Changes in microbiological characteristics of bloodstream infections before and after the COVID-19 pandemic in a tertiary teaching hospital in Iran

The COVID-19 pandemic has significantly impacted healthcare systems, leading to increased antibiotic misuse and multidrug-resistant (MDR) infections. This study assessed microorganisms and antibiotic-resistance patterns in bloodstream infections at a tertiary teaching hospital in Shiraz, Iran. This retrospective cohort study analyzed blood culture results from the Professor Alborzi Clinical Microbiology Research Center (2017–2023), comparing two periods: pre-pandemic (Period 1) (2017–2020) and pandemic/post-pandemic (Period 2) (2020–2023). Among 5,970 pathogenic strains, Gram-negative bacteria predominated (81.2% Period 1, 77.3% Period 2), maintaining a 1:5 ratio with Gram-positive bacteria. Stenotrophomonas maltophilia was the most common pathogen (23%) but declined significantly from 30 to 11% post-2020. In contrast, Escherichia coli (8–14%), Klebsiella spp. (6–13%), and Enterobacter spp. (6–13%) increased significantly. Staphylococcus aureus (7–12%) and Enterococcus spp. (6–8%) showed non-significant increases, while Pseudomonas spp. (10–6%) and streptococcus viridans (5–2%) declined without statistical significance. Acinetobacter spp. remained stable at 6%. Time-series analysis revealed pandemic-induced shifts: Enterobacter spp. reversed its pre-pandemic rise post-COVID (p < 0.001), while Stenotrophomonas maltophilia exhibited a declining trend in Period 1 and an increasing one in Period 2 (p = 0.016). No significant trend changes were observed for the remaining examined bacterial species. Among Gram-negative fermenters, carbapenems and amikacin remained the most effective antibiotics. While Acinetobacter spp. demonstrated elevated resistance to all tested antibiotics, including carbapenems, other Gram- negative non-fermenter bacteria displayed mixed resistance patterns, with increases, decreases, and stable resistance levels observed when comparing data from two periods. Methicillin-resistant S. aureus (MRSA) rates doubled (24% to 50%, p < 0.0001), with increased resistance to ciprofloxacin. High 54 rates of vancomycin-resistant Enterococci (VRE, 62%) were observed, with a 55 non-significant upward change (67%). The study highlights shifts in pathogen epidemiology and antibiotic resistance during the pandemic, emphasizing the need for continuous surveillance and targeted antibiotic stewardship programs.