Reductions in drug susceptibility of more than 10-fold were rare among this cohort of recently HIV-infected subjects and were distributed among each of the 3 major classes of ARV drugs tested. Reductions in susceptibility of more than 2.5- to 10-fold to certain ARV drugs of unknown clinical significance were highly prevalent among newly infected patients. Resistance testing may be warranted to monitor the frequency of drug resistance over time and to assess the potential for geographic variability.

The extended-spectrum P-lactamases are believed to arise by mutations which alter the configuration around the active site of TEM-and SHV-type enzymes so as to increase their efficiency with otherwise nonhydrolyzable cephalosporins and monobactams. This hypothesis predicts that the genes for these new enzymes should be found on the same wide variety of plasmids that encode TEM-1, TEM-2, and SHV-1 P-lactamases and that at least some of them should be mediated by transposons. Fifteen plasmids, each encoding an extended-spectrum 13-lactamase, were examined. Unlike the average TEM plasmid, all were large, ranging in size from 80 to 300 kb. All determined resistance to multiple antimicrobial agents, ranging from 5 to 11, and some conferred resistance to heavy metals and UY radiation as well. The plasmids belonged to a limited number of incompatibility (Inc) groups, including IncC, IncFI, IncHI2, and IncM. Because most of the mutations giving rise to extended-spectrum activity are G * C-*A T transitions and some of the mutant genes have as many as four base substitutions, a plasmid-determined mutator gene was searched for, but no such property was found.Several techniques were used to detect transposition of the extended-spectrum 13-lactamase genes, but a mobile genetic element could not be demonstrated even though eight of the plasmids hybridized with a DNA probe derived from the tnpR gene of Tn3. The genesis of extended-spectrum (-lactamases may not be as simple as has been supposed.Extended-spectrum 1-lactamases are enzymes that confer resistance to cefotaxime, ceftazidime, and other broadspectrum cephalosporins and to monobactams such as aztreonam (43). They appear to arise from genes for common plasmid-mediated SHV-1, TEM-1, and TEM-2 3-lactamases by mutations that alter the amino acid configuration around the active site of these enzymes so as to expand their spectrum of activity (19,30,42,49).TEM-1 and TEM-2 3-lactamases are determined by plasmids belonging to many different incompatibility (Inc) groups in enteric organisms (35) and are often carried on transposons (22). SHV-1 is also encoded by several plasmid types (36), but whether it is (40) or is not (33) transposon determined is in dispute.To explore the mutation hypothesis, we have characterized plasmids responsible for extended-spectrum P-lactamase production. The aims of this study were to establish whether the plasmids belonged to many or a few Inc groups, to determine whether they carried transposons encoding the extended-spectrum activity, and to investigate whether they possessed genes facilitating the mutational alterations observed.MATERIALS AND METHODS Bacterial strains, plasmids, and bacteriophages. line, 50; thiamine, 0.34; and L-threonine, 43. Antimicrobial agents were used at the indicated concentrations (in micrograms per milliliter): ampicillin, 50; gentamicin, 20; kanamycin, 25; nalidixic acid, 100; rifampin, 100; streptomycin, 25; tetracycline, 5 or 25; and tobramycin, 10.Antibiotic resistance was evaluated by disk or agar diffusion usin...

The relative replicative fitness of human immunodeficiency virus type 1 (HIV-1) mutants selected by different protease inhibitors (PIs) in vivo was determined. Each mutant was compared to wild type (WT), NL4-3, in the absence of drugs by several methods, including clonal genotyping of cultures infected with two competing viral variants, kinetics of viral antigen production, and viral infectivity/virion particle ratios. A nelfinavir-selected protease D30N substitution substantially decreased replicative capacity relative to WT, while a saquinavir-selected L90M substitution moderately decreased fitness. The D30N mutant virus was also outcompeted by the L90M mutant in the absence of drugs. A major natural polymorphism of the HIV-1 protease, L63P, compensated well for the impairment of fitness caused by L90M but only slightly improved the fitness of D30N. Multiply substituted indinavir-selected mutants M46I/L63P/V82T/I84V and L10R/M46I/L63P/V82T/I84V were just as fit as WT. These results indicate that the mutations which are usually initially selected by nelfinavir and saquinavir, D30N and L90M, respectively, impair fitness. However, additional mutations may improve the replicative capacity of these and other drug-resistant mutants. Hypotheses based on the greater fitness impairment of the nelfinavir-selected D30N mutant are suggested to explain observations that prolonged responses to delayed salvage regimens, including alternate PIs, may be relatively common after nelfinavir failure.

HIV type 1 (HIV-1) drug resistance mutations were selected during antiretroviral therapy successfully suppressing plasma HIV-1 RNA to <50 copies͞ml. New resistant mutant subpopulations were identified by clonal sequencing analyses of viruses cultured from blood cells. Drug susceptibility tests showed that biological clones of virus with the mutations acquired during successful therapy had increased resistance. Each of the five subjects with new resistant mutants had evidence of some residual virus replication during highly active antiretroviral therapy (HAART), based on transient episodes of plasma HIV-1 RNA > 50 copies͞ml and virus env gene sequence changes. Each had received a suboptimal regimen before starting HAART. Antiretroviral-resistant HIV-1 can be selected from residual virus replication during HAART in the absence of sustained rebound of plasma HIV-1 RNA.