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PubMed2006

Prevalence and evolution of methicillin-resistant Staphylococcus aureus in Spanish hospitals between 1996 and 2002.

Pulsed-field gel electrophoretic analysis of 2,144 methicillin-resistant Staphylococcus aureus (MRSA) strains isolated from patients in Spanish hospitals over a 7-year period revealed 17 predominant profiles. Typing showed the replacement of Iberian clone E1 (ST247-MRSA-I) by two prevalent clones, E7 and E8, that are closely related to each other and have the same genetic background as ST125-MRSA-IV.

Biological Evolution↗

PubMed1986

Evolution and transfer of aminoglycoside resistance genes under natural conditions.

3'-Aminoglycoside phosphotransferases [APH(3')] were chosen as a model to study the evolution and the transfer of aminoglycoside resistance genes under natural conditions. Comparison of the amino acid sequences of APH(3') enzymes from transposons Tn903 (type I) and Tn5 (type II) detected in Gram-negative bacteria, from the Gram-positive Staphylococcus and Streptococcus (type III), from the butirosin-producing Bacillus circulans (type IV) and from a neomycin-producing Streptomyces fradiae (type V) indicate that they have diverged from a common ancestor. These structural data support the hypothesis that the antibiotic-producing strains were the source of certain resistance determinants. We have shown that kanamycin resistance in Campylobacter coli BM2509 was due to the synthesis of an APH(3')-III, an enzyme not detected previously in a Gram-negative bacterium. The genes encoding APH(3')-III in Streptococcus and Campylobacter are identical. These findings constitute evidence for a recent in-vivo transfer of DNA between Gram-positive and Gram-negative bacteria.

Aminoglycosides↗

PubMed2002

Combination prophylaxis with Hepatitis B immunoglobulin and lamivudine after liver transplantation minimizes HBV recurrence rates unless evolution of pretransplant lamivudine resistance.

BACKGROUND: Survival rates of hepatitis B patients after liver transplantation improved significantly by introduction of passive immunoprophylaxis. Due to viral escape mutations recurrence still occurs, but recently a combination prophylaxis with hepatitis B immunoglobuline plus lamivudine is evaluated in transplant centers in terms of a further reduction of recurrence rates. PATIENTS AND METHODS: Between 1996 and 2000 a postoperative combination prophylaxis with HBIg and lamivudine was initiated in 44 HBsAg positive liver transplant recipients. In total 14 patients were HBV-DNA negative and 30 were HBV-DNA positive at the time of evaluation. In 22 HBV-DNA positive patients a pre-operative lamivudine treatment (150 mg/die) was started. Five of them developed pre-transplant lamivudine resistance with high viral replication (mean HBV-DNA prior to transplantation 728 +/- 219 pg/ml). In all patients passive immunoprophylaxis was started in the anhepatic phase with application of 10.000 units hepatitis B immunoglobuline. It was continued after seroconversion to HBsAg negativity with an aimed titer of more than 100 U/l and only stopped in case of HBV recurrence. Lamivudine was also continued indefinitely after liver transplantation. RESULTS: Overall recurrence rate in the 44 patients, including retransplantations and patients with pretransplant lamivudine resistance, was 11.5 % under combination prophylaxis. Recurrence was seen only in one of 39 patients (2.6 %) without preoperative lamivudine resistance, in contrast 4 out of 5 patients (80 %) with pre-existing lamivudine resistance suffered from early hepatitis B recurrence. The single patient without preoperative lamivudine resistance, who developed recurrence was pre-transplant HBV-DNA negative without lamivudine treatment, but a postoperative seroconversion to negative HBsAg could not be achieved. The overall 3 year patient survival rate was 91 % in the study population. One patient, who was retransplanted with preoperative lamivudine resistance, died 4.5 months after retransplantation due to hepatitis B recurrence and sepsis, three other patients died for reasons not related to hepatitis B recurrence. Combination prophylaxis was well tolerated in all patients and no severe side effects were observed. CONCLUSION: Combination prophylaxis with hepatitis B immunoglobulin and lamivudine is safe and highly effective in prevention of HBV recurrence after liver transplantation, even in case of positive viral replication. In accordance with the results of other centers it should therefore be the standard regimen. However it fails in the majority of patients with preoperative evolution of YMDD mutations, in which the optimal management has to be determined yet. To minimize preoperative resistance formation universal preoperative antiviral treatment of HBV-DNA positive patients should be replaced by individualized indication for preoperative treatment.

Adult↗

PubMed2004

Chemical control of ticks on cattle and the resistance of these parasites to acaricides.

Toward the end of the nineteenth century a complex of problems related to ticks and tick-borne diseases of cattle created a demand for methods to control ticks and reduce losses of cattle. The discovery and use of arsenical solutions in dipping vats for treating cattle to protect them against ticks revolutionized tick and tick-borne disease control programmes. Arsenic dips for cattle were used for about 40 years before the evolution of resistance of ticks to the chemical, and the development and marketing of synthetic organic acaricides after World War II provided superior alternative products. Most of the major groups of organic pesticides are represented on the list of chemicals used to control ticks on cattle. Unfortunately, the successive evolution of resistance of ticks to acaricides in each chemical group with the concomitant reduction in the usefulness of a group of acaricides is a major reason for the diversity of acaricides. Whether a producer chooses a traditional method for treating cattle with an acaricide or uses a new method, he must recognize the benefits, limitations and potential problems with each application method and product. Simulation models and research were the basis of recommendations for tick control strategies advocating approaches that reduced reliance on acaricides. These recommendations for controlling ticks on cattle are in harmony with recommendations for reducing the rate of selection for acaricide resistance. There is a need to transfer knowledge about tick control and resistance mitigation strategies to cattle producers.

Animals↗

PubMed2003

Multidrug resistant Acinetobacter baumannii isolates from a teaching hospital.

Abstract Acinetobacter baumannii is the most common nosocomial pathogen among all Acinetobacter spp. A. baumannii tend to be resistant to multiple antibiotics, and represent a severe threat in the treatment of hospitalized patients. The purpose of this study was to evaluate the prevalence, biotyping, and antibiotic resistance status of A. baumannii isolates recovered from submitted hospital clinical specimens. The in-vitro activity of 29 currently used antimicrobial agents was studied in 180 isolates of A. baumannii. The prevalence was higher in wound exudates (32%) and urine samples (22%) than in other specimens. Forty-six percent of the isolates were of biotype 9. The evolution of resistance from November 1996 through October 1998 was studied. More than 75% of the isolates were multidrug resistant (MDR) and more than 70% were #-lactamase producers. Amikacin, ampicillin + sulbactam, and imipenem are still effective antimicrobial agents, but a steady rise in the values of the minimum inhibitory concentration at which 50% of the isolates were inhibited (MIC50 )and MIC90 was observed. Resistance to aztreonam (60%), enrofloxacin (60%), imipenem (29%), moxalactam (73%), and sparfloxacin (25%) was noted; enrofloxacin and sparfloxacin are not generally used in this hospital. From the findings of the present study, no single drug appears to be suitable for empirical therapy for this nosocomial pathogen. Abstract Acinetobacter baumannii is the most common nosocomial pathogen among all Acinetobacter spp. A. baumannii tend to be resistant to multiple antibiotics, and represent a severe threat in the treatment of hospitalized patients. The purpose of this study was to evaluate the prevalence, biotyping, and antibiotic resistance status of A. baumannii isolates recovered from submitted hospital clinical specimens. The in-vitro activity of 29 currently used antimicrobial agents was studied in 180 isolates of A. baumannii. The prevalence was higher in wound exudates (32%) and urine samples (22%) than in other specimens. Forty-six percent of the isolates were of biotype 9. The evolution of resistance from November 1996 through October 1998 was studied. More than 75% of the isolates were multidrug resistant (MDR) and more than 70% were #-lactamase producers. Amikacin, ampicillin + sulbactam, and imipenem are still effective antimicrobial agents, but a steady rise in the values of the minimum inhibitory concentration at which 50% of the isolates were inhibited (MIC50) and MIC90 was observed. Resistance to aztreonam (60%), enrofloxacin (60%), imipenem (29%), moxalactam (73%), and sparfloxacin (25%) was noted; enrofloxacin and sparfloxacin are not generally used in this hospital. From the findings of the present study, no single drug appears to be suitable for empirical therapy for this nosocomial pathogen.

Acinetobacter baumannii↗

PubMed1991

Molecular evolution of tetracycline-resistance plasmids carrying TetM found in Neisseria gonorrhoeae from different countries.

High level tetracycline resistant strains of Neisseria gonorrhoeae (TRNG) have been shown to carry a 40.6 kb (25.2 MDa) conjugative plasmid with a Class M tetracycline resistance determinant. Restriction endonuclease analysis mapping showed that there were at least two different TRNG plasmid types which were found in geographically distinct locations. The physical maps of these two plasmids were compared to a gonococcal conjugative plasmid which did not encode tetracycline resistance. The plasmid type which is endemic in the Netherlands was found to be closely related to the gonococcal conjugative plasmid, which supports the established hypothesis that the 40.6 kb plasmid has evolved by transposition of the TetM determinant into the conjugative plasmid. The plasmid found in the United States has either evolved by substantial divergent evolution or it results from a different transposition event. In the UK there have been isolations of TRNGs carrying either of the two plasmid types reflecting a flow of people both across the Atlantic and in Europe. It is possible that further TetM-containing plasmids will be found in N. gonorrhoeae paralleling the family of TEM beta-lactamase encoding plasmids already described.

Bacterial Proteins↗

PubMed1992

Selection of dieldrin-resistant strains of Lucilia cuprina (Diptera: Calliphoridae) after ethyl methanesulfonate mutagenesis of a susceptible strain.

After ethyl methanesulfonate (EMS) mutagenesis of a susceptible strain (SWT), selective screening of Lucilia cuprina (Wiedemann) resulted in four strains that were resistant to the insecticide dieldrin. Concentrations used for selection were greater than LC99 of susceptible phenotypes. No resistant variants were screened from the standard laboratory strain (SWT) not treated with EMS. The resistance phenotypes of the four resistant strains were similar to each other and to that of a field-selected resistant strain. The genetic basis of resistance is monogenic in all strains and the data are consistent with the same locus, Rdl, determining resistance status in each strain. The Rdl locus maps to chromosome V, approximately 3.5 map units distal to the Sut locus. Dieldrin resistance may be caused by less effective blocking of insect neuronal GABA receptors by the chemical in resistant strains. The data indicate that the evolution of resistance to an insecticide in the field may be constrained by a limited number of genetical and biochemical options if a monogenic response is selected for and that the spontaneous mutation rate to the Rdl allele is less than 1 in 10(6) in the laboratory.

Animals↗

PubMed1997

Inheritance of Resistance to the Bacillus thuringiensis Toxin Cry1C in the Diamondback Moth.

Laboratory selection increased resistance to the Bacillus thuringiensis toxin Cry1C in a strain of diamondback moth (Plutella xylostella). The selected strain was derived from a field population that had evolved high levels of resistance to Bacillus thuringiensis subsp. kurstaki and moderate resistance to Cry1C. Relative to the responses of a susceptible strain of diamondback moth, the resistance to Cry1C of the selected strain increased to 62-fold after six generations of selection. The realized heritability of resistance was 0.10. Analysis of F(inf1) hybrid progeny from reciprocal crosses between the selected strain and a susceptible strain showed that resistance to Cry1C was autosomally inherited. The dominance of resistance to Cry1C depended on the concentration; inheritance was increasingly dominant as the concentration decreased. Responses of progeny from single-pair families showed that resistance to Cry1C and resistance to Cry1Ab were inherited independently, which enhances opportunities for managing resistance. However, compared with projections based on previously reported recessive inheritance of resistance to Cry1A toxins, the potentially dominant inheritance of resistance to Cry1C observed here could accelerate evolution of resistance.

Journal Article↗

PubMed1999

Diversity and molecular evolution of the RPS2 resistance gene in Arabidopsis thaliana.

The RPS2 gene in Arabidopsis thaliana governs resistance to strains of the bacterial pathogen, Pseudomonas syringae pv. tomato, that express the avrRpt2 gene. The two loci are involved in a gene-for-gene interaction. Seventeen accessions of A. thaliana were sequenced to explore the diversity present in the coding region of the RPS2 locus. An unusually high level of nucleotide polymorphisms was found (1.26%), with nearly half of the observed polymorphisms resulting in amino acid changes in the RPS2 protein. Seven haplotypes (alleles) were identified and their evolutionary relationships deduced. Several of the alleles conferring resistance were found to be closely related, whereas susceptibility to disease was conferred by widely divergent alleles. The possibility of selection at the RPS2 locus is discussed.

Arabidopsis↗

PubMed1990

Evolution of enzymatic mechanisms of resistance among beta-lactam antibiotics.

Resistance to third-generation cephalosporins occurs as a result of either the production of high concentrations of chromosomal cephalosporinase or, increasingly, the presence of broad-spectrum plasmid-mediated beta-lactamases. Both cases represent the response of bacteria in the hospital setting to the selection pressure brought to bear by the use of these antibiotics. Continued evolution of the plasmid-mediated enzymes is occurring as new antibiotics are introduced, probably reflecting the process that began when the first beta-lactamase apparently evolved from the penicillin-binding proteins. beta-Lactamase inhibitors offer one approach to dealing with the evolution of resistance to previously beta-lactamase-stable antibiotics.

Amino Acid Sequence↗

PubMed2003

Role of mobile DNA in the evolution of vancomycin-resistant Enterococcus faecalis.

The complete genome sequence of Enterococcus faecalis V583, a vancomycin-resistant clinical isolate, revealed that more than a quarter of the genome consists of probable mobile or foreign DNA. One of the predicted mobile elements is a previously unknown vanB vancomycin-resistance conjugative transposon. Three plasmids were identified, including two pheromone-sensing conjugative plasmids, one encoding a previously undescribed pheromone inhibitor. The apparent propensity for the incorporation of mobile elements probably contributed to the rapid acquisition and dissemination of drug resistance in the enterococci.

Adhesins, Bacterial↗

PubMed2000

Evolution of parasite virulence against qualitative or quantitative host resistance.

We analysed the effects of two different modes of host resistance on the evolution of parasite virulence. Hosts can either adopt an all-or-nothing qualitative response (i.e. resistant hosts cannot be infected) or a quantitative form of resistance (i.e. which reduces the within-host growth rate of the parasite). We show that the mode of host resistance greatly affects the evolutionary outcome. Specifically, a qualitative form of resistance reduces parasite virulence, while a quantitative form of resistance generally selects for higher virulence.

Animals↗

PubMedDate not supplied

Importance of integrons in the diffusion of resistance.

Horizontal transfer of resistance genes is a successful mechanism for the transmission and dissemination of multiple drug resistance among bacterial pathogens. The impact of horizontally transmitted genetic determinants in the evolution of resistance is particularly evident when resistance genes are physically associated in clusters and transferred en bloc to the recipient cell. Recent advances in the molecular characterisation of antibiotic resistance mechanisms have highlighted the existence of genetic structures. called integrons, involved in the acquisition of resistance genes. These DNA elements have frequently been reported in multi-drug resistant strains isolated from animals and humans, and are located either on the bacterial chromosome or on broad-host-range plasmids. The role of integrons in the development of multiple resistance relies on their unique capacity to cluster and express drug resistance genes. Moreover, the spread of resistance genes among different replicons and their exchange between plasmid and bacterial chromosome are facilitated by the integration of integrons into transposable elements. The association of a highly efficient gene capture and expression system, together with the capacity for vertical and horizontal transmission of resistance genes represents a powerful weapon used by bacteria to combat the assault of antibiotics.

Animal Diseases↗

PubMed1999

Directed evolution of thermostable kanamycin-resistance gene: a convenient selection marker for Thermus thermophilus.

The whole-genome sequencing of an extreme thermophile, Thermus thermophilus, is now in progress. Like other genome projects, major concern is shifting from the sequence itself to post-sequencing research such as functional or structural genomics. Under such circumstances, the demand for convenient genetic-engineering tools is increasing. In this study we have increased the thermostability of a kanamycin-resistance gene product using strategies based on directed evolution in T. thermophilus to the upper limit of its growth temperature. The most thermostable mutant has 19 amino-acid substitutions, whereby the thermostability is increased by 20 degrees C, but the enzymatic activity is not significantly changed. Most of the mutated residues are located on the surface of the protein molecule, and, interestingly, five of the 19 substitutions are those to proline residues. The evolved kanamycin-resistance gene products could be used as selection markers at the optimum growth temperature of T. thermophilus. The development of such a convenient genetic-engineering tool would facilitate post-sequencing research on T. thermophilus.

Amino Acid Substitution↗

PubMed2002

Organization, expression and evolution of a disease resistance gene cluster in soybean.

PCR amplification was previously used to identify a cluster of resistance gene analogues (RGAs) on soybean linkage group J. Resistance to powdery mildew (Rmd-c), Phytophthora stem and root rot (Rps2), and an ineffective nodulation gene (Rj2) map within this cluster. BAC fingerprinting and RGA-specific primers were used to develop a contig of BAC clones spanning this region in cultivar "Williams 82" [rps2, Rmd (adult onset), rj2]. Two cDNAs with homology to the TIR/NBD/LRR family of R-genes have also been mapped to opposite ends of a BAC in the contig Gm_Isb001_091F11 (BAC 91F11). Sequence analyses of BAC 91F11 identified 16 different resistance-like gene (RLG) sequences with homology to the TIR/NBD/LRR family of disease resistance genes. Four of these RLGs represent two potentially novel classes of disease resistance genes: TIR/NBD domains fused inframe to a putative defense-related protein (NtPRp27-like) and TIR domains fused inframe to soybean calmodulin Ca(2+)-binding domains. RT-PCR analyses using gene-specific primers allowed us to monitor the expression of individual genes in different tissues and developmental stages. Three genes appeared to be constitutively expressed, while three were differentially expressed. Analyses of the R-genes within this BAC suggest that R-gene evolution in soybean is a complex and dynamic process.

Amino Acid Sequence↗

PubMed2003

Directed evolution towards protease-resistant hirudin variants.

Hirudin, a thrombin-specific inhibitor, is efficiently digested and inactivated by proteases with pepsin- and chymotrypsin-like specificity. Using a combination of phage display selection and high-throughput screening methods, several variants of recombinant hirudin were generated. Only very few variants comprising amino acid substitutions in the amino-terminal domain (residues 1-5) and in the carboxyl-terminal tail (residues 49, 50, and/or 56, 57, 62-64) were identified that showed thrombin inhibition activities similar to those of the wild-type polypeptide. Analysis of protease susceptibility, however, revealed that mutations, which conferred protease resistance, simultaneously diminish thrombin inhibition activity. This is particularly apparent for substitutions in the region of residues 56-64, which forms a large number of electrostatic and hydrophobic interactions with thrombin in the crystal structure of the complex. Unlike wild-type hirudin, the variant comprising Pro(50)- ...-His(56)-Asp(57)- ...-Pro(62)-Pro(63)-His(64) is completely resistant to pepsin and chymotrypsin cleavage; however, this is at the expense of thrombin inhibition activity where there is a 100-fold increase in the IC50 value. The frequent replacement of wild-type amino acids by proline at major protease cleavage sites indicates that at least pepsin- and chymotrypsin-like enzymes may exhibit a (conformational) specificity concerning the P1 and P2 positions. On the basis of these results, proline substitutions appear to be a general strategy to design polypeptides that are not susceptible to digestion by a broader range of different proteases.

Amino Acid Sequence↗

PubMed2003

Dynamics of molecular markers linked to the resistance loci in a mosquito-Plasmodium system.

Models on the evolution of resistance to parasitism generally assume fitness tradeoffs between the costs of being parasitized and the costs associated with resistance. This study tested this assumption using the yellow fever mosquito Aedes aegypti and malaria parasite Plasmodium gallinaceum system. Experimental mosquito populations were created by mixing susceptible and resistant strains in equal proportions, and then the dynamics of markers linked to loci for Plasmodium resistance and other unlinked neutral markers were determined over 12 generations. We found that when the mixed population was maintained under parasite-free conditions, the frequencies of alleles specific to the susceptible strain at markers closely linked to the loci for resistance (QTL markers) as well as other unlinked markers increased significantly in the first generation and then fluctuated around equilibrium frequencies for all six markers. However, when the mixed population was exposed to an infected blood meal every generation, allele frequencies at the QTL markers for resistance were not significantly changed. Small population size caused significant random fluctuations of allele frequencies at all marker loci. Consistent allele frequency changes in the QTL markers and other unlinked markers suggest that the reduced fitness in the resistant population has a genome-wide effect on the genetic makeup of the mixed population. Continuous exposure to parasites promoted the maintenance of alleles from the resistant Moyo-R strain in the mixed population. The results are discussed in relation to the proposed malaria control strategy through genetic disruption of vector competence.

Aedes↗

PubMed1978

Instability of plasmid DNA sequences: macro and micro evolution of the antibiotic resistance plasmid R6-5.

Detailed examination of the structure of cloned DNA fragments of the R6-5 antibiotic resistance plasmid has revealed a substantial degree of polynucleotide sequence heterogeneity and indicates that sequence rearrangements in plasmids and possible other replicons occur more frequently than has hitherto been appreciated. The sequences changes in cloned R6-5 fragments were shown in some instances to have occurred prior to cloning, i.e. existing in the original population of R6-5 molecules that was obtained from a single bacterial clone and by several different criteria judged to be homogeneous, and in others to have occurred either during the cloning procedure or during subsequent propagation of hybrid molecules. The molecular changes that are described involved insertion/deletion of the previously characterized IS2 insertion element, formation of a new inverted repeat structure probably by duplication of a preexisting R6-5 DNA sequence, sequence inversion, and loss and gain of restriction endonuclease cleavage sites.

Anti-Bacterial Agents↗