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...

Neurofibromatosis 1 (NF1) is one of the most common autosomal dominant diseases. Although there is a considerable variability in clinical expression, NF1 is almost fully penetrant in adult patients and may be associated with a variety of skeletal anomalies. Spinal deformities are the most common skeletal manifestation, with an incidence estimated from 10-25% in various studies. Some NF1 patients have a dystrophic form of scoliosis, which is characterized by early age at onset and rapid progression. Complications have been reported during spinal instrumentation of dystrophic curves due to soft, non-resistant vertebral bony tissue, suggesting that an alteration of bone quality may occur in NF1 patients. Recent studies have suggested that decreased bone mineral density (BMD) may occur among patients with NF1. We performed a cross-sectional study on 104 adults with NF1, using quantitative ultrasonometry (QUS) to investigate whether decreased BMD is a general phenomenon in NF1 patients. The data reveal that BMD, as measured by age- and gender- adjusted Z-scores, is significantly lower in NF1 patients than in the normal reference population. The decrease in BMD appears to be even more marked among NF1 patients with scoliosis that requires surgical treatment. The findings indicate that NF1 produces a generalized alteration of bone in addition to the focal osseous dysplasias of the vertebrae, tibia, and sphenoid wing that characterize this condition. The pathological mechanism underlying these bony changes remains to be elucidated.

The GluR2 flop subunit of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors greatly determines calcium permeability and kinetic properties of heteromeric AMPA subunit assemblies. Post-transcriptional editing of this subunit at the Q/R/N site controls calcium permeability whereas editing at the R/G site is involved in the regulation of biophysical properties. We used patch-clamp techniques with ultrafast solution exchange to examine the kinetics of recombinant human homomeric GluR2 flop channels transiently expressed in HEK293 cells [edited at the R/G site and Q/R/N site (GR), and unedited (RN) and edited (GN) at the R/G site both with asparagine (N) at the Q/R/N site]. The time constant of desensitization after application of 10 mm glutamate was 1.38 +/- 0.05 ms (n = 10), 5.53 +/- 0.57 ms (n = 7) and 1.33 +/- 0.06 ms (n = 12) for the GluR2 flop GR, RN and GN channels, respectively. The time constant of resensitization was 75 ms for the GluR2 flop RN and 30 ms for the GN channels. The dose-dependence of the peak current amplitude, kinetics of activation and deactivation, and peak open probability did not differ between RN and GN channels. The study shows that desensitization and resensitization kinetics of homomeric GluR2 flop channels are controlled by a single amino acid exchange (glycine by arginine) at the R/G site. Quantitative analysis by computer simulation using a circular kinetic scheme allows the prediction of the main experimental results.