Trends in the prevalence and antibiotic susceptibility of anaerobic Gram-negative bacteria causing clinical infections in Estonia

Objective: The aim of our study was to describe the species distribution and changes in the resistance profile of anaerobic bacteria isolated at Tartu University Hospital.

Methods: The data for 2010, 2016 and 2020 were analysed retrospectively. The strains were identified by Vitek2 (2010) and MALDI-TOF MS (2016, 2020); and the MIC values for the antibiotics were determined using gradient tests. Resistance was interpreted using EUCAST breakpoints.

Results: The average number of anaerobic cultures received during the 10 years increased from 1551 to 5983; (386%).. The most common pathogens were Gram negative anaerobic rod-shaped bacteria (Bacteroides fragilis, Bacteroides spp., Fusobacterium spp. and Prevotella spp.). The percentage of susceptible strains in the three years studied was stable for the majority of drug-bug combinations tested, except beta-lactam antibiotic sensitivity (Bacteroides spp. sensitivity to imipenem and ampicillin-sulbactam, Fusobacterium spp. and Prevotella spp. to penicillin).

There were significant differences in the MIC values (p < 0.05) when comparing 2020 with 2016. The MIC was higher among Bacterioides spp. ampicillin-sulbactam, Bacteriodes fragilis for imipenem and Prevotella for imipenem, ampicillin-sulbactam and clindamycin in 2020 than in 2016. The same was observed in 2020 vs 2010 among Prevotella spp. MIC for metronidazole, penicillin and cefoxitin. In contrast, the MIC values were surprisingly lower in 2020 than in 2010 for other Gram-negative rods.

Conclusions: There was no shift in the spectrum of microbial groups as causative agents of clinical infections during the 2010-2020 period. However, due to the improvement of identification methods, the number of identified species increased. The resistance pattern of anaerobes was stable, but the changes in MIC values may indicate a further steady increase in resistance. The surveillance of antibiotic resistance of anaerobes is important to predict the efficiency of empirical treatment.