How antibiotics work together: molecular mechanisms behind combination therapy

•Combination therapy is an important tool for tackling antibiotic resistance.•In depth molecular studies for combination therapy are lacking.•We summarise six major mechanisms mediating the effects seen in synergy and antagony.•Combination therapy resistance mechanisms are complex on a population level.•Resistance mutations relate to general as well as specific resistance strategies. Antibiotics used in combination are an effective strategy for combatting numerous infectious diseases in clinical and veterinary settings, particularly as a last-line therapy for difficult-to-treat cases. Combination therapy can either increase or slow the rate of killing, broaden the antibiotic spectrum, reduce dosage and unwanted side-effects, and even control the emergence of resistance. The administration of antibiotics in combination has been used effectively against bacterial infections for >70 years, first used to treat tuberculosis. However, effective antibiotic combinations and their dosage regimes have been largely determined empirically in the clinic, and the molecular mechanisms underpinning how these combinations work remains surprisingly elusive. This review focuses on studies that have outlined the genetics and molecular mechanisms of action underlying antibiotic combinations, as well as those that examine how resistance develops. We highlight the need for further experimentation and genetic validation to fully realise the potential of combination therapy.