De novo resistance mutations at sub-MIC levels

Abstract: Antibiotic resistance is a kind of achievable drug resistance meaning that microorganisms are able to sustain and survive exposure to antibiotics. A major contribution to development of antibiotic resistance is the abuse of antibiotics in human and in veterinary medicine. Antibiotics have been used for many decades but only during recent years studies have been published that have increased the understanding in the area of antibiotic input and consequences thereof on bacterial resistance. Thus, it was just recently published that bacterial strains can undergo rapid selection of resistant mutants at antibiotic concentrations far below MIC (Minimal Inhibitory Concentration). However, presently there are no studies at low antibiotic concentrations that have been able to show whether growth and development of de novo resistant mutants competitively can out grow the normal bacterial population. Therefore, the aim of this project was to investigate whether de novo resistant mutants of Salmonella typhimurium and Escherichia coli can appear and take over the normal bacterial population, and if this occurs also to isolate the resistant mutations for characterization. These are closely related bacteria but they differ in their target sites for antibiotic resistance i.e. S. typhimurium carries a cryptic aminoglycoside resistance gene, aadA, that can be up-regulated and thereby causing streptomycin resistance, while the most common low-cost mutations conferring resistance to ciprofloxacin in E. coli are in the gyrase gene gyrA. The results showed that concentrations of antibiotics far below MIC can select for de novo mutants with high antibiotic resistance both in S. typhimurium and E. coli. The novel findings regarding these resistant mutants are that they are likely to have lower fitness cost than previously studied resistance mutations (such as rpsL mutants for streptomycin) which means that these mutations may be new and different from those previously described.