Combination therapy is the treatment of infections with two or more drugs usually to increase the clinical efficacy of the treatment, for example as described above for quinupristin-dalfopristin, or to minimize the development of resistant strains of the infective organism. Where the infection is of unknown origin, then multiple therapies are advisable to fight the most likely pathogens. With mixed infections involving two or more known pathogens, it is desirable to target each microorganism with one or more different antibiotics. Combination therapy may be used even if only a single infective pathogen is present as using combinations of drugs can prevent or delay the development of resistance to the drugs being used. For example, some bacteria are resistant toA-lactams because theyproduce aA-lactamase that catalyzes the breakdown of the A-lactam ring. Combining an inhibitor of A-lactamase, such as clavulanic acid, with the A-lactam antibiotic helps preserve the drug in vivo. Other drugs commonly combined in therapeutic use are sulphamethoxazole and trimethoprim that synergistically inhibit the synthesis of folate by blocking different steps in its synthesis. The cocktail of isoniazid, rifampicin and pyrazinamide is used in the treatment of TB, while clofazimine, dapsone and rifampicin are used in combination in the therapy of leprosy.