in the following descending order; 19A, 11A/D, 19F, 3, 7F, 1/6B while two strains remained non-typable. Penicillin susceptibility was shown in <80% of the isolates, while parenteral cephalosporins (cefuroxime and ceftriaxone) demonstrated activity in 83.3 and 95.8% respectively. High resistance frequency was noted for macrolides and sulfonamides, but the strains were uniformly sensitive to respiratory fluroquinolones, vancomycin and linezolid. The macrolide-resistant strain exhibited a high growth rate, suggesting a possible beneficial effect. Phenotypes with mono-resistance to sulfonamides and clindamycin were equally fit as the susceptible counterpart strains. Resistance to multiple antimicrobial agents resulted in a high degree of fitness deficit, while other resistant phenotypes were less fit.

Conclusions: The pneumococcal conjugate vaccine PCV13 serotypes still circulate in the community. The data indicate that resistance to certain antimicrobials incurs an apparent fitness cost in pneumococci which may limit the dissemination of such strains while low fitness cost, seen in case of resistance to macrolides, may contribute to the spread of resistant clones.