Bacterioides fragilis: resistance is rising

Posted on December 5, 2012   by Naomi Osborne, ThermoFisher Scientific

The emergence of multi-drug resistant (MDR) micro-organisms is ever increasing; Staphylococcus aureus, Clostrium difficile, Mycobacterium tuberculosis, Acinetobacter baumanii – the list seems endless.

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A review published in the Journal of Medical Microbiology discusses the rise in the prevalence of an anaerobic MDR organism, Bacteroides fragilis.

Members of the Bacteroides group – the most prevalent of which is B. fragilis – are anaerobic commensals of the gastrointestinal (GI) tract. B. fragilis is an opportunistic pathogen, the most frequent anaerobic bacterium isolated in the clinical setting, and a common cause of abscesses and bacteremia. Some strains termed enterotoxigenic Bacteroides fragilis cause inflammatory diarrhoea by secreting toxin

B. fragilis infections are treated with antibiotics such as metronidazole, clindamycin and carbapenems but resistance is rapidly increasing. This review includes a case report of a patient infected with MDR B. fragilis, the first time it was identified in Scandinavia. The isolate was resistant to all antibiotics usually used to treat anaerobic infections, including metronizadole, carbapenems, clindamycin and tetracycline.

Often, carbapenem antibiotics are our last line of defense against infections caused by MDR micro-organisms, so emerging resistance is extremely concerning. Carbapenem resistance is usually attributed to a metallo-betalactamase enzyme, which was found in this isolated B. fragilis strain. The strain also possessed resistance genes against metronidazole, a commonly used antibiotic used to treat anaerobic infections. Although resistance to this antibiotic is still low, it has slowly been rising as the bacterium has acquired novel resistance genes. As B. fragilis is a commensal of the GI tract, it is possible that resistance genes have been transferred from other residents of the gut flora. The transfer of these genes further limits our therapeutic options to treat B. fragilis infections and there is also concern that these genes can be transferred to other organisms in the gut.

The study highlights that we do not know how prevalent resistance is amongst B. fragilis, and how much resistance is transferred, partly because of lack of monitoring; antibiotic resistance trends are usually monitored through surveys, and susceptibility testing is only carried out on strains from seriously infected patients. In addition, resistance in anaerobic organisms can go unnoticed due to poor isolation and identification methods.

The increasing resistance of B. fragilis is worrying as the pipeline of antibiotics to treat anaerobic organisms is limited. Awareness of resistance in aerobic organisms such as methicillin-resistant Staphylococcus aureus and extended spectrum beta-lactamase-producing Enterobacteriacae has risen in recent years but increasing resistance in anaerobic organisms should not be overlooked. Improved identification methods and surveillance of antibiotic resistance is needed for a better understanding of the transfer of resistance genes. This will ensure patients suffering from drug-resistant infections receive effective and timely treatment.