Horizontal transfer of multiple penicillin-binding protein genes, and capsular biosynthetic genes, in natural populations of Streptococcus pneumoniae

Multiply antibiotic-resistant serotype 23F isolates of Streptococcus pneumoniae are prevalent in Spain and have also been recovered recently in the United Kingdom and the United States. Analysis of populations of these isolates by multilocus enzyme electrophoresis, and restriction endonuclease cleavage electrophoretic profiling of penicillin-binding protein (PBP) genes, has demonstrated that these isolates are a single clone (Muñoz et al., 1991). Here we report studies of non-serotype 23F penicillin-resistant pneumococci isolated in Spain and the United Kingdom. One of the isolates expressed serotype 19 capsule but was otherwise indistinguishable from the serotype 23F clone on the basis of multilocus enzyme electrophoresis, antibiotic resistance profiling, and restriction endonuclease patterns of genes encoding PBP1A, PBP2B and PBP2X, a result which suggests that horizontal transfer of capsular biosynthesis genes had occurred. These same techniques revealed that six other resistant isolates, all expressing serotype 9 polysaccharide capsule, represent a clone. Interestingly, the chromosomal lineage of this clone is not closely related to the 23F clone; however, the serotype 9 and 23F clones harbour apparently identical PBP1 A, -2B and -2X genes. To explain these data, we favour the interpretation that horizontal gene transfer in natural populations has distributed genes encoding altered forms of PBP1A, -2B and -2X to distinct evolutionary lineages of S. pneumoniae.     

Horizontal transfer of antibiotic resistance genes in a membrane bioreactor

Growing attention has been paid to the dissemination of antibiotic resistance genes (ARGs) in wastewater microbial communities. The application of membrane bioreactors (MBRs) in wastewater treatment is becoming increasingly widespread. We hypothesized that the transfer of ARGs among bacteria could occur in MBRs, which combine a high density of bacterial cells, biofilms, and antibiotic resistance bacteria or ARGs. In this study, the transfer discipline and dissemination of the RP4 plasmid in MBRs were investigated by the counting plate method, the MIDI microorganism identification system, and quantitative polymerase chain reaction (qPCR) techniques. The results showed that the average transfer frequency of the RP4 plasmid from the donor strain to cultivable bacteria in activated sludge was 2.76 × 10⁻⁵ per recipient, which was greater than the transfer frequency in wastewater and bacterial sludge reported previously. In addition, many bacterial species in the activated sludge had received RP4 by horizontal transfer, while the genera of Shewanella spp., Photobacterium spp., Pseudomonas spp., Proteus spp., and Vibrio spp. were more likely to acquire this plasmid. Interestingly, the abundance of the RP4 plasmid in total DNA remained at high levels and relatively stable at 10⁴ copies/mg of biosolids, suggesting that ARGs were transferred from donor strains to activated sludge bacteria in our study. Thus, the presence of ARGs in sewage sludge poses a potential health threat.     

Horizontal gene transfer of stress resistance genes through plasmid transport

The horizontal gene transfer of plasmid-determined stress tolerance was achieved under lab conditions. Bacterial isolates, Enterobacter cloacae (DGE50) and Escherichia coli (DGE57) were used throughout the study. Samples were collected from contaminated marine water and soil to isolate bacterial strains having tolerance against heavy metals and antimicrobial agents. We have demonstrated plasmid transfer, from Amp⁺Cu⁺Zn⁻ strain (DGE50) to Amp⁻Cu⁻Zn⁺ strain (DGE57), producing Amp⁺Cu⁺Zn⁺ transconjugants (DGE_TC50→57) and Amp⁺Cu⁻Zn⁺ transformants (DGE_TF50→57). DGE57 did not carry any plasmid, therefore, it can be speculated that zinc tolerance gene in DGE57 is located on chromosome. DGE50 was found to carry three plasmids, out of which two were transferred through conjugation into DGE57, and only one was transferred through transformation. Plasmid transferred through transformation was one out of the two transferred through conjugation. Through the results of transformation it was revealed that the genes of copper and ampicillin tolerance in DGE50 were located on separate plasmids, since only ampicillin tolerance genes were transferred through transformation as a result of one plasmid transfer. By showing transfer of plasmids under lab conditions and monitoring retention of respective phenotype via conjugation and transformation, it is very well demonstrated how multiple stress tolerant strains are generated in nature.     

Plasmid transfer to indigenous marine bacterial populations by natural transformation

Abstract Horizontal gene transfer among microbial populations has been assumed to occur in the environment, yet direct observations of this phenomenon are rare or limited to observations where the mechanism(s) could not be explicitly determined. Here we demonstrate the transfer of exogenous plasmid DNA to members of indigenous marine bacterial populations by natural transformation, the first report of this process for any natural microbial community. Ten percent of marine bacterial isolates examined were transformed by plasmid DNA while 14% were transformed by chromosomal DNA. Transformation of mixed marine microbial assemblages was observed in 5 of 14 experiments. In every case, acquisition of the plasmid by members of the indigenous flora was accompanied by modification (probably from genetic rearrangement or methylation) that altered its restriction enzyme digestion pattern. Estimation of transformation rates in estuarine environments based upon the distribution of competency and transformation frequencies in isolates and mixed populations ranged from 5 × 10⁻⁴ to 1.5 transformants/1 day. Extrapolation of these rates to ecosystem scales suggests that natural transformation may be an important mechanism for plasmid transfer among marine bacterial communities.     

Horizontal gene transfer in bacteria: laboratory simulation, natural populations, genomic data

Various aspects of horizontal gene transfer among bacteria are considered: modeling of this phenomenon in microcosms and natural environments; influence of gene migration on the composition of natural bacterial populations; peculiarities of bacterial chromosome evolution.     

Horizontal gene transfer and bacterial diversity

Bacterial genomes are extremely dynamic and mosaic in nature. A substantial amount of genetic information is inserted into or deleted from such genomes through the process of horizontal transfer. Through the introduction of novel physiological traits from distantly related organisms, horizontal gene transfer often causes drastic changes in the ecological and pathogenic character of bacterial species and thereby promotes microbial diversification and speciation. This review discusses how the recent influx of complete chromosomal sequences of various microorganisms has allowed for a quantitative assessment of the scope, rate and impact of horizontally transmitted information on microbial evolution.     

Persistence of antibiotic resistance in bacterial populations

Unfortunately for mankind, it is very likely that the antibiotic resistance problem we have generated during the last 60 years due to the extensive use and misuse of antibiotics is here to stay for the foreseeable future. This view is based on theoretical arguments, mathematical modeling, experiments and clinical interventions, suggesting that even if we could reduce antibiotic use, resistant clones would remain persistent and only slowly (if at all) be outcompeted by their susceptible relatives. In this review, we discuss the multitude of mechanisms and processes that are involved in causing the persistence of chromosomal and plasmid-borne resistance determinants and how we might use them to our advantage to increase the likelihood of reversing the problem. Of particular interest is the recent demonstration that a very low antibiotic concentration can be enriching for resistant bacteria and the implication that antibiotic release into the environment could contribute to the selection for resistance. Several mechanisms are contributing to the stability of antibiotic resistance in bacterial populations and even if antibiotic use is reduced it is likely that most resistance mechanisms will persist for considerable times.     

Preparation of a DNA gene probe for detection of mercury resistance genes in gram-negative bacterial communities

A DNA gene probe was prepared to study genetic change mechanisms responsible for adaptation to mercury in natural bacterial communities. The probe was constructed from a 2.6-kilobase NcoI-EcoRI DNA restriction fragment which spans the majority of the mercury resistance operon (mer) in the R-factor R100. The range of specificity of this gene probe was defined by hybridization to the DNA of a wide variety of mercury-resistant bacteria previously shown to possess the mercuric reductase enzyme. All of the tested gram-negative bacteria had DNA sequences homologous to the mer probe, whereas no such homologies were detected in DNA of the gram-positive strains. Thus, the mer probe can be utilized to study gene flow processes in gram-negative bacterial communities.     

Selection on protein-coding genes of natural cyanobacterial populations

We examined the distribution of synonymous and non-synonymous changes in 12 protein-coding genes of natural populations of cyanobacteria to infer changes in gene functionality. By comparing mutation distributions within and across species using the McDonald-Kreitman test, we found data sets to contain evidence for purifying selection (hetR of Trichodesmium, nifH of Cylindrospermopsis raceborskii and rpoC1 of Anabaena lemmermannii) and positive selection (kaiC of Microcoleus chthonoplastes and rbcX of Anabaena and Aphanizomenon sp.). Other genes from the same set of clonal isolates (petB and rbcL in M. chthonoplastes and Anabaena/Aphanizomenon, respectively) did not harbour evidence for either form of selection. The results of branch models of codon evolution agreed fully with the results of the McDonald-Kreitman test in terms of significance and absolute value of the dN/dS estimates. The high frequency of gene-specific mutation patterns and their association with branches that separate closely related cyanobacterial genera suggest that evolutionary tests are suited to uncover gene-specific selective differentiation in cyanobacterial genomes. At the same time, given the lack of information about the history of cyanobacteria, analysis of larger numbers of protein-coding genes of clonal cyanobacterial isolates will produce more detailed pictures of the effects of natural selection.     

A network-based approach for resistance transmission in bacterial populations

Horizontal transfer of mobile genetic elements (conjugation) is an important mechanism whereby resistance is spread through bacterial populations. The aim of our work is to develop a mathematical model that quantitatively describes this process, and to use this model to optimize antimicrobial dosage regimens to minimize resistance development. The bacterial population is conceptualized as a compartmental mathematical model to describe changes in susceptible, resistant, and transconjugant bacteria over time. This model is combined with a compartmental pharmacokinetic model to explore the effect of different plasma drug concentration profiles. An agent-based simulation tool is used to account for resistance transfer occurring when two bacteria are adjacent or in close proximity. In addition, a non-linear programming optimal control problem is introduced to minimize bacterial populations as well as the drug dose. Simulation and optimization results suggest that the rapid death of susceptible individuals in the population is pivotal in minimizing the number of transconjugants in a population. This supports the use of potent antimicrobials that rapidly kill susceptible individuals and development of dosage regimens that maintain effective antimicrobial drug concentrations for as long as needed to kill off the susceptible population. Suggestions are made for experiments to test the hypotheses generated by these simulations.     

Mapping spot blotch resistance genes in four barley populations

Bipolaris sorokiniana (teleomorph: Cochliobolus sativus) is the fungal pathogen responsible for spot blotch in barley (Hordeum vulgare L.) and occurs worldwide in warmer, humid growing conditions. Current Australian barley varieties are largely susceptible to this disease and attempts are being made to introduce sources of resistance from North America. In this study we have compared chromosomal locations of spot blotch resistance reactions in four North American two-rowed barley lines; the North Dakota lines ND11231-12 and ND11231-11 and the Canadian lines TR251 and WPG8412-9-2-1. Diversity arrays technology-based PCR, expressed sequence tag and SSR markers have been mapped across four populations derived from crosses between susceptible parental lines and these four resistant parents to determine the location of resistance loci. Quantitative trait loci (QTL) conferring resistance to spot blotch in adult plants (APR) were detected on chromosomes 3HS and 7HS. In contrast, seedling resistance (SLR) was controlled solely by a locus on chromosome 7HS. The phenotypic variance explained by the APR QTL on 3HS was between 16 and 25% and the phenotypic variance explained by the 7HS APR QTL was between 8 and 42% across the four populations. The SLR QTL on 7HS explained between 52 and 64% of the phenotypic variance. An examination of the pedigrees of these resistance sources supports the common identity of resistance in these lines and indicates that only a limited number of major resistance loci are available in current two-rowed germplasm.     

Horizontal transfer of CS1 pilin genes of enterotoxigenic Escherichia coli

CS1 is one of a limited number of serologically distinct pili found in enterotoxigenic Escherichia coli (ETEC) strains associated with disease in people. The genes for the CS1 pilus are on a large plasmid, pCoo. We show that pCoo is not self-transmissible, although our sequence determination for part of pCoo shows regions almost identical to those in the conjugative drug resistance plasmid R64. When we introduced R64 into a strain containing pCoo, we found that pCoo was transferred to a recipient strain in mating. Most of the transconjugant pCoo plasmids result from recombination with R64, leading to acquisition of functional copies of all of the R64 transfer genes. Temporary coresidence of the drug resistance plasmid R64 with pCoo leads to a permanent change in pCoo so that it is now self-transmissible. We conclude that when R64-like plasmids are transmitted to an ETEC strain containing pCoo, their recombination may allow for spread of the pCoo plasmid to other enteric bacteria.     

食物链中抗生素耐药性基因的转移

抗生素的使用,一方面起到预防和治疗疾病的作用,另一方面,饲料、食品和环境中抗生素残留增加,使抗生素耐药性菌产生并进化,从而导致将来治疗某些疾病时无有效抗生素可用,比如,结核病已经卷土重来,而且现在就有许多患者就不能用抗生素治愈。随着人们生活水平的提高,安全、健康问题受到人们广泛的关注和重视。本文初步对耐药性基因在食物链中怎样产生和转移进行综述。