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PubMed1996

Characterization of the drug resistance plasmid R2418: restriction map and role of insertion and deletion in its evolution.

Escherichia coli 2418 strain is resistant to beta-lactam antibiotics (ampicillin, carbenicillin, and cephalothin), streptomycin, tetracycline, kanamycin, and chloramphenicol. This strain contains at least two conjugative plasmids (R2418 and R2418S) encoding resistance to beta lactam antibiotics and resistance to both beta-lactam antibiotics and streptomycin, respectively. Restriction endonuclease mapping of plasmid DNAs indicates that the plasmid R2418S has evolved from R2418 DNA by the insertion of 2.5-kb DNA between BamHI and PvuII sites, and deletion of 0.5-kb DNA within the EcoRI-EcoRV region. The 2.5-kb DNA insert is responsible for streptomycin resistance. This evolution is also associated with a reduction in the efficiency of conjugal transfer for the plasmid R2418S. The conjugal transfer of streptomycin resistance occurs only through the coresidence of the conjugative plasmid R2418 or R2418S in the donor cell. In accordance with the hypothesis that the Smr determinant is due to a putative transposon, plasmid-free transconjugants resistant to streptomycin only were isolated. Southern blot analysis of HindIII chromosomal digests extracted from these transconjugants shows that the Smr determinant is inserted into different sites in chromosomal DNA.

Anti-Bacterial Agents↗
PubMed2005

Antibiotic resistance in outpatient urinary isolates: final results from the North American Urinary Tract Infection Collaborative Alliance (NAUTICA).

The goal of the North American Urinary Tract Infection Collaborative Alliance (NAUTICA) study was to determine antibiotic susceptibility to commonly used agents for urinary tract infections against outpatient urinary isolates obtained in various geographic regions in the USA and Canada. Forty-one medical centres (30 from the USA and 11 from Canada) participated, with each centre submitting up to 50 consecutive outpatient midstream urine isolates. Isolates were identified to species level by the standard protocol of each laboratory. Susceptibility testing was determined using the National Committee for Clinical Laboratory Standards (NCCLS) microdilution method. Resistance breakpoints used were those published by the NCCLS, including: ampicillin (resistant > or = 32 microg/mL), sulphamethoxazole/trimethoprim (SMX/TMP) (resistant > or = 4 microg/mL), nitrofurantoin (resistant > or = 128 microg/mL), ciprofloxacin (resistant > or = 4 microg/mL) and levofloxacin (resistant > or = 8 microg/mL). Of the 1990 isolates collected, 75.1% (1494) were collected from the USA and 24.9% (496) were collected from Canada. The mean age of the patients was 48.3 years (range 1 month to 99 years), and 79.5% and 20.5% of isolates were obtained from women and men, respectively. The most common organisms were Escherichia coli (57.5%), Klebsiella pneumoniae (12.4%), Enterococcus spp. (6.6%), Proteus mirabilis (5.4%), Pseudomonas aeruginosa (2.9%), Citrobacter spp. (2.7%), Staphylococcus aureus (2.2%), Enterobacter cloacae (1.9%), coagulase-negative staphylococci (1.3%), Staphylococcus saprophyticus (1.2%), Klebsiella spp. (1.2%), Enterobacter aerogenes (1.1%) and Streptococcus agalactiae (1.0%). Among all 1990 isolates, 45.9% were resistant to ampicillin, 20.4% to SMX/TMP, 14.3% to nitrofurantoin, 9.7% to ciprofloxacin and 8.1% to levofloxacin. Fluoroquinolone resistance was highest in patients > or = 65 years of age. For the 1142 E. coli isolates, resistance rates were: ampicillin 37.7%, SMX/TMP 21.3%, ciprofloxacin 5.5%, levofloxacin 5.1% and nitrofurantoin 1.1%. For all 1990 isolates and for the 1142 E. coli only, resistance rates were significantly higher in US compared with Canadian medical centres. This study reports higher rates of antibiotic resistance in US versus Canadian outpatient urinary isolates and demonstrates the continuing evolution of resistance to antimicrobial agents.

Adolescent↗
PubMed2000

Chlamydia pneumoniae and cardiovascular disease: an evolutionary perspective on infectious causation and antibiotic treatment.

Evolutionary considerations implicate infectious causation of atherosclerosis and help to resolve different risk factors as parts of an overall process of disease causation. An evolutionary approach also provides insight for the timing of research efforts to provide better control of pathogen evolution. In particular, evolutionary considerations emphasize the need to understand the transmissibility of Chlamydia pneumoniae from systemic infections in order to control the evolution of antibiotic resistance.

Anti-Bacterial Agents↗
PubMed2005

Analysis of the dynamics of adaptation to transgenic corn and crop rotation by western corn rootworm (Coleoptera: Chrysomelidae) using a daily time-step model.

Western corn rootworm, Diabrotica virgifera virgifera LeConte, has overcome crop rotation in several areas of the north central United States. The effectiveness of crop rotation for management of corn rootworm has begun to fail in many areas of the midwestern United States, thus new management strategies need to be developed to control rotation-resistant populations. Transgenic corn, Zea mays L., effective against western corn rootworm, may be the most effective new technology for control of this pest in areas with or without populations adapted to crop rotation. We expanded a simulation model of the population dynamics and genetics of the western corn rootworm for a landscape of corn; soybean, Glycine max (L.); and other crops to study the simultaneous development of resistance to both crop rotation and transgenic corn. Results indicate that planting transgenic corn to first-year cornfields is a robust strategy to prevent resistance to both crop rotation and transgenic corn in areas where rotation-resistant populations are currently a problem or may be a problem in the future. In these areas, planting transgenic corn only in continuous cornfields is not an effective strategy to prevent resistance to either trait. In areas without rotation-resistant populations, gene expression of the allele for resistance to transgenic corn, R, is the most important factor affecting the evolution of resistance. If R is recessive, resistance can be delayed longer than 15 yr. If R is dominant, resistance may be difficult to prevent. In a sensitivity analysis, results indicate that density dependence, rotational level in the landscape, and initial allele frequency are the three most important factors affecting the results.

Adaptation, Physiological↗
PubMed2006

Evolution of mutation rates in bacteria.

Evolutionary success of bacteria relies on the constant fine-tuning of their mutation rates, which optimizes their adaptability to constantly changing environmental conditions. When adaptation is limited by the mutation supply rate, under some conditions, natural selection favours increased mutation rates by acting on allelic variation of the genetic systems that control fidelity of DNA replication and repair. Mutator alleles are carried to high frequency through hitchhiking with the adaptive mutations they generate. However, when fitness gain no longer counterbalances the fitness loss due to continuous generation of deleterious mutations, natural selection favours reduction of mutation rates. Selection and counter-selection of high mutation rates depends on many factors: the number of mutations required for adaptation, the strength of mutator alleles, bacterial population size, competition with other strains, migration, and spatial and temporal environmental heterogeneity. Such modulations of mutation rates may also play a role in the evolution of antibiotic resistance.

Bacteria↗
PubMed2006

Evolutionary history of Salmonella typhi.

For microbial pathogens, phylogeographic differentiation seems to be relatively common. However, the neutral population structure of Salmonella enterica serovar Typhi reflects the continued existence of ubiquitous haplotypes over millennia. In contrast, clinical use of fluoroquinolones has yielded at least 15 independent gyrA mutations within a decade and stimulated clonal expansion of haplotype H58 in Asia and Africa. Yet, antibiotic-sensitive strains and haplotypes other than H58 still persist despite selection for antibiotic resistance. Neutral evolution in Typhi appears to reflect the asymptomatic carrier state, and adaptive evolution depends on the rapid transmission of phenotypic changes through acute infections.

Adaptation, Physiological↗
PubMed2006

Model selection for mixtures of mutagenetic trees.

The evolution of drug resistance in HIV is characterized by the accumulation of resistance-associated mutations in the HIV genome. Mutagenetic trees, a family of restricted Bayesian tree models, have been applied to infer the order and rate of occurrence of these mutations. Understanding and predicting this evolutionary process is an important prerequisite for the rational design of antiretroviral therapies. In practice, mixtures models of K mutagenetic trees provide more flexibility and are often more appropriate for modelling observed mutational patterns. Here, we investigate the model selection problem for K-mutagenetic trees mixture models. We evaluate several classical model selection criteria including cross-validation, the Bayesian Information Criterion (BIC), and the Akaike Information Criterion. We also use the empirical Bayes method by constructing a prior probability distribution for the parameters of a mutagenetic trees mixture model and deriving the posterior probability of the model. In addition to the model dimension, we consider the redundancy of a mixture model, which is measured by comparing the topologies of trees within a mixture model. Based on the redundancy, we propose a new model selection criterion, which is a modification of the BIC. Experimental results on simulated and on real HIV data show that the classical criteria tend to select models with far too many tree components. Only cross-validation and the modified BIC recover the correct number of trees and the tree topologies most of the time. At the same optimal performance, the runtime of the new BIC modification is about one order of magnitude lower. Thus, this model selection criterion can also be used for large data sets for which cross-validation becomes computationally infeasible.

Bayes Theorem↗
PubMed2026

Overcoming cancer resistance in pancreatic cancer: toward dynamic precision oncology.

Pancreatic ductal adenocarcinoma (PDAC) remains a highly lethal malignancy, largely because of its profound and evolving therapeutic resistance. Resistance is not determined by a single molecular alteration but arises from interconnected mechanisms, including intrinsic resistance, treatment-induced adaptive resistance, acquired resistance, genomic evolution, clonal selection, cancer stemness, phenotypic plasticity, metabolic adaptation, and tumor microenvironment-mediated effects. Emerging therapeutic approaches targeting KRAS/RAS signaling, stromal and immune components, metabolic dependencies, and DNA damage repair pathways offer opportunities to address these mechanisms, although durable efficacy remains limited by biological heterogeneity and adaptive responses. In this review, we examine therapeutic resistance as an evolutionary and multidimensional process and summarize emerging strategies for overcoming resistance. We further propose a Dynamic Precision Oncology (DPO) framework that extends conventional precision oncology beyond baseline molecular profiling by integrating longitudinal assessment of tumor genomics, circulating tumor DNA, CA19-9, imaging, radiomics, and clinical characteristics. This framework emphasizes iterative detection and characterization of emerging resistance, mechanism-informed treatment adaptation, and subsequent reassessment rather than automatic treatment modification based on a single biomarker. DPO may provide a conceptual framework for integrating evolving tumor biology into treatment decision-making, while prospective studies are needed to validate biomarkers, define actionable thresholds, and determine whether longitudinal resistance-guided strategies improve clinical outcomes in PDAC.

Humans↗
PubMed2000

Molecular evolution in a multidrug-resistant lineage of Streptococcus pneumoniae: emergence of strains belonging to the serotype 6B Icelandic clone that lost antibiotic resistance traits.

Since their first detection in 1988, penicillin-resistant Streptococcus pneumoniae isolates have rapidly spread in Iceland to account for close to 20% of all pneumococcal disease in that country by 1993. The major component (70%) of the resistant pneumococci identified from 1989 to 1992 was the progeny of a single multidrug-resistant clone (Icelandic clone) with a homogeneous chromosomal macrorestriction profile and identical multilocus enzyme type expressing serotype 6B and resistance to penicillin, tetracycline, chloramphenicol, erythromycin, and trimethoprim-sulfamethoxazole. The rest of the non-penicillin-susceptible isolates included bacteria with serotype 6A and serogroups 19 and 23. The unique geographic and epidemiological setting and the availability of a complete collection of all non-penicillin-susceptible isolates of S. pneumoniae in Iceland prompted us to carry out a molecular epidemiological study to monitor the fate of the Icelandic clone between 1989 and 1996; in addition, we wished to extend the characterization to representative groups of all non-penicillin-susceptible serotype 6B pneumococci which showed variations in antibiotype and which were recovered in Iceland between late 1989 and the end of 1996. Also included in the study were non-penicillin-susceptible isolates of serogroup 23. Pulsed-field gel electrophoresis of SmaI-restricted chromosomal DNA and Southern hybridization with the lytA DNA probe and probes specific for antibiotic resistance genes were used to characterize pneumococcal isolates. The results show that (i) the Icelandic clone remained the predominant type among penicillin-resistant S. pneumoniae through 1996; (ii) the emergence of variants of the Icelandic clone which had lost one or more of the antibiotic resistance phenotypes and/or resistant genes, singly or in combination, was documented during the surveillance period; and (iii) isolates belonging to the internationally spread multidrug-resistant serotype 23F clone were present in the Icelandic collection since late 1989 but did not increase in number during the subsequent years.

Adult↗
PubMed2000

Aldehyde oxidase is coamplified with the World's most common Culex mosquito insecticide resistance-associated esterases.

The evolution and spread of insecticide resistance is an important factor in human disease prevention and crop protection. The mosquito Culex quinquefasciatus is the main vector of the disease filariasis and a member of a species complex which is a common biting nuisance worldwide. The common insecticide resistance mechanism in this species involves germline amplification of the esterases estalpha21 and estbeta21. This amplification has arisen once and rapidly spread worldwide. Less common and more variable resistance phenotypes involve coamplification of estalpha3 and estbeta1, or individual amplification of a single estbeta1, different alleles of the same estalpha and estbeta gene loci. Estalpha21 and estbeta21 are on the same large fragment of amplified DNA (amplicon) 2.7 kb apart. We have now shown that this amplicon contains another full-length gene immediately 5' of estalpha21 which codes for a molybdenum-containing hydroxylase, with highest homology to aldehyde oxidase (AO) from other organisms. The full-length putative AO gene is not present on the estalpha3/estbeta1 or estbeta1 amplicons, but multiple truncated 5' ends of this gene are present around the presumed estalpha3/estbeta1 amplicon breakpoint. Polymerase chain reaction (PCR) analysis of insecticide-susceptible genomic DNA demonstrated that a different allele of the putative AO gene in its non-amplified form is immediately 5' of estalpha. The 'AO' gene on the estalpha21/estbeta21 amplicon is expressed and resistant insects have greater AO activity. This AO activity is sensitive to inhibition by an aldehyde-containing herbicide and pesticide. This enzyme may confer a selective advantage to these insects in the presence of insecticide, as AO in mammals is believed to be important in the detoxification process of several environmental pollutants.

Aldehyde Oxidase↗
PubMed2002

Resistance and tolerance in a host plant-holoparasitic plant interaction: genetic variation and costs.

Host organisms are believed to evolve defense mechanisms (i.e., resistance and/or tolerance) under selective pressures exerted by natural enemies. A prerequisite for the evolution of resistance and tolerance is the existence of genetic variation in these traits for natural selection to act. However, selection for resistance and/or tolerance may be constrained by negative genetic correlations with other traits that affect host fitness. We studied genetic variation in resistance and tolerance against parasitic infection and the potential fitness costs associated with these traits using a novel study system, namely the interaction between a flowering plant and a parasitic plant. In this system, parasitic infection has significant negative effects on host growth and reproduction and may thus act as a selective agent. We conducted a greenhouse experiment in which we grew host plants, Urtica dioica, that originated from a single natural population and represented 20 maternal families either uninfected or infected with the holoparasitic dodder, Cuscuta europaea. that originated from the same site. We calculated correlations among resistance, tolerance, and host performance to test for costs of resistance and tolerance. We measured resistance as parasite performance (quantitative resistance) and tolerance as the slopes of regressions relating the vegetative and reproductive biomass of host plants to damage level (measured as parasite biomass). We observed significant differences among host families in parasite resistance and in parasite tolerance in terms of reproductive biomass, a result that suggests genetic variation in these traits. Furthermore, we found differences in resistance and tolerance between female and male host plants. In addition, the correlations indicate costs of resistance in terms of host growth and reproduction and costs of tolerance in terms of host reproduction. Our results thus indicate that host tolerance and resistance can evolve as a response to infection by a parasitic plant and that costs of resistance and tolerance may be one factor maintaining genetic variation in these traits.

Analysis of Variance↗
PubMed1996

Trends in antibiotic resistance of respiratory pathogens: an analysis and commentary on a collaborative surveillance study.

The evolution of antibiotic resistance was studied among common respiratory tract pathogens in five countries of the European Union and in the USA during 1992-1993. The data obtained from a collaborative surveillance study were submitted to population analysis, to detect possible shifts in antibiotic susceptibility and, therefore, associated mechanisms of resistance. Among the emerging haemophilus influenzae phenotypes were isolates that did not correspond to the beta-lactamase negative, amino-penicillin resistant (BLNAR) phenotype, but were beta-lactamase producers showing low level ceftriaxone resistance (early extended spectrum beta-lactamases?) amoxycillin susceptible strains with low level ceftriaxone resistance (PBP modification?) and isolates with high-level fluoroquinolone resistance. Moraxella catarrhalis resistance to ceftriaxone erythromycin or fluoroquinolones was noted. The quantitative evolution of antibiotic resistance may reach saturation in some countries with a very high proportion of resistant strains, for example, Spain and France. Qualitatively, resistant strains may be selected that have broader or more effective mechanisms of resistance, particularly under the recently introduced pressure of more active antibiotics of the same family. In countries with modest levels of antibiotic resistance (UK, Germany, Italy), attention should be paid to the misuse of antibiotics with a propensity to select low-level resistant strains. In this respect, the relative prescribing of aminopenicillins and oral cephalosporins in the UK (a high ratio and low prevalence of Streptococcus pneumoniae) and resistance to penicillin in the USA (a low ratio and high prevalence of resistance) is of potential importance.

Anti-Bacterial Agents↗
PubMed2002

High prevalence of oxacillin-resistant Staphylococcus aureus isolates from hospitalized patients in Asia-Pacific and South Africa: results from SENTRY antimicrobial surveillance program, 1998-1999.

As part of the SENTRY antimicrobial surveillance program, we examined the prevalence rates, types, and antibiograms of oxacillin-resistant Staphylococcus aureus from hospitalized patients from 17 institutions in eight countries in Asia-Pacific and South Africa (APAC). From April 1998 to December 1999, a total of 1,711 isolates of S. aureus (814 from blood, 392 from the respiratory tract, 467 from skin and skin structures, and 38 from urine) were collected from hospitalized patients within the APAC region. Multidrug-resistant oxacillin-resistant S. aureus (MORSA) isolates, defined as strains with three or more resistances to drug classes other than beta-lactams, were the most common type of oxacillin-resistant S. aureus (ORSA). They were the most frequently identified pathogen in wound infections and were common in bloodstream and lower respiratory tract infections. In all contributing institutions combined, more than 45% (range, 4 to 74%) of S. aureus isolates were oxacillin resistant, and in six institutions, this rate exceeded 60%. MORSA accounted for 91.2% of all oxacillin-resistant isolates. Distinct resistance patterns predominated at various sites within the APAC region, suggesting the local evolution of resistant clones. Non-multidrug-resistant strains were frequent in one part of Australia. No vancomycin-intermediate strains were detected, and no strains were resistant to linezolid or quinupristin-dalfopristin. MORSA strains are a very common cause of infection in hospitalized patients in the APAC region.

Asia↗
PubMedDate not supplied

Clinical implications of HIV drug resistance to nucleoside and nucleotide reverse transcriptase inhibitors.

This review focuses on issues pertinent to the epidemiology and clinical interpretation of resistance to nucleos(t)ide reverse transcriptase inhibitors. Nucleoside reverse transcriptase inhibitor resistance mutations, and especially thymidine analog mutations, remain the most common form of resistance detected in drug-naive patients. At the same time, improved treatment strategies, changes in prescribing policies, and prompt management of treatment failure are changing the prevalence and patterns of nucleoside reverse transcriptase inhibitor resistance in treatment-experienced patients. The clinical interpretation of nucleoside reverse transcriptase inhibitor resistance is being increasingly refined, helped by improved understanding of resistance pathways, development of sophisticated methods of analysis of genotypic resistance patterns, and introduction of clinically relevant cutoffs. Correlation of genotypic and phenotypic resistance data to clinical outcomes is essential to allow appropriate interpretation. In some cases, phenotypic data, either obtained directly by phenotypic tests or extrapolated from genotypic results, provide the most immediate predictors of virologic response. In other cases, genotypic analyses identify mutations that impact on responses without showing a marked effect on the phenotype, by either acting as "sentinel" markers for the presence of resistance undetectable by standard methods, or by lowering the genetic barrier to the evolution of resistance. The potential benefits of nucleoside reverse transcriptase inhibitor resistance, through hypersusceptibility and fitness effects, are also increasingly understood and exploited in clinical practice. Although progress has been significant, there remain many challenges. It is often questioned whether genotypic scores and clinical cutoffs obtained by various methods and from frequently small datasets can be reliably extrapolated to the general population of treated patients. At the same time, there is a need to define the role of reverse transcriptase mutations that are identified by statistical analyses as being associated with nucleoside reverse transcriptase inhibitor exposure, but have unknown effects on virus phenotype and clinical outcome. Novel mechanisms have also been proposed to play a role in nucleoside reverse transcriptase inhibitor resistance, including changes in RNaseH that are not targeted by routine testing at present. One additional, currently unresolved issue is the clinical relevance of minority resistant species and the feasibility of introducing ultra-sensitive resistance tests in routine diagnostic settings. The most appropriate viral-load cutoff for performing resistance tests and the reliability of results obtained at low copy numbers are similarly controversial. In spite of these limitations, resistance testing with appropriate interpretation provides an important guide to successful treatment outcomes and necessary support to the introduction of new treatment strategies.

Drug Resistance, Multiple, Viral↗
PubMed2001

Reversing insect adaptation to transgenic insecticidal plants.

The refuge-high-dose strategy for delaying insect adaptation to transgenic plants produces non-transgenic plants that enable survival of susceptible individuals. Previous theoretical work has suggested three requirements for success of the refuge-high-dose strategy: a low initial frequency of the resistance allele, extensive mating between resistant and susceptible adults and recessive inheritance of resistance. In order to understand an observed decrease in resistance frequency and improve the potential for managing resistance better, we used analytical and simulation models for exploring the conditions that prevent or reverse the evolution of resistance, even when resistance is not rare initially. Assuming random mating and recessive or nearly recessive inheritance of resistance, the factors favouring reversal of resistance are non-recessive costs of resistance, low initial resistance allele frequency, large refuges, incomplete resistance and density-independent population growth in refuges.

Adaptation, Biological↗
PubMed2026

Evolution of Candidaemia and azole resistance in Italy: A multicentre retrospective study.

PURPOSE: Candidaemia is the most common healthcare-associated invasive fungal infection. The evolution of the epidemiology of candidaemia in Italy has not been assessed, except at the local level. The primary objective of this study is monitoring changes in the epidemiology of candidaemia and in the susceptibility profiles of Candida isolates between 2015 and 2023. METHODS: This retrospective multicentre study (2015-2023), involved 11 tertiary-care hospital microbiology laboratories across the Italian country. The confirmed candidaemia episodes were included and demographic data, hospital ward, species identification, and antifungal susceptibility profiles (Sensititre Yeast One) were collected. RESULTS: 6,927 candidaemia cases were identified; incidence increased from 1.1/1000 hospitalisations in 2017 to 2.3/1000 in 2020-2021, peaking during the COVID-19 pandemic, and declined in 2023 while remaining above prepandemic levels. Patients older than 65 years accounted for most infections. Medical wards represented the main setting of occurrence, followed by intensive care units, especially during pandemic years. C. albicans remained the most common species (45.4%), followed by C. parapsilosis (24.7%), C. glabrata (11.8%), and C. tropicalis (11.4%). Echinocandin resistance remained low (<&#x2009;2% for C. albicans and C. glabrata), whereas azole resistance increased markedly, particularly in C. parapsilosis, reaching fluconazole resistance rates of 25.6% in 2022. CONCLUSIONS: Candidaemia increased during the 9-year study period in Italy, particularly in the COVID-19 pandemic, in medical wards and ICU. Emerged a growing azole resistance, underscoring the need for enhanced surveillance and informed empirical treatment strategies.

Candida species etiology↗
PubMed1997

Distribution, diversity and evolution of the bacterial mercury resistance (mer) operon.

Mercury and its compounds are distributed widely across the earth. Many of the chemical forms of mercury are toxic to all living organisms. However, bacteria have evolved mechanisms of resistance to several of these different chemical forms, and play a major role in the global cycling of mercury in the natural environment. Five mechanisms of resistance to mercury compounds have been identified, of which resistance to inorganic mercury (HgR) is the best understood, both in terms of the mechanisms of resistance to mercury and of resistance to heavy metals in general. Resistance to inorganic mercury is encoded by the genes of the mer operon, and can be located on transposons, plasmids and the bacterial chromosome. Such systems have a worldwide geographical distribution, and furthermore, are found across a wide range of both Gram-negative and Gram-positive bacteria from both natural and clinical environments. The presence of mer genes in bacteria from sediment cores suggest that mer is an ancient system. Analysis of DNA sequences from mer operons and genes has revealed genetic variation both in operon structure and between individual genes from different mer operons, whilst analysis of bacteria which are sensitive to inorganic mercury has identified a number of vestigial non-functional operons. It is hypothesised that mer, due to its ubiquity with respect to geographical location, environment and species range, is an ancient system, and that ancient bacteria carried genes conferring resistance to mercury in response to increased levels of mercury in natural environments, perhaps resulting from volcanic activity. Models for the evolution of both a basic mer operon and for the Tn21-related family of mer operons and transposons are suggested. The study of evolution in bacteria has recently become dominated by the generation of phylogenies based on 16S rRNA genes. However, it is important not to underestimate the roles of horizontal gene transfer and recombinational events in evolution. In this respect mer is a suitable system for evaluating phylogenetic methods which incorporate the effects of horizontal gene transfer. In addition, the mer operon provides a model system in the study of environmental microbiology which is useful both as an example of a genotype which is responsive to environmental pressures and as a generic tool for the development of new methodology for the analysis of bacterial communities in natural environments.

Biological Evolution↗