Susceptibility testing is not routinely performed on anaerobes due to perceptions of predictable susceptibility, and lack access to reference methods. In recent years however, emerging resistance of anaerobes to antimicrobial agents have been noted, hence periodical monitoring of antimicrobial resistance patterns is important.
The study aimed to analyse the resistance patterns of Bacteroides spp from clinical specimens to amoxicillin-clavulanate, clindamycin and metronidazole.
A total of 61 clinical isolates were screened from sterile sites from 2015-2019. 93% of isolates analysed were from Blood cultures and 7% were from intra-abdominal and soft tissue infections.
An E-test was performed on all isolates of Bacteroides spp isolated from these specimens. The final MICs were read at 48 hours and were categorised as R, S or I using the CLSI breakpoints.
57% of the isolates tested were B. fragilis sp, and the remainder 43% were other Bacteroides spp. Amoxicillin-clavulanate and metronidazole non susceptibility was noted in only 3 and 4 isolates respectively. Clindamycin non-susceptibility was most frequently detected, being present in 37% of isolates.
This survey found an increased non-susceptibility to clindamycin when compared to previous published Australian data in 1992 and 2018 by Chen et al. and Hughes et al. In these surveys Clindamycin Susceptibility was noted at higher rate of 86-89% compared to 63% in this study. The overall non-susceptibility data of Bacteroides spp to amoxicillin-clavulanate and metronidazole was in concordance to previous studies. However direct comparison with the studies is difficult given the differences in methodology and clinical breakpoints used.
Based on this study, clindamycin should not be used for empiric therapy of anaerobic infection given the high rate of non-susceptibility. Amoxicillin-clavulanate and metronidazole remain suitable choices for empirical treatment, but routine testing should be considered for sterile sites to reduce treatment failure and decrease the morbidity rate of these infections.