Can susceptibility to an antimicrobial be restored by halting its use? The case of streptomycin versus Enterobacteriaceae.

To test the widespread view that resistance disappears in the absence of antimicrobial use, we tested streptomycin against 477 Enterobacteriaceae from the Royal London Hospital. Twenty per cent proved resistant although streptomycin is little used at the hospital and streptomycin resistance in gram-negative bacteria is caused by mechanisms that do not compromise the drugs that are used. Up to 70% of the observed resistance was associated with cross-resistance to spectinomycin and the presence of ant(3")-Ia, an integron-associated gene carried in Tn21-type transposons. This genetic organization may have conserved streptomycin resistance in the absence of direct selection pressure.