Fitness Cost of Chromosomal Drug Resistance-Conferring Mutations

Abstract

To study the cost of chromosomal drug resistance mutations to bacteria, we investigated the fitness cost of mutations that confer resistance to different classes of antibiotics affecting bacterial protein synthesis (aminocyclitols, 2-deoxystreptamines, macrolides). We used a model system based on an in vitro competition assay with defined Mycobacterium smegmatis laboratory mutants; selected mutations were introduced by genetic techniques to address the possibility that compensatory mutations ameliorate the resistance cost. We found that the chromosomal drug resistance mutations studied often had only a small fitness cost; compensatory mutations were not involved in low-cost or no-cost resistance mutations. When drug resistance mutations found in clinical isolates were considered, selection of those mutations that have little or no fitness cost in the in vitro competition assay seems to occur. These results argue against expectations that link decreased levels of antibiotic consumption with the decline in the level of resistance.The increasing rates of recovery of antimicrobial-resistant microorganisms in hospital and community settings are of growing concern (1, 42). Resistance may emerge from a mutation in an intrinsic chromosomal gene or by acquisition of exogenous genetic material bearing resistance determinants. Resistance to antibiotics frequently reduces the fitness of bacteria in the absence of antibiotics; this is referred to as the "cost" of resistance (38). In mathematical models, the fitness cost of resistance is the primary parameter that determines both the frequency of resistance at any given level of antibiotic use and the rate at which that frequency will change with changes in antibiotic use patterns (3,20,21).Restricted use of antibiotics is advocated not only to contain the dissemination of resistance but also to favor the nonexpansion and, finally, the disappearance of the resistant bacteria already present in human and environmental reservoirs (3, 38). As a consequence of decreased use of antibiotics, rates of drug resistance usually fall but do not vanish, and stable rates of resistance in the apparent absence of direct selection pressure has been observed (9, 12, 32). It is not clear whether this persistence of resistant bacteria is due to (i) low-level antibiotic contamination that maintains the selective pressure, (ii) selection by means other than antibiotics, or (iii) the stability of resistance genes.Analogous to the resistance mediated by exogenous genetic elements (13,14,19), chromosomal drug resistance-conferring mutations are commonly assumed to carry a fitness cost (38). This is supported by the observation that some drug resistance mutations selected in vitro involve a significant decrease in bacterial fitness (4,20,36); this fitness burden can subsequently be ameliorated by compensatory mutations (4, 5, 36). However, for streptomycin resistance-conferring rpsL mutations, a high level of selection for no-cost drug resistance mutations was suggested to exist in vivo (6). In order...

Artificial gene amplification reveals an abundance of promiscuous resistance determinants in Escherichia coli

“…Furthermore, most strains showed no fitness cost associated with expressing an ASKA ORF in the absence of toxin (and indeed, many out-competed the negative control) ( Table 3). Strong selection pressure for cost-free resistance mutations has been observed in clinical isolates of Mycobacterium tuberculosis (36). Together, our MIC and fitness data suggest that up-regulating the expression of preexisting, latent resistance determinants may play an important role in the emergence of drug-resistant pathogens.…”

The Fitness Cost of Streptomycin Resistance Depends on rpsL Mutation, Carbon Source and RpoS (σS)

“…This particular mutation (Str R ¼ K42N in S12) affects the 30S ribosomal subunit and confers a substantial fitness cost in S. enterica (Bjö rkman et al 1998;Maisnier-Patin et al 2002), in Escherichia coli (Kurland 1992;Schrag and Perrot 1996), and in Mycobacterium tuberculosis (Sander et al 2002). These fitness costs were apparent in the absence of antibiotic during growth in rich medium, in minimal-glucose or -glycerol media, and in animal models (Bjö rkman et al 1998;Paulander et al 2007).…”

Lytic phages obscure the cost of antibiotic resistance in Escherichia coli

“…Phages and antibiotic resistance SJ Tazzyman and AR Hall genotypes typically have mutation rates orders of magnitude higher than nonmutators (LeClerc et al, 1996;Chopra et al, 2003;Maciá et al, 2005), but costs of antibiotic resistance measured in vitro or in vivo are usually o50% (Björkman et al, 2000;Sander et al, 2002;Gagneux et al, 2006). We therefore suggest that hypermutable drug-resistant genetic backgrounds will typically lie in the white region of Figure 2.…”