Objective: Our aim was to document and characterize the hetero-resistant phenotypes, also known for methicillin and vancomycin-resistant Staphylococcus aureus and penicillin-resistant Streptococcus pneumoniae, of Bacteroides strains to cefoxitin and carbapenems noticed using Etests for antimicrobial susceptibility testing.
Methods: Eight hetero-resistant B. fragilis strains collected during our 10 years experience and 100 clinical B. fragilis group strains recently isolated in Hungary were studied. For in vitro susceptibility measurements of cefoxitin and carbapenems agar dilution and Etest were used. Heteroresistant colonies growing in the ellipse between the E-test strip and main population were further analyzed. The occurrence of cfxA and cfiA resistance genes and their regulatory regions were investigated by PCR and nucleotide sequencing. Population analysis profiles were also investigated.
Results: We detected 8 B. fragilis strains hetero-resistant to carbapenems using Etest susceptibility measurements (from 0.25-4 µg/ml of continous growth up to 16-32 µg/ml). All of them were cfiA-positive but did not harbour insertion sequence elements in the upstream region of the resistance genes. Of 100 recently isolated Bacteroides strains, 21 strains hetero-resistant to cefoxitin were observed. Their Etest patterns usually displayed a continuous growth of the less susceptible subpopulation from 8-128 µg/ml up to 256 µg/ml. Of these 21 isolates 11 harbored cfxA genes and their upstream regions were usually altered to the common 1.2 kb fragment, as seen in our previous studies. The population profile analysis demonstrated presence of more resistant subpopulations in the culture of strains corresponding to the more resistant colonies in the Etest ellipse zones. Repeated experiments of subcultures from single colonies taken from the heteroresistant zones resulted in the original hetero-resistant appearance using the Etest method.
Conclusion: Hetero-resistance to important β-lactam antibiotics appear among Bacteroides strains but the phenotype could not yet be linked to any particular genetic constitution.
This study was supported by a Hungarian National Research Fund grant (K69044).
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