Antimicrobial susceptibility testing of anaerobic bacteria for various beta-lactam antibiotics in combination with beta-lactamase inhibitors

Odpornost proti antibiotikom med anaerobnimi bakterijami je močno narasla v zadnjih tridesetih letih. Najobsežnejša slovenska raziskava o protimikrobni odpornosti anaerobnih bakterij iz leta 2015 je pokazala, da je odpornost proti amoksicilinu s klavulansko kislino pri najpogosteje izolirani skupini B. fragilis znašala 8 %, odpornost proti imipenemu pa 1 %. Podatkov o občutljivosti za ostale betalaktamske antibiotike v kombinaciji z zaviralci betalaktamaz ni, saj se v rutinski diagnostiki pri nas testiranje zanje ne izvaja. Zato smo se odločili, da v našem magistrskem delu z gradient difuzijskim testom preverimo občutljivost izbranih anaerobnih bakterij, iz rodu Bacteroides in Parabacteroides, za amoksicilin s klavulansko kislino, piperacilin s tazobaktamom in ceftolozan s tazobaktamom. Skupno smo testirali 120 prospektivno zbranih izolatov (104 izolati rodu Bacteroides, 16 izolatov rodu Parabacteroides), ki smo jih izolirali iz kužnin in 87 izbranih izolatov iz zbirke IMI z znano odpornostjo proti amoksicilinu s klavulansko kislino (77 izolatov rodu Bacteroides, 10 izolatov rodu Parabacteroides). Odpornost prospektivno zbranih izolatov rodu Bacteroides za amoksicilin s klavulansko kislino je bila 17,3 %, med izolati rodu Parabacteroides je bilo odpornih 31,3 % testiranih izolatov. Odpornost izolatov rodu Bacteroides proti piperacilinu s tazobaktamom je znašala 11,5 %, za izolate rodu Parabacteroides pa kar 43,8 %. Odstotek odpornosti pri izbranih izolatih z znano odpornostjo proti amoksicilinu s klavulansko kislino je bil še višji in je za izolate rodu Bacteroides znašal 58,4 %, za izolate rodu Parabacteroides pa 50 %. Ker o ceftolozanu s tazobaktamom še ni smernic za interpretacijo mejnih vrednosti, smo določili distribucijo vrednosti MIK. Za prospektivno zbrane izolate je bila vrednost MIK50 16 μg/mL, MIK90 pa 256 μg/mL. Vsi izbrani izolati rodu Parabacteroides z znano odpornostjo proti amoksicilinu s klavulansko kislino so imeli vrednost MIK 256 μg/mL, medtem ko je bil razpon vrednosti za izolate rodu Bacteroides od 1,5 do 256 μg/mL.Antibiotic resistance among anaerobic bacteria has risen sharply in the last thirty years. The most extensive Slovenian research about antimicrobial resistance of anaerobic bacteria from 2015 showed that resistance to amoxicillin/clavulanic acid of the most frequently isolated B. fragilis group was 8%, and the resistance to imipenem was 1%. There is no data about susceptibility for the other beta-lactam antibiotics in combination with beta-lactamase inhibitors as testing for them is not performed in our routine diagnostics. For this reason, we decided to test the susceptibility of selected anaerobic bacteria from the genus Bacteroides and Parabacteroides to amoxicillin/clavulanic acid, piperacilin/tazobactam, and ceftolozan/tazobactam with a gradient diffusion method. We tested a total of 120 prospectively collected clinically significant anaerobic bacteria (104 isolates of the genus Bacteroides, 16 isolates of the genus Parabacteroides) and 87 selected isolates from the IMI collection with known resistance to amoxicillin/clavulanic acid (77 isolates of the genus Bacteroides, 10 isolates of the genus Parabacteroides). The resistance of prospectively collected Bacteroides isolates to amoxicillin/clavulanic acid was 17.3%, and 31.3% for the Parabacteroides isolates. The resistance of isolates of the genus Bacteroides to piperacillin/tazobactam was 11.5%, and among isolates of the genus Parabacteroides resistance was as high as 43.8%. The percentage of resistance of the selected isolates with known resistance to amoxicillin with clavulanic acid was even higher: 58.4% for Bacteroides isolates and 50% for isolates of the genus Parabacteroides. As there are no guidelines for interpretive breakpoints for ceftolozane/ tazobactam, we only determined MIC distribution. For prospectively collected isolates, the MIC50 value was 16 μg/mL, and MIC90 was 256 μg/mL. All selected isolates of the genus Parabacteroides with known resistance to amoxicillin/clavulanic acid had a MIC value of 256 μg/mL, while the value range for isolates of the genus Bacteroides ranged from 1.5 to 256 μg/mL