Antimicrobial resistance profiles of clinical Streptococcus pneumoniae isolates and serotype distribution among invasive isolates: a single-center retrospective study from Türkiye, 2024–2025

Streptococcus pneumoniae ( S. pneumoniae ) remains an important cause of invasive and non-invasive infections. Monitoring changes in serotype distribution and antimicrobial resistance is important for updating vaccination and treatment strategies. This study aimed to compare the antimicrobial resistance profiles of invasive and non-invasive S. pneumoniae isolates and to evaluate the serotype distribution and potential vaccine coverage of invasive isolates. A total of 462 S. pneumoniae isolates recovered from clinical specimens between January 2024 and December 2025 were retrospectively evaluated. Isolates recovered from normally sterile sites were classified as invasive, whereas those recovered from non-sterile sites were classified as non-invasive. Species identification was performed using MALDI-TOF MS and confirmed by optochin susceptibility testing. Antimicrobial susceptibility was determined using the Phoenix M50 system and interpreted according to EUCAST version 14.0 criteria. Multidrug resistance (MDR) was defined as resistance to at least one antimicrobial agent in three or more antimicrobial categories. Serotyping of invasive isolates was performed at the National Respiratory Pathogens Reference Laboratory. Resistance profiles were compared between invasive and non-invasive isolates and between pediatric and adult groups. Potential vaccine coverage was assessed for PCV7, PCV10, PCV13, PCV15, PCV20, PCV21, and PPV23. Of the 462 isolates, 112 (24.2%) were invasive and 350 (75.8%) were non-invasive. Among the 79 invasive isolates subjected to serotyping, a definitive serotype was identified in 41. Among adults with definitive serotype identification ( n = 37), serotype 3 was the most frequently identified (21.6%; 8/37), followed by serotype 8 (13.5%; 5/37) and serotypes 14 and 9 N (10.8%; 4/37 each). Potential vaccine coverage was 86.5% (32/37) for PPV23, 67.6% (25/37) for PCV20, and 64.9% (24/37) for PCV21. MDR was detected in 251/462 (54.3%) isolates and was more frequent among non-invasive than invasive isolates (57.4% vs. 44.6%). Erythromycin resistance was 52.8% (243/460) among all isolates, whereas fluoroquinolone resistance remained below 10%. When non-meningitis breakpoints were applied, penicillin resistance was 32.0% (146/456), increasing to 57.8% (63/109) when meningitis breakpoints were applied to invasive isolates. Similarly, cefotaxime and meropenem resistance increased from 7.9% to 28.2% and from 4.8% to 43.6%, respectively. Clindamycin resistance was significantly higher in children than in adults (55.2% vs. 36.2%; p = 0.0051). Our findings show that S. pneumoniae isolates continue to exhibit high antimicrobial resistance. Serotype 3 was the most frequently identified serotype among adult isolates, while PPV23, PCV20, and PCV21 showed broad potential serotype coverage. Among the antimicrobial resistance proportions compared between children and adults, only clindamycin resistance differed significantly and was higher in children. More than half of the isolates were MDR, highlighting the importance of antimicrobial resistance surveillance. Continued surveillance of pneumococcal serotypes, vaccine coverage, and antimicrobial resistance patterns is needed in Türkiye.