Prevention of emergence of bacterial resistance: no advantage in combining rifampicin and tetracycline over use of tetracycline alone.

Experiments have been performed using rifampicin (RIF) and tetracycline (TET) to establish whether the use of the combination prevents the emergence of bacterial resistance to either drug. Experiments were performed using repeated 4-hour cycles of exposure of large bacterial populations to varying concentrations of antibiotics in order to reproduce in vitro the conditions of treatment of a systemic infection in man. When exposed to both RIF and TET the doubly-sensitive test organism was eliminated from the test cultures in 92 h without emergence of resistance. When the organism was exposed to RIF alone it became massively RIF-resistant in 20 h. The organism was eliminated in 104 h when exposed to TET alone. Under the test conditions there was no significant difference in experimental outcome when RIF was added to TET compared to that when TET alone was used.