Molecular Epidemiology, Antimicrobial Resistance, and Clinical Characteristics of Streptococcus pneumoniae Isolated from Adult Patients with Invasive Pneumococcal Disease

<b>Background/Objectives</b>: <i>Streptococcus pneumoniae</i> is a major human pathogen causing illnesses that range from mild respiratory infections to severe invasive diseases. More than 100 known <i>S. pneumoniae</i> serotypes differ in their virulence, prevalence, and levels of drug resistance. Additionally, different clonal types within the same serotype may exhibit varying disease potential and genetic characteristics. This study aimed to determine phenotypic and molecular characteristics of <i>S. pneumoniae</i> isolated from patients with invasive pneumococcal disease (IPD). <b>Methods</b>: The serotypes of invasive <i>S. pneumoniae</i> isolates collected between 2022 and 2025 from adult patients hospitalized in a tertiary hospital were determined. Multilocus sequence typing (MLST) was performed on isolates with reduced susceptibility to penicillin to assess their molecular epidemiology. <b>Results</b>: Serotype 3 was the most common among all invasive isolates (29/85; 34.1%), followed by serotype 19A (22/85; 25.9%). Most penicillin-resistant isolates belonged to serotypes 19A and 19F. Three of the eight 19A isolates with reduced penicillin susceptibility were assigned to ST320 (37.5%), a clinically significant clone due to its high virulence and antibiotic resistance. While 15.3% of all isolates were multidrug-resistant (MDR), nearly half of the isolates with reduced penicillin susceptibility were MDR, most frequently exhibiting the erythromycin-clindamycin-tetracycline resistotype. <b>Conclusions</b>: This study highlights the predominance of serotype 19A, particularly the highly virulent and resistant ST320 clone, among invasive isolates with reduced penicillin susceptibility. These findings underscore the ongoing threat of antimicrobial resistance in IPD and the importance of continued surveillance of serotype distribution and resistance patterns to guide treatment strategies and vaccination policy decisions.