Antimicrobial activity of β-lactam antibiotics against pneumococcal isolates causing pneumococcal disease in adults immediately before and after the COVID-19 pandemic in Spain (2019–2020)

Background Streptococcus pneumoniae remains a major respiratory pathogen. Although pneumococcal conjugate vaccines (PCVs) have reduced the burden of resistant vaccine-covered serotypes, serotype replacement has led to the emergence of non-vaccine types. Methods This study assessed the activity of β-lactam antibiotics against 1,018 clinical isolates of S. pneumoniae with reduced penicillin susceptibility collected across Spain in 2019–2020. Minimum inhibitory concentrations (MICs) were determined for oral cephalosporins (cefditoren, cefuroxime, cefixime, cefpodoxime), intravenous cephalosporins (cefotaxime, cefuroxime), and penicillins (penicillin, amoxicillin). MIC 50 and MIC 90 values were analyzed by year, serotype (PCV20- vs. PCV21-specific), and clinical presentation. Results Cefditoren showed the lowest MIC 50 / 90 values among oral β-lactams, and cefotaxime was the most active intravenous cephalosporin. Most isolates were resistant to cefuroxime (oral and intravenous) based on EUCAST breakpoints. While penicillin derivatives showed susceptibility with increased exposure at MIC 50 , MIC 90 values indicated resistance to oral amoxicillin. A rise in antibiotic use during the COVID-19 pandemic’s first year was associated with an increased proportion of resistant isolates, particularly among non-invasive respiratory infections. Serotype analysis revealed that PCV20-specific types (9V, 14 and 19A) and shared PCV20 and PCV21 serotypes (11A and 19A) had higher MIC 90 values and bigger proportion of resistant strains to β-lactams than PCV21-unique ones (15A, 23B and 24F). Conclusion Despite the overall increase in resistance, cefditoren and cefotaxime retained favorable in vitro activity, while cefuroxime, cefixime, and cefpodoxime exhibited poor efficacy. Newer PCVs may broaden protection against emerging resistant serotypes, yet continued antimicrobial resistance surveillance is essential, especially regarding non-vaccine serotypes. These findings support cefditoren and cefotaxime as viable therapeutic options against non-susceptible S. pneumoniae isolates.