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Introduction to Clinical Microbiology
Published in Keith Struthers, Clinical Microbiology, 2017
As many plasmids carry genes coding for antibiotic resistance, the spread of plasmids is central to the problem of antibiotic resistance. There are various types of mobile genetic elements that can move between the bacterial chromosome and a plasmid. Insertion sequences (IS) are one example. They are about 1000 base pairs in length and consist of short inverted repeat sequences on either side of a gene that enables the element to move to different sites in chromosomal or plasmid DNA. Bacteria contain many copies of one or more IS structures; Escherichia coli has more than 40 scattered throughout its chromosome. When two IS domains combine at either end of an antibiotic resistance gene, this forms a transposon, and the resistance gene gains mobility. Integrons add a further level to the mechanisms of antibiotic resistance. These are sequences of DNA where different antibiotic resistance genes are linked together in an operon, under the control of a single promotor. Integrons can be integrated into transposons. These mobile genetic elements can be transferred via plasmids between members of the same species, or different species.
Problematic Beta-Lactamases: An Update
Published in Robert C. Owens, Lautenbach Ebbing, Antimicrobial Resistance, 2007
Marion S. Helfand, Louis B. Rice
Beta-lactamases are encoded by genes, and so the genetic structure of the regions surrounding the beta-lactamase genes is of considerable interest. Information about the genetic environment can give clues to the likely mobility of the gene, the extent to which it may be expressed, and to its origins and evolution. Unfortunately, the terminology surrounding genetic structures in bacteria is often arcane and difficult for the non-expert to follow. It is therefore worthwhile to start with a few definitions. A bacterial gene is a sequence of nucleotides that includes a promoter region (to get the RNA polymerase started), a ribosome-binding site (to get the tRNA started on protein synthesis), an open reading frame encoding a series of amino acids that make up the eventual protein, and a stop codon to tell the tRNAribosome complex to disassociate, completing the process of protein synthesis. A gene cassette is generally a ribosome binding site, open reading frame, and stop codon associated with a specific sequence that allows the gene to be incorporated into the integron by the integron’s integrase. An integron is a “sink” for collecting stray gene cassettes. It contains a place for the cassette to enter and an integrase to facilitate entry. It also contains a promoter to facilitate transcription of the integrated gene cassettes. Since more than one gene cassette can be incorporated into an integron, expression of genes that are further from the promoter is less than those closer (it is never good to be last in line). By themselves, integrons are generally not mobile, but they may be incorporated into transposons or plasmids. Transposons are segments of DNA that contain genes that facilitate mobility of the segment between different replicons in the cell. They may be integrated into plasmids or into the chromosome. Plasmids are closed, circular, supercoiled segments of DNA that replicate independently of the chromosome. They may be transferable to other bacteria, and may integrate into the chromosome and use their transfer functions to facilitate chromosomal gene transfer.
Mechanisms of Antibiotic Resistance in Acinetobacter spp. — Genetics of Resistance
Published in E. Bergogne-Bénézin, M.L. Joly-Guillou, K.J. Towner, Acinetobacter, 2020
Several other dhfr genes have also been shown to reside within transposons. All those which have been characterised can be classified as either Tn7-like or Tn21-like in structure. The extended array of different dhfr genes which are found incorporated within Tn7 and Tn21-like transposons results from the presence of integron structures within these two classes of transposon. Integrons can be defined as genetic elements, found within plasmids and transposons, which encode a site-specific recombination system enabling the insertion, deletion and rearrangement of discrete genetic cassettes within the integron element (Stokes and Hall, 1989). The majority of gene cassettes identified within integron structures encode resistance to antibiotics (Bissonnette and Roy, 1992). The integron structure identified within Tn21 and related transposons comprises a variable central region of DNA flanked by two conserved regions (Figure 8.2). The 3’ conserved segment generally includes the sulphonamide resistance gene, sull, flanked by two open reading frames orf4 — identified as a qac gene encoding a transporter protein mediating resistance to antiseptics and disinfectants (Paulsen et al., 1993) — and orf5 (function unknown). The 5’ conserved segment encodes an integrase (int) that mediates the sitespecific integration, deletion or rearrangement of genetic cassettes into a core site within the variable region of the integron (Collis and Hall, 1992; Collins et al., 1993). The 5’ conserved segment also provides the promoter for transcription of integrated cassettes in the variable region. Each genetic cassette comprises three conserved features: a central coding region, all except the last seven bases of a 59-bp element (comprising a short imperfect repeat sequence associated with the 3’ end of the gene), and a core site comprising the last seven bases of the 59-bp element located 5’ to the gene. These 59-bp elements act as recombination hot-spots (RHS) for integrase action (Hall et al., 1991). The integron identified within Tn7 and related transposons has not been characterised fully, but is known to comprise a pseudo-integrase gene (Pelletier and Roy, 1990; H-K. Young and S. Rosser, unpublished data; L. Sundstrom, personal communication) and up to three genetic cassettes in the cen-tral variable region (Sundstrom et al., 1991). Full characterisation of the 3’ end of the element has not yet been achieved.
In vitro activity of hyperthermia on swarming motility and antimicrobial susceptibility profiles of Proteus mirabilis isolates
Published in International Journal of Hyperthermia, 2021
Deniz Gazel, Hadiye Demirbakan, Mehmet Erinmez
P. mirabilis is the most commonly isolated species from clinical samples, with 90% being from urinary infections, but also from other infections including those of the respiratory system, eye, ear, nose, and skin, and burn and wound infections [18]. Resistance genes to antibiotic families including beta-lactams, aminoglycosides, quinolones and carbapenems, are increasingly identified in P. mirabilis. These genes may be carried on genetic elements such as plasmids, transposons and on integrons that can be mobilized [12]. Previously, the emergence of multidrug-resistant P. mirabilis-producing extended spectrum beta lactamase, AmpC, and carbapenemases has been reported [19,20]. Since there is a trend of developing resistance to various antimicrobial drugs, it is important to investigate new drugs, combination therapies or methods to overcome antimicrobial resistance problems caused by P. mirabilis bacteria.
Genotyping and molecular characterization of clinical Acinetobacter baumannii isolates from a single hospital in Southwestern Iran
Published in Pathogens and Global Health, 2020
Ahmad Farajzadeh Sheikh, Mohammad Savari, Effat Abbasi Montazeri, Saeed Khoshnood
PCR was performed for each pair of primers in a total volume of 25 µl in a reaction tube containing 2 µl of DNA template, 10 µl of Amplicon Master Mix, 1 µl of each forward and reverse primer (2.5 pmol), and 11 µl of distilled water. PCR reactions included 30 cycles of amplification in a Mastercycler (Eppendorf, Germany) under the following conditions: denaturation at 95 °C for five min, annealing at 55 °C for 30 s, and extension at 72 °C for 45 s, with a final extension at 72 °C for 6 min. Amplified products were visualized following electrophoresis on a 1% agarose gel stained with safe stain in a Tris-Borate-EDTA (TBE) buffer (Promega, USA). PCR was also performed to detect the presence of class I, II, and III integrons by the amplification of integrase genes, including intI1-, intI2-, and intI3-specific primers [24].
Characterization of integrons, extended-spectrum β-lactamases, AmpC cephalosporinase, quinolone resistance, and molecular typing of Shigella spp. from Iran
Published in Infectious Diseases, 2018
Sajjad Zamanlou, Mohammad Ahangarzadeh Rezaee, Mohammad Aghazadeh, Reza Ghotaslou, Farhad Babaie, Younes Khalili
PCR amplification of the internal variable region of the class 1 integron in 78 ESC-resistant isolates using CS-F/CS-R primers produced two different products of approximately 750 and 1600 bp in 2 (2.6%) and 19 (24.3%) isolates, respectively. Sequence analysis of the variable region indicated the presence of dfrA7 and dfrA17-aadA5 resistance gene cassettes among the isolates, which corresponded to 750 and 1600 bp PCR products, respectively. Furthermore, amplification of the internal variable region for the class 2 integron using hep74 and hep51 primers showed that 63 (80.8%) isolates harbored the class 2 integron. In addition, two types of gene cassettes were identified in class 2 integrons. Fifty-two (66.7%) strains had a 2300-bp fragment (with the sequence of dfrA1-sat1-aadA1), and 11 (14.1%) strains showed a 1500-bp fragment (with the sequence of dfrA1-sat1) (Figures in supplement data).