Comparison of Various Methods for Identification of Anaerobic Bacteria and Investigation of Sensitivity to Antibiotics

Introduction: In this study, it was aimed to identify anaerobic bacteria isolated from clinical specimens admitted to Necmettin Erbakan University Meram Faculty of Medicine Hospital, Microbiology Laboratory with suspicion of anaerobic infection via different methods and to determine the antibiotic susceptibility against several antibiotics.Materials and Methods: The study included 197 clinical specimens sent for anaerobic culture between August 2019 and March 2020. The samples were incubated using anaerobic culture techniques and the isolates obtained were identified by conventional methods, an-ident discs, API 20 a panel and Vitek 2 semi-automatic system compatible ANC cards. Penicillin G, cefoxitin, clindamycin, metro-nidazole, imipenem, piperacillin/tazobactam, ceftolozane /tazobactam susceptibilities of isolates were determined by concentration gradient method.Results: A total of 46 anaerobic bacteria were isolated from 37 (18.8%) samples; 69.6% of these isolates were gram-positive anaerobic bacteria and 30.4% were gram-negative anaerobes. The most frequently isolated anaerobic bacteria were Clostridium spp. (n= 9) and Peptostreptococcus spp. (n= 8) in gram-positives; Prevotella spp. (n= 7) and Bacteroides fragilis group (n= 5) in gram-negatives. The compatibility between API20A panels and ANC cards was determined 79.5% (35/44), 43.2% (19/44) of test strains at the genus and species levels respectively. The highest rate of resistance was detected against penicillin G (38.6%). Cefoxitin, clindamycin, metronidazole, piperacillin/tazobactam and imipenem sensitivity rates were 93.2%, 65.9%, 84.1%, 93.2% and 100% respectively. The MIC value of ceftolozane-tazobactam, one of the new antimicrobials, varied in a wide range (0.032-12mg/L) according to the bacterial species. While the highest MIC values were found in B. fragilis group strains, the lowest levels were detected in Veillonella spp. (MIC= 0.25) and Fusobacterium spp. (MIC= 0.5 mg/L) isolates.Conclusion: As a result, the data of this study, which is one of the limited number of studies evaluating the performance among anaerobic diagnostic tests frequently used in routine; suggested that the diagnosis should be supported by a different diagnostic test in case of inconsistency between the identification result and clinical or antibiotic susceptibility profiles. One of our isolates was the first strain of metronidazole resistant B. fragilis isolated in our laboratory in which requires monitoring of susceptibility profiles. To our knowledge, this is the first study to evaluate the efficacy of ceftolozane/tazobactam on anaerobic clinical isolates from our country. Ceftolozane/tazobactam activity was detected in great variability between bacterial species in accordance with the literature data. So, it is need optimized studies including more bacterial species and numbers.

Automated summary

This study aimed to identify anaerobic bacteria isolated from clinical specimens and determine their antibiotic susceptibility. The most commonly isolated anaerobic bacteria were Clostridium spp. and Peptostreptococcus spp. The highest resistance was found against penicillin G, while the sensitivity to ceftolozane/tazobactam varied widely among different bacterial species.