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Despite the large number of in vitro mutations that increase resistance to extended-spectrum cephalosporins in TEM-type beta-lactamases, only a small number occur in naturally occurring enzymes. In nature, and particularly in the hospital, bacteria that contain beta-lactamases encounter simultaneous or consecutive selective pressure with different(More)
The choice of appropriate antimicrobial agents should take into consideration not only the interests of the individual patient, but also the ecological impact of different drugs and their delivery schedules. Selection of antibiotic-resistant organisms is a key aspect to remember. Bacterial populations harboring determinants of antibiotic resistance will be(More)
In this paper, the first robust experimental evidence of in vitro and in vivo concentration-dependent selection of low-level antibiotic-resistant genetic variants is described. The work is based on the study of an asymmetric competition assay with pairs of isogenic Escherichia coli strains, differing only (apart from a neutral chromosomal marker) in a(More)
The frequency of mutation to rifampin resistance of 200 clinical Streptococcus pneumoniae isolates was examined. Two peaks were observed in the distribution, with mode frequencies of 2.5 x 10(-7) (20% of isolates) and 2.5 x 10(-8). The hexA and hexB gene entire sequences were analyzed in 13 isolates. Sequences from both hypermutable and "normomutable"(More)
The biochemistry and genetics of antibiotic resistance are far better known than the equally important events underlying the selection of resistant populations. The hidden selection of low-level resistant variants may be a key process in the emergence of high-level antibiotic resistance. Different low-level resistant bacterial subpopulations may be(More)
The evolution and spread of antibiotic resistance depends on the antibiotic pressure exerted in the microbial environment. Selective effects occur in selective compartments, where particular antibiotic concentrations result in a differential growth rate of resistant bacterial variants. This may happen even at very low antibiotic concentrations able to(More)
A TEM-1 beta-lactamase derivative containing the single amino acid substitution A237T slightly increased (from 24 to 32 microg/ml) the cephalothin MIC for Escherichia coli RYC1000 but did not influence the activities of cefotaxime, ceftazidime, and aztreonam (MICs of 0.03, 0.12, and 0.06 microg/ml, respectively). Despite its apparent neutrality, addition of(More)
Therapeutic regimens containing beta-lactam antibiotics are selecting penicillin-resistant Streptococcus pneumoniae populations all over the world. The selective pressure after 4 h of exposure to different concentrations of amoxicillin, cefixime, cefuroxime, and cefotaxime for low-level or high-level penicillin-resistant S. pneumoniae was evaluated in an in(More)
The development of bacterial resistance to antibiotics is one of the best documented examples of contemporary biological evolution. Variability in the mechanisms of resistance depends on the diversity of genotypes in the huge bacterial populations, and also on the diversity of selective pressures that are produced along the antibiotic concentration(More)
A new extended-spectrum beta-lactamase was detected in a lactose-positive Salmonella enterica subsp. enterica strain that caused a nosocomial outbreak involving eight patients in a pediatric cardiology unit. This strain showed high levels of resistance to ceftazidime and aztreonam and relatively low levels of resistance to cefotaxime and ceftriaxone.(More)