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PubMed2006

Antibiotic resistance, integrons and Salmonella genomic island 1 among non-typhoidal Salmonella serovars in The Netherlands.

The objective of this study was to investigate the antimicrobial resistance patterns, integron characteristics and gene cassettes as well as the presence of Salmonella genomic island 1 (SGI1) in non-typhoidal Salmonella (NTS) isolates from human and animal origin. Epidemiologically unrelated Dutch NTS strains (n=237) originating from food-producing animals and human cases of salmonellosis were tested for their susceptibility to 15 antimicrobial agents. Resistance to 14 of these antimicrobials, including the third-generation cephalosporins, was detected. Resistance to sulphonamides, ampicillin, tetracycline, streptomycin, trimethoprim and nalidixic acid was common (>/=10% of the strains were resistant). Resistance against three or more antimicrobials was observed in 57 isolates. The same 237 strains were studied for the prevalence of class 1 integrons, their gene cassettes and the presence of SGI1. Thirty-six isolates (15.2%) carried class 1 integrons. These integrons had ten distinct profiles based on the size of the integron and restriction fragment length polymorphism analysis. Integrons were detected for the first time in serovars Indiana and Senftenberg. Multidrug resistance was strongly associated with the presence of class 1 integrons in which the aadA2, aadA1, bla(PSE-1), dfrA1, dfrA5, dfrA14 or sat genes were present, as determined by nucleotide sequence determination. The presence of gene cassettes or combinations of gene cassettes not previously found in integrons in Salmonella was observed. SGI1 or its variants (SGI-B, -C and -F) were present in 16 isolates belonging to either serovar Typhimurium, Derby or Albany. Regardless of whether the isolate was of human or animal origin, the same resistance phenotype, integron profile and SGI1 structure could be observed.

Animals↗
PubMed2005

Identification of antimicrobial resistance and class 1 integrons in Shiga toxin-producing Escherichia coli recovered from humans and food animals.

OBJECTIVES: The objective of this study was to identify antimicrobial resistance and class 1 integrons among Shiga toxin-producing Escherichia coli (STEC). METHODS: Two-hundred and seventy-four STEC recovered from poultry, cattle, swine and humans were characterized by antimicrobial susceptibility testing, screened for the presence of class 1 integrons by PCR, and assayed for integron transfer by conjugation. RESULTS: Ninety-three (34%) of the isolates were resistant to streptomycin, followed by 89 (32%) to sulfamethoxazole, 83 (30%) to tetracycline, 48 (18%) to ampicillin, 29 (11%) to cefalothin, 22 (8%) to trimethoprim/sulfamethoxazole, 18 (7%) to gentamicin, 13 (5%) to chloramphenicol and 10 (4%) to cefoxitin. Class 1 integrons were detected in 43 (16%) of the 274 isolates. The adenyl acetyltransferase gene, aadA, which confers resistance to streptomycin, was identified in integrons from 41 (95%) of these 43 isolates, and the dfrA12 gene, which confers resistance to trimethoprim, was identified in integrons from eight (19%) of the isolates. The sat1 gene, which confers resistance to streptothricin, an antimicrobial that has never been approved for use in the United States, was identified in integrons from three (7%) of the isolates. Transfer of integrons by conjugation between strains of E. coli resulted in transfer of antimicrobial-resistant phenotypes for ampicillin, chloramphenicol, cefalothin, gentamicin, tetracycline, trimethoprim, sulfamethoxazole and streptomycin. CONCLUSIONS: Antimicrobial resistance is common in STEC. Class 1 integrons located on mobile plasmids have facilitated the emergence and dissemination of antimicrobial resistance among STEC in humans and food animals.

Animals↗
PubMed2006

Class 1 integrons potentially predating the association with tn402-like transposition genes are present in a sediment microbial community.

Integrons are genetic elements that contribute to lateral gene transfer in bacteria as a consequence of possessing a site-specific recombination system. This system facilitates the spread of genes when they are part of mobile cassettes. Most integrons are contained within chromosomes and are confined to specific bacterial lineages. However, this is not the case for class 1 integrons, which were the first to be identified and are one of the single biggest contributors to multidrug-resistant nosocomial infections, carrying resistance to many antibiotics in diverse pathogens on a global scale. The rapid spread of class 1 integrons in the last 60 years is partly a result of their association with a specific suite of transposition functions, which has facilitated their recruitment by plasmids and other transposons. The widespread use of antibiotics has acted as a positive selection pressure for bacteria, especially pathogens, which harbor class 1 integrons and their associated antibiotic resistance genes. Here, we have isolated bacteria from soil and sediment in the absence of antibiotic selection. Class 1 integrons were recovered from four different bacterial species not known to be human pathogens or commensals. All four integrons lacked the transposition genes previously considered to be a characteristic of this class. At least two of these integrons were located on a chromosome, and none of them possessed antibiotic resistance genes. We conclude that novel class 1 integrons are present in a sediment environment in various bacteria of the beta-proteobacterial class. These data suggest that the dispersal of this class may have begun before the "antibiotic era."

Bacteria↗
PubMed2006

Recovery and evolutionary analysis of complete integron gene cassette arrays from Vibrio.

BACKGROUND: Integrons are genetic elements capable of the acquisition, rearrangement and expression of genes contained in gene cassettes. Gene cassettes generally consist of a promoterless gene associated with a recombination site known as a 59-base element (59-be). Multiple insertion events can lead to the assembly of large integron-associated cassette arrays. The most striking examples are found in Vibrio, where such cassette arrays are widespread and can range from 30 kb to 150 kb. Besides those found in completely sequenced genomes, no such array has yet been recovered in its entirety. We describe an approach to systematically isolate, sequence and annotate large integron gene cassette arrays from bacterial strains. RESULTS: The complete Vibrio sp. DAT722 integron cassette array was determined through the streamlined approach described here. To place it in an evolutionary context, we compare the DAT722 array to known vibrio arrays and performed phylogenetic analyses for all of its components (integrase, 59-be sites, gene cassette encoded genes). It differs extensively in terms of genomic context as well as gene cassette content and organization. The phylogenetic tree of the 59-be sites collectively found in the Vibrio gene cassette pool suggests frequent transfer of cassettes within and between Vibrio species, with slower transfer rates between more phylogenetically distant relatives. We also identify multiple cases where non-integron chromosomal genes seem to have been assembled into gene cassettes and others where cassettes have been inserted into chromosomal locations outside integrons. CONCLUSION: Our systematic approach greatly facilitates the isolation and annotation of large integrons gene cassette arrays. Comparative analysis of the Vibrio sp. DAT722 integron obtained through this approach to those found in other vibrios confirms the role of this genetic element in promoting lateral gene transfer and suggests a high rate of gene gain/loss relative to most other loci on vibrio chromosomes. We identify a relationship between the phylogenetic distance separating two species and the rate at which they exchange gene cassettes, interactions between the non-mobile portion of bacterial genomes and the vibrio gene cassette pool as well as intragenomic translocation events of integrons in vibrios.

DNA, Bacterial↗
PubMed2000

Characterisation of drug resistance gene cassettes associated with class 1 integrons in clinical isolates of Escherichia coli from Taiwan, ROC.

The presence of class 1 integrons in clinical isolates of Escherichia coli was detected by PCR. Of 104 E. coli isolates from Kaohsiung, 54 (52%) carried class 1 integrons, with inserted DNA regions of 1-3 kb. These integrons were located on plasmids, as demonstrated by Southern hybridisation. DNA sequencing was used to identify the genetic content of the integron-variable regions. Different class 1 integrons contained various numbers, kinds and combinations of gene cassettes within their variable regions. These gene cassettes included those encoding resistance to trimethoprim (dfrIa, dfrV, dfr12 and dfr17), aminoglycosides (aadA1a, aadA2, aadA4 and aadB), chloramphenicol (cmlA), erythromycin (ereA2) and beta-lactams (blaP1). An integron carrying three inserted cassettes - dfr12-orJF-aadA2 - was present in 33 (61%) of the 54 isolates with class 1 integrons. Gene cassettes encoding resistance were expressed phenotypically. The results indicate that class 1 integrons are widespread in clinical E. coli isolates in Taiwan. The types, combinations and frequency of the gene cassettes in integrons may reflect the specific selective pressures to which the isolates were exposed and could provide useful surveillance data for relation to antibiotic usage information.

Anti-Bacterial Agents↗
PubMed1995

Mobile gene cassettes and integrons: capture and spread of genes by site-specific recombination.

An integron is a genetic unit that includes the determinants of the components of a site-specific recombination system capable of capturing and mobilizing genes that are contained in mobile elements called gene cassettes. An integron also provides a promoter for expression of the cassette genes, and integrons thus act both as natural cloning systems and as expression vectors. The essential components of an integron are an int gene encoding a site-specific recombinase belonging to the integrase family, an adjacent site, attI, that is recognized by the integrase and is the receptor site for the cassettes, and a promoter suitably oriented for expression of the cassette-encoded genes. The cassettes are mobile elements that include a gene (most commonly an antibiotic-resistance gene) and an integrase-specific recombination site that is a member of a family of sites known as 59-base elements. Cassettes can exist either free in a circularized form or integrated at the attI site, and only when integrated is a cassette formally part of an integron. A single site-specific recombination event involving the integron-associated attI site and a cassette-associated 59-base element leads to insertion of a free circular cassette into a recipient integron. Multiple cassette insertions can occur, and integrons containing several cassettes have been found in the wild. The integrase also catalyses excisive recombination events that can lead to loss of cassettes from an itegron and generate free circular cassettes. Due to their ability to acquire new genes, integrons have a clear role in the evolution of the genomes of the plasmids and transposons that contain them.

Aspergillosis↗
PubMed2001

Rapid screening technique for class 1 integrons in Enterobacteriaceae and nonfermenting gram-negative bacteria and its use in molecular epidemiology.

A screening technique for integrons in members of the family Enterobacteriaceae and nonfermenting gram-negative bacteria by real-time PCR is reported. A total of 226 isolates of gram-negative bacteria obtained from a variety of clinical specimens were screened for class 1 integrons by real-time PCR performed on a LightCycler instrument. This technique used a primer pair specific for a 300-bp conserved region at the 5' ends of class 1 integrons. The screening assay was evaluated by comparison with results obtained by the conventional, thermal-block PCR (long PCR) by using established conditions and primers for the detection of class 1 integrons, and the real-time PCR technique was thus shown to be both sensitive and specific. DNA from 50 of 226 (22%) isolates screened was identified as containing an integron by the screening PCR, and sequence data were obtained across the integron for 34 of 50 (68%) of these isolates. In an attempt to study the molecular epidemiology of antimicrobial resistance genes carried within integrons, a comparison of the types of gene cassettes carried by isolates from different patients was made. Adenyltransferase genes conferring resistance to streptomycin and spectinomycin were the predominant gene cassettes amplified in the study. Resistance to trimethoprim was also frequently found to be encoded within integrons. Furthermore, multiple bacterial isolates obtained from one patient over a 5-month period were all shown to carry an integron containing the same single adenyltransferase gene cassette, suggesting that these elements were relatively stable in this case.

Community-Acquired Infections↗
PubMed2001

Identification and characterization of integron-mediated antibiotic resistance among Shiga toxin-producing Escherichia coli isolates.

A total of 50 isolates of Shiga toxin-producing Escherichia coli (STEC), including 29 O157:H7 and 21 non-O157 STEC strains, were analyzed for antimicrobial susceptibilities and the presence of class 1 integrons. Seventy-eight (n = 39) percent of the isolates exhibited resistance to two or more antimicrobial classes. Multiple resistance to streptomycin, sulfamethoxazole, and tetracycline was most often observed. Class 1 integrons were identified among nine STEC isolates, including serotypes O157:H7, O111:H11, O111:H8, O111:NM, O103:H2, O45:H2, O26:H11, and O5:NM. The majority of the amplified integron fragments were 1 kb in size with the exception of one E. coli O111:H8 isolate which possessed a 2-kb amplicon. DNA sequence analysis revealed that the integrons identified within the O111:H11, O111:NM, O45:H2, and O26:H11 isolates contained the aadA gene encoding resistance to streptomycin and spectinomycin. Integrons identified among the O157:H7 and O103:H2 isolates also possessed a similar aadA gene. However, DNA sequencing revealed only 86 and 88% homology, respectively. The 2-kb integron of the E. coli O111:H8 isolate contained three genes, dfrXII, aadA2, and a gene of unknown function, orfF, which were 86, 100, and 100% homologous, respectively, to previously reported gene cassettes identified in integrons found in Citrobacter freundii and Klebsiella pneumoniae. Furthermore, integrons identified among the O157:H7 and O111:NM strains were transferable via conjugation to another strain of E. coli O157:H7 and to several strains of Hafnia alvei. To our knowledge, this is the first report of integrons and antibiotic resistance gene cassettes in STEC, in particular E. coli O157:H7.

Animals↗
PubMed2004

Class 1 integrons in various Salmonella enterica serovars isolated from animals and identification of genomic island SGI1 in Salmonella enterica var. Meleagridis.

OBJECTIVES: To determine the prevalence of integron-mediated antibiotic resistance in a diverse sample set of Salmonella enterica isolated from animals. MATERIALS AND METHODS: Multiplex PCR was used to detect class 1 integron gene sequences, and integron gene cassettes were identified by PCR mapping. Susceptibility to 18 antibiotics or antibiotic combinations commonly used in either human or veterinary medicine was measured using a microdilution method, and statistical comparisons of the frequency of resistance between groups were made using Fisher's two-sided probability test. Genotypic comparisons of isolates were made following pulsed-field gel electrophoresis of genomic DNA. RESULTS: Thirty-two (30.8%) of 104 isolates contained class 1 integron sequences. Integron-positive isolates represented 15 different S. enterica serovars, were obtained from nine different animal species and had a higher frequency of non-integron-mediated antibiotic resistance (P < 0.05) compared with integron-negative isolates. One non-Typhimurium isolate (S. enterica Meleagridis) contained an SGI1 genomic island, including the antibiotic resistance gene cluster. CONCLUSIONS: These data demonstrate that integron-mediated antibiotic resistance is common among diverse Salmonella serovars, many of them rare. In addition, SGI1 is not limited to Salmonella enterica Typhimurium DT104 or other commonly isolated serovars.

Animals↗
PubMed2005

Characterization of antimicrobial resistance and class 1 integrons found in Escherichia coli isolates from humans and animals in Korea.

OBJECTIVES: Antimicrobial resistance and class 1 integrons found in Escherichia coli isolates from humans and animals in Korea were characterized. METHODS: E. coli isolates were examined for susceptibility to antimicrobial agents. Integrase genes were amplified. Gene cassette regions for classes 1 and 2 integrons were amplified and sequenced. Conjugal transfer and Southern hybridization were performed to determine the genetic localization of class 1 integrons. The clonal relationship of E. coli isolates carrying an identical cassette array was analysed by PFGE. RESULTS: Commensal E. coli isolates from animals were highly resistant to commonly used antimicrobial agents such as tetracycline, sulfamethoxazole, streptomycin, ampicillin and carbenicillin. Integrons were most prevalent in commensal E. coli isolates from poultry (44%), followed by clinical isolates from humans (33%), commensal isolates from swine (23%) and humans (13%). dfrA17-aadA5, dfrA12-orfF-aadA2 and aadA1 were found most frequently in E. coli isolates from humans, poultry and swine, respectively. Class 1 integrons were mostly located in conjugative plasmids. E. coli isolates carrying an identical cassette array were phylogenetically unrelated. CONCLUSIONS: The use of antibiotics is strongly associated with antimicrobial resistance. E. coli isolates from different sources may select a specific gene cassette by antibiotic selective pressure, which results in differences in class 1 integrons. The horizontal transfer of class 1 integrons through conjugative plasmids seems to be responsible for wide dissemination of a particular type of class 1 integron.

Animals↗
PubMed2004

Integron analysis and genetic mapping of antimicrobial resistance genes in Salmonella enterica serotype Typhimurium.

Antimicrobial resistance determinants may be transferred among bacteria via mobile genetic elements including plasmids, transposons, and the more recently explored integrons. Integrons are naturally occurring genetic elements found as part of the Tn21 transposon family or located on various broad host-range plasmids. The fundamental integron structure consists of a 5'-conserved segment (5'-CS) of 1.4-kbp and a 2-kbp 3'-CS. Between these conserved regions are DNA sequences of variable length and molecular complexity. These intervening sequences are known as gene cassettes, and several have now been characterized. Acquisition and dissemination of these genes located within the integron structure, results in an increase in antimicrobial resistance. Three classes of integron structure have been described. Class 1 integrons are of principal importance in clinical isolates. The 5'-CS of class 1 integrons includes an intI 1 gene of 1358 bp, which encodes a specific recombinase, a member of the DNA integrase family. This gene contains the att1 recombination site, required for specifically integrating gene cassettes. Classes 2 and 3 also contain integrase genes (intI 2 and intI 3), with the former showing 40% sequence identity to those of class 1, and the latter showing 61%. All three classes of integrons contain similar gene cassettes from the same families, which suggests the existence of a common pool of gene cassette with cross-specificity between the classes. When the 3'-CS region is examined in detail, it contains several open reading frames (ORFs). These include qacEDelta1, which confers resistance to quaternary ammonium compounds, often associated with antiseptics, along with a sul1 gene expressing resistance to sulphonamide antimicrobial agents. The sul1 gene encodes the enzyme hydopteroate synthase. Transcription of the sul1 gene begins at a promoter located in the 5'-CS. The latter is also responsible for the transcription of the inserted gene cassette(s). Two additional ORFs, ORF-5 and -6, are located toward the distal end of the 3'-CS. The gene product of ORF 5 appears to share some sequence similarity with puromycin acetyltransferase, and this feature suggests a possible role in antimicrobial resistance. A biological function has yet to be ascribed to ORF-6.

Base Sequence↗
PubMed2003

Integron content of Salmonella enterica serotype Typhimurium strains isolated in Hungary in the years 1997-1999.

The integron content of 52 DT104/U302 phage type strains and 53 non-DT104/U302 strains of Salmonella enterica serotype Typhimurium (S. Typhimurium) was studied in PCR experiments using a 5'-CS/3'-CS primer pair (Lévesque et al., 1995). Forty-three out of 44 streptomycin- and/or ampicillin-resistant DT104 and related phage type strains were found to carry a 1 kb and/or 1.2 kb long integron. The other resistance markers did not affect the number and size of integrons; no integron-free multidrug-resistant (MDR) DT104 strains were found. The two large groups of DT104 strains (Felix-Callow's phage types 2 and 2c) proved to be identical in respect of integron patterns (IPs), supporting the views of those authors who consider DT104 a single clone. Strains of human and animal origin did not differ from each other in their IPs. Within the non-DT104 phage types, ampicillin- and/or streptomycin-resistant, integron-free MDR strains were also found. Based on amplicons varying between 290 and 3500 bp an IP system was suggested. The commonest amplicon sizes in non-DT104 strains were 1450 and 2050 bp. The IPs of DT104 strains and of non-DT104 strains containing an integron of 1 and 1.2 kb size were stable. In contrast, the IPs of other non-DT104 strains showed a varying degree of instability. Integron loss was frequently associated with spontaneous plasmid elimination and changes of R-type among the descendants of a given strain.

Animals↗
PubMed2003

Class 1 and class 2 integrons in poultry carcasses from broiler house and poultry processing environments.

Integrons have been identified as major genetic contributors to the dissemination of antimicrobial resistance in bacteria. The objective of this study was to examine the prevalence of integrons in poultry processing at the broiler house and in processing plants. Class 1 and class 2 integrons were found throughout the processing environment. Of the two classes of integrons, class 1 was the most prevalent in all processing areas. The levels of both classes of integrons decreased from the farm to the processing plant. Within the chiller tank in the processing plant, the persistence of these sequences appears to be related to the free chlorine concentration of the chiller tank water. The variable regions of the amplified integrons showed size diversity (from 680 to 2,000 bp), suggesting diversity in types of antibiotic-resistance-coding gene cassettes. The presence of the class 1 and class 2 integrons in the chlorinated chiller tank suggests that these sequences are capable of withstanding this critical step in the reduction of microbial loads on poultry carcasses. The persistence of the integron gene sequences on the farm and throughout processing highlights the stability of these transmissible antibiotic-resistance-coding nucleotide sequences and their potential role as reservoirs of antibiotic-resistance-coding genetic elements within the poultry rearing and processing environments.

Animals↗
PubMed2003

[Integrons and antimicrobial resistance gene cassettes in Shigella flexneri strains ].

BACKGROUND: The resistance of Shigella flexneri to antimicrobial agents can be associated to the presence of integrons that may contain and express antimicrobial resistance gene cassettes. AIM: To study antimicrobial resistance and the presence of integrons and antimicrobial gene cassettes in Shigella flexneri strains. MATERIAL AND METHODS: In vitro susceptibility to 27 antimicrobials was studied in twenty four Shigella flexneri strains isolated from stools. The presence of integrons class 1, 2 and 3 and antimicrobial resistance gene cassettes was investigated by polymerase chain reaction (PCR) using specific primers for each gene. RESULTS: Most strains were resistant to one of the following antimicrobials: ampicillin, sulphonamide, trimethoprim, tetracycline, streptomycin, sulfamethoxazole-trimethoprim or chloramphenicol. Twenty nine percent were simultaneously resistant to all these antimicrobials. Integrons class 1 and 2 were found in 19 strains (79%). Class 3 integrons were not found. Gene cassettes dfrA1 and ant(3")I were associated to integrons class 2 in most strains (15/20, 75%). Genes cat, tetB and blarTEM were detected in 18/24 (75%), 7/24 (29%) and 4/24 (17%) of the strains, respectively and were not associated to any of the studied integrons. Genes that codify enzymes AAC(6')Ib and APH(3')VI were not detected in any strain. CONCLUSIONS: The high frequency of integrons found in the studied strains, could partly explain the increasing antimicrobial resistance of Shigella flexneri strains, isolated in Chile.

Amino Acid Sequence↗
PubMed2003

[Class 1 integrons in Enterobacteriaceae and its association with multidrug resistance and conjugative plasmids].

The prevalence of antimicrobial resistance genes is a cause of concern. The combination of antimicrobial resistance genes and mobile genetic elements leads to their widespread presence in different bacterial species, in which integrons are a new and important element. We studied the presence of integrons in 123 unrelated enterobacteria and identified them in 20.3% of the strains. The combination of integrons and multidrug resistance was shown to be statistically significant (p <0.001). Integron-positive isolates were statistically (p <0.05) more likely to be resistant to amoxicillin-clavulanic acid, quinolones and trimethoprim-sulfamethoxazole. All the integrons were identified in conjugative plasmids. The prevalence of integrons increased from 21.2% in 1992-1994 to 72% in 1995-1997 (p <0.001). The aacC1 and aacC2 genes were identified in 80% of the integrons. The relationship between integrons and conjugative plasmids is a matter of concern because it could contribute to the dissemination of antimicrobial resistance genes among different bacterial populations.

Conjugation, Genetic↗
PubMed1997

Mobile gene cassettes and integrons: moving antibiotic resistance genes in gram-negative bacteria.

In Gram-negative pathogens, multiple antibiotic resistance is common and many of the known resistance genes are contained in mobile gene cassettes. Cassettes can be integrated into or deleted from their receptor elements, the integrons, or infrequently may be integrated at other locations via site-specific recombination catalysed by an integron-encoded recombinase. As a consequence, arrays of several different antibiotic resistance genes can be created. Over 40 gene cassettes and three distinct classes of integrons have been identified to date. Cassette-associated genes conferring resistance to beta-lactams, aminoglycosides, trimethoprim, chloramphenicol, streptothricin and quaternary ammonium compounds used as antiseptics and disinfectants have been found. In addition, most members of the commonest family of integrons (class 1) include a sulfonamide resistance determinant in the backbone structure. Integrons are themselves translocatable, though most are defective transposon derivatives. Integron movement allows transfer of the cassette-associated resistance genes from one replicon to another or into another active transposon which facilitates spread of integrons that are transposition defective. Horizontal transfer of the resistance genes can be achieved when an integron containing one or more such genes is incorporated into a broad-host-range plasmid. Likewise, single cassettes integrated at secondary sites in a broad-host-range plasmid can also move across species boundaries.

Anti-Bacterial Agents↗
PubMed1998

Antibiotic resistance in gram-negative bacteria: the role of gene cassettes and integrons.

Resistance of gram-negative organisms to antibiotics such as beta-lactams, aminoglycosides, trimethoprim and chloramphenicol is caused by many different acquired genes, and a substantial proportion of these are part of small mobile elements known as gene cassettes. A gene cassette consists of the gene and a downstream sequence, known as a 59-base element (59-be), that acts as a specific recombination site. Gene cassettes can move into or out of a specific receptor site (attl site) in a companion element called an integron, and integration or excision of the cassettes is catalysed by a site-specific recombinase (Intl) that is encoded by the integron. At present count there are 40 different cassette-associated resistance genes and three distinct classes of integron, each encoding a distinct Intl integrase. The same cassettes are found in all three classes of integron, indicating that cassettes can move freely between different integrons. Integrons belonging to class I often contain a further antibiotic resistance gene, sull, conferring resistance to sulphonamides. The sull gene is found in a conserved region (3'-CS) that is not present in all members of this class. Class I integrons of the sull type are most prevalent in clinical isolates and have been found in many different organisms. Even though most of them are defective transposon derivatives, having lost at least one of the transposition genes, they are none the less translocatable and consequently found in many different locations. The transposon Tn7 is the best known representative of class 2 integrons, and Tn7 and relatives are also found in many different species.

Journal Article↗
PubMed1999

Incidence and characterization of integrons, genetic elements mediating multiple-drug resistance, in avian Escherichia coli.

Antibiotic resistance among avian bacterial isolates is common and is of great concern to the poultry industry. Approximately 36% (n = 100) of avian, pathogenic Escherichia coli isolates obtained from diseased poultry exhibited multiple-antibiotic resistance to tetracycline, oxytetracycline, streptomycin, sulfonamides, and gentamicin. Clinical avian E. coli isolates were further screened for the presence of markers for class 1 integrons, the integron recombinase intI1 and the quaternary ammonium resistance gene qacEDelta1, in order to determine the contribution of integrons to the observed multiple-antibiotic resistance phenotypes. Sixty-three percent of the clinical isolates were positive for the class 1 integron markers intI1 and qacEDelta1. PCR analysis with the conserved class 1 integron primers yielded amplicons of approximately 1 kb from E. coli isolates positive for intI1 and qacEDelta1. These PCR amplicons contained the spectinomycin-streptomycin resistance gene aadA1. Further characterization of the identified integrons revealed that many were part of the transposon Tn21, a genetic element that encodes both antibiotic resistance and heavy-metal resistance to mercuric compounds. Fifty percent of the clinical isolates positive for the integron marker gene intI1 as well as for the qacEDelta1 and aadA1 cassettes also contained the mercury reductase gene merA. The correlation between the presence of the merA gene with that of the integrase and antibiotic resistance genes suggests that these integrons are located in Tn21. The presence of these elements among avian E. coli isolates of diverse genetic makeup as well as in Salmonella suggests the mobility of Tn21 among pathogens in humans as well as poultry.

Animals↗