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...

Staphylococcal exoproteins can be divided into two groups. One group comprises proteins bearing only a signal peptide, the other group requires an additional propeptide for secretion. The secretion signals of the propeptide-requiring lipase from Staphylococcus hyicus (Lip) have been frequently used to produce recombinant secretory proteins in the food-grade species Staphylococcus carnosus. However, it has been unclear whether recombinant proteins can be secreted using signal peptides of staphylococcal proteins without propeptide. The human growth hormone protein (hGH) was fused to various staphylococcal secretion signals of proteins without propeptide (Seb, SceA, and SceB) and of proteins requiring a propeptide (lipase, lysostaphin, and glycerol ester hydrolase). Secretory hGH was efficiently produced by S. carnosus after fusion with any propeptide-containing secretion signal, whereas precursor proteins were retained in the cells when only a signal peptide was used. Addition of the first six amino acid residues of mature SceA to the signal peptide did also not lead to secretion of hGH. It was concluded that the properties of the mature protein domains determine whether a propeptide is required for secretion or not. The Lip propeptide could be efficiently removed from hGH after introduction of an enterokinase cleavage site between the two protein domains.