A new retrospective study from Türkiye, published in BMC Microbiology, offers a detailed two-year snapshot of how this pathogen is responding to antibiotics and how its circulating serotypes align with available vaccines.
The research team, based at a single center, classified isolates recovered from normally sterile sites as invasive and those from non-sterile sites as non-invasive. Of the 462 isolates, 112 (24.2 percent) were invasive and 350 (75.8 percent) were non-invasive. Species identification was performed using MALDI-TOF mass spectrometry and confirmed by optochin susceptibility testing, while antimicrobial susceptibility was determined with the Phoenix M50 system and interpreted according to EUCAST version 14.0 criteria. Multidrug resistance was defined as resistance to at least one antimicrobial agent in three or more antimicrobial categories.
Serotyping of invasive isolates was carried out at the National Respiratory Pathogens Reference Laboratory. Among the 79 invasive isolates subjected to serotyping, a definitive serotype could be identified in only 41. Within the adult subset with definitive serotype identification, numbering 37 isolates, serotype 3 was the most frequently identified at 21.6 percent (8 of 37), followed by serotype 8 at 13.5 percent (5 of 37), with serotypes 14 and 9N each accounting for 10.8 percent (4 of 37). This distribution matters because serotype composition directly determines how much of the circulating pneumococcal population current vaccines can potentially prevent.
The researchers assessed potential vaccine coverage for seven formulations: PCV7, PCV10, PCV13, PCV15, PCV20, PCV21, and PPV23. Among adults with a definitive serotype, potential coverage was 86.5 percent (32 of 37) for the 23-valent polysaccharide vaccine PPV23, 67.6 percent (25 of 37) for the 20-valent conjugate vaccine PCV20, and 64.9 percent (24 of 37) for PCV21.
On the resistance front, the headline finding is that multidrug resistance was detected in 251 of 462 isolates, or 54.3 percent of the total. Resistance was more frequent among non-invasive isolates than invasive ones, at 57.4 percent versus 44.6 percent.
Macrolide resistance emerged as a particular concern. Erythromycin resistance stood at 52.8 percent (243 of 460 isolates) across all isolates, meaning that more than half of the pneumococci collected at this center could not be treated reliably with this class. By contrast, fluoroquinolone resistance remained below 10 percent, suggesting that these agents retain better activity against the local pneumococcal population, at least for now.
Penicillin susceptibility, a cornerstone of pneumococcal treatment decisions, depended heavily on which breakpoints were applied. Using non-meningitis breakpoints, penicillin resistance was 32.0 percent (146 of 456 isolates). When meningitis breakpoints were applied to invasive isolates, resistance rose sharply to 57.8 percent (63 of 109). The same pattern held for other beta-lactam agents: cefotaxime resistance increased from 7.9 percent to 28.2 percent, and meropenem resistance climbed from 4.8 percent to 43.6 percent, when meningitis criteria were applied. These figures underscore how the same isolate population can appear moderately or severely resistant depending on the clinical context and the laboratory standards used.
Comparing age groups revealed one statistically significant difference. Clindamycin resistance was significantly higher in children than in adults, at 55.2 percent versus 36.2 percent, with a p-value of 0.0051. Among the resistance proportions compared between children and adults, this was the only one that differed significantly, indicating that while macrolide and beta-lactam resistance burdens are broadly similar across ages, lincosamide resistance is disproportionately concentrated in pediatric isolates.
The authors conclude that S. pneumoniae isolates in their setting continue to exhibit high antimicrobial resistance, with more than half of all isolates being multidrug resistant. They emphasize that continued surveillance of pneumococcal serotypes, vaccine coverage, and resistance patterns is needed in Türkiye. As a single-center retrospective study, the work captures one institution’s experience over two years rather than a national picture, and the relatively small number of invasive isolates with definitive serotype results limits the precision of the vaccine coverage estimates. Nevertheless, the combination of high macrolide resistance, substantial multidrug resistance, and a serotype distribution led by serotype 3 among adults provides a concrete evidence base for clinicians and public health authorities weighing vaccination strategies and empiric treatment choices against pneumococcal disease.
Subject of Research: Antimicrobial resistance profiles and serotype distribution of Streptococcus pneumoniae isolates in Türkiye
Article Title: Antimicrobial resistance profiles of clinical Streptococcus pneumoniae isolates and serotype distribution among invasive isolates: a single-center retrospective study from Türkiye, 2024–2025
Article References: Genç Bahçe, Y., Hortaç İştar, E., Öztürk, E., Gürbüz, O. A., Ünal Şahin, N., & Coşgun, Y. (2026). Antimicrobial resistance profiles of clinical Streptococcus pneumoniae isolates and serotype distribution among invasive isolates: a single-center retrospective study from Türkiye, 2024–2025. BMC Microbiology. https://doi.org/10.1186/s12866-026-05792-0
Image Credits: AI Generated
DOI: 10.1186/s12866-026-05792-0
Keywords: Streptococcus pneumoniae, antimicrobial resistance, multidrug resistance, serotype distribution, pneumococcal vaccines, PCV20, PPV23, erythromycin resistance, penicillin resistance, Türkiye, invasive pneumococcal disease, BMC Microbiology
News Source: Kristina Jarvis. (October 29, 2026). Pneumococcal resistance runs high in Türkiye, with serotype 3 leading in adults. Scienmag.
