Nosocomial A. baumannii infections are a rising problem globally. Remarkable genome plasticity facilitates the rapid acquisition of resistance determinants and swift adaptation to new antibiotic therapies. Clinical A. baumannii frequently possess an impressively MDR phenotype, which can make it extremely difficult to treat with even our last line therapies.
In the absence of effective therapeutics, combination therapy which uses two or more antibiotics at once, has many advantages that make it an enticing option for sustainably treating drug-resistant A. baumannii like revitalising of our existing arsenal of drugs, exploiting antibiotic synergy, and broadened antimicrobial targeting. Despite the widespread use of combination therapy in hospitals and veterinary clinics, very little is known about how resistance develops on a molecular level.
The Australian Therapeutic Guidelines recommends the use of co-trimoxazole (trimethoprim-sulfamethoxazole) for the treatment of many infections due to strong bactericidal activity and synergy between its components. Both drugs target sequential steps of the folate synthesis pathway, thus mutually potentiating each other’s effects. Importantly, this combination has shown promise in treating colistin- and carbapenem-resistant A. baumannii.
We employed transposon directed insertion-site sequencing (TraDIS), a high-throughput genome-wide assay to simultaneously assess the fitness contribution of every single gene under subinhibitory trimethoprim, sulfamethoxazole and trimethoprim-sulfamethoxazole stress, using two clinical A. baumannii strains (ATCC 17978 and BAL062). We identified a range of expected resistance genes (e.g. RND efflux component genes) as well as some novel genes not previously linked to either trimethoprim or sulfamethoxazole resistance (ihfB, cpdA). Using EggNOG, genes were grouped into functional categories. Several categories including “cell membrane/envelope/biogenesis” and “replication, recombination and repair” were enriched under trimethoprim-sulfamethoxazole. Differences in the genes and functional classes implicated in combination treatment compared to single-antibiotic treatment, indicated that resistance to trimethoprim-sulfamethoxazole combination therapy has a complex, pleotropic mechanism, not merely mediated by the addition of mechanisms that provide resistance to trimethoprim or sulfamethoxazole monotherapies.