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PubMed2006

Evolution of multidrug-resistant Acinetobacter baumannii isolates obtained from elderly patients with respiratory tract infections.

OBJECTIVES: To study the evolution between 1999 and 2002 and mechanisms of antibiotic resistance in a multidrug-resistant Acinetobacter baumannii clone predominant in isolates from elderly patients with respiratory tract infections. METHODS: Susceptibility to antimicrobials was determined using an agar dilution method. Bacterial clones were identified by PCR-fingerprinting and PFGE with ApaI. Carbapenemases were detected by phenotypic tests; by PCR with primers specific for bla (OXA-40), bla(IMP), bla(VIM-1) and bla(VIM-2); and by hybridization with DNA probes. Class 1 integrons were detected using PCR. RESULTS: In 1999 isolates were grouped into two main genotypes: clone I (33%) and clone II (55%). These were also detected in 2002 with a different distribution: clone I (69%), clone II (22%). Resistance to amikacin, meropenem and imipenem increased significantly in clone I over this time, whereas clone II was not affected. In 2002, the incidence of bla(OXA-40) rose to 91% in clone I isolates with some also harbouring bla(VIM-2) and bla(IMP) genes. Different class 1 integrons were detected ranging in size from 550 to 1200 bp. No relationship was found between carbapenemases and class 1 integrons. CONCLUSIONS: In elderly patients, a single clone became predominant among A. baumannii isolates, coinciding with an increase in antibiotic resistance rates. The majority of isolates harboured the bla(OXA-40) carbapenemase gene and some of them also harboured bla(VIM-2) and bla(IMP) genes. The presence of class 1 integrons also increased over time.

Acinetobacter Infections↗
PubMed2006

The tetracycline resistance determinant Tet 39 and the sulphonamide resistance gene sulII are common among resistant Acinetobacter spp. isolated from integrated fish farms in Thailand.

OBJECTIVES: To determine the genetic basis for tetracycline and sulphonamide resistance and the prevalence of class I and II integrons in oxytetracycline-resistant Acinetobacter spp. from integrated fish farms in Thailand. METHODS: A total of 222 isolates were screened for tetracycline resistance genes [tet(A), tet(B), tet(H), tet(M) and tet(39)] and class II integrons by PCR. One hundred and thirty-four of these isolates were also sulphonamide resistant and these isolates were screened for sulphonamide resistance genes (sulII and sulIII) as well as class I integrons. Plasmid extraction and Southern blots with sulII and tet(39) probes were performed on selected isolates. RESULTS: The recently identified tetracycline resistance gene tet(39) was demonstrated in 75% (166/222) of oxytetracycline-resistant Acinetobacter spp. from integrated fish farms in Thailand. Isolates that were also sulfamethoxazole-resistant contained sulII (96%; 129/134) and/or sulI (14%; 19/134) (as part of class I integrons). sulII and tet(39) were located on plasmids differing in size in the isolates tested. CONCLUSIONS: The study shows tet(39) and sulII to be common resistance genes among clonally distinct Acinetobacter spp. from integrated fish farms and these bacteria may constitute reservoirs of resistance genes that may increase owing to a selective pressure caused by the use of antimicrobials in the overlaying animal production.

Acinetobacter↗
PubMed2006

Molecular evolution of metallo-beta-lactamase-producing Pseudomonas aeruginosa in a nosocomial setting of high-level endemicity.

An outbreak of multidrug-resistant Pseudomonas aeruginosa strains producing VIM-type metallo-beta-lactamases (MBLs) has occurred in an Italian hospital since 2000 (C. Lagatolla, E. A. Tonin, C. Monti-Bragadin, L. Dolzani, F. Gombac, C. Bearzi, E. Edalucci, F. Gionechetti, and G. M. Rossolini, Emerg. Infect. Dis. 10:535-538, 2004). In this work, using molecular methods, we characterized 128 carbapenem-resistant isolates (including 98 VIM-positive isolates) collected from that hospital from 2000 to 2002 to investigate the dynamics of the dissemination of MBL producers in the clinical setting. Genotyping by random amplification of polymorphic DNA and pulsed-field gel electrophoresis showed that most VIM-positive isolates belonged to two different clonal lineages, producing either a VIM-1- or a VIM-2-like MBL, whose ancestors were detected for the first time in the hospital in 1999, suggesting that clonal expansion played a predominant role in the dissemination of these isolates. The 86 clonally related isolates carrying a blaVIM-1-like gene on an In70-like integron were clearly related to a VIM-1-positive P. aeruginosa clone circulating in various Italian hospitals since the late 1990s. VIM-negative P. aeruginosa strains related to the VIM-1-positive clone were detected during the same period, suggesting that the latter strain was derived from a clonal lineage already circulating in the hospital. In the VIM-2-like positive clone, the MBL gene was carried by an unusual class 1 integron, named In71, lacking the 3' conserved sequence region typical of sul1-associated integrons. A different class 1 integron with an original structure carrying a blaVIM-2 determinant, named In74, was detected in a sporadic isolate. A retrospective investigation did not reveal the presence of strains related to any of the VIM-producing isolates earlier than 1997.

Blotting, Southern↗
PubMed1997

The resistance and integrase genes of pACM1, a conjugative multiple-resistance plasmid, from Klebsiella oxytoca.

pACM1 is an 85-kb conjugative plasmid from a clinical isolate of Klebsiella oxytoca that encodes resistance to beta-lactams (mediated by SHV-5 extended spectrum beta-lactamase), trimethoprim, sulfonamides, tetracycline, aminoglycosides, and mercuric chloride. The expression of the aminoglycoside resistance is difficult to detect, which could have clinical implications. A region of pACM1 containing five resistance genes and two putative integrons was characterized by restriction mapping and partial DNA sequencing. One integron appears to be class I (sull type); the second lacks a recognizable 3' conserved segment. Neither integron has the BamHI site predicted for the 5' conserved segment. Plasmids encoding SHV-5 from other bacterial strains appear to be closely related to pACM1 by restriction enzyme analysis, but have resistance/ integron regions that vary in size and content from that of pACM1. Integrase-mediated recombination might be responsible for genetic divergence in a widely distributed family of pACM1-like plasmids.

Amino Acid Sequence↗
PubMed2004

Novel GES/IBC extended-spectrum beta-lactamase variants with carbapenemase activity in clinical enterobacteria.

Two clinical isolates, an Escherichia coli and a Klebsiella pneumoniae, with decreased susceptibility to carbapenems were studied. This phenotype was associated with production of novel GES/IBC variant beta-lactamases, designated GES-3 (from E. coli) and GES-4 (from K. pneumoniae), exhibiting carbapenemase activity. Both enzymes possessed Ser at Ambler's position 170 instead of Gly found in the beta-lactamases GES-1 and IBC-1 that lack carbapenemase activity. Additionally, position 104 in GES-4 was occupied by a Lys as in IBC-1. bla(GES-3) and bla(GES-4) occurred as gene cassettes in the variable regions of class 1 integrons carried by plasmids. The structure of the GES-4-encoding integron was similar to that of previously described IBC-1 integrons. The GES-3-encoding integron was, most likely, truncated at the 3' conserved segment.

Anti-Bacterial Agents↗
PubMed2002

Molecular epidemiology of cystic fibrosis-linked Burkholderia cepacia complex isolates from three national referral centres in Ireland.

AIMS: Burkholderia cepacia is a Gram-negative bacterium associated with increasing morbidity and mortality and is readily transmitted among infected cystic fibrosis (CF) patients. The B. cepacia complex consists of five distinct subgroups, termed genomovars. A collection of 17 presumptive B. cepacia isolates, obtained from three national CF referral centres located in different geographical regions in Ireland, was studied. The aim of this study was to investigate these isolates using molecular subtyping protocols for evidence of genetic relationships and for the presence of antibiotic resistance-encoding class 1 integron structures. METHODS AND RESULTS: Genomovar classifications were assigned to each isolate based on HaeIII enzyme profiles of their recA locus. Genetic relationships among this collection were also assessed after restriction fragment length polymorphism (RFLP)-mediated analysis of the 16S rDNA locus and DNA amplification fingerprinting (DAF). The surface expression of the cable pilus gene (cblA) may facilitate an early step in the infection process. All isolates were tested by amplification strategies for this marker. Burkholderia cepacia is known to be resistant to several antimicrobial agents. Resistance typing showed that the majority were resistant to three or more common antimicrobial agents. Five of the 17 isolates were resistant to sulphonamide, a characteristic linked with the presence of class 1 integrons. Gene cassettes containing beta-lactamase (oxa) and aminoglycoside acetyltransferase (aac(6')-1a) encoding genes were identified by polymerase chain reaction amplification. CONCLUSIONS: Most of the isolates in this study were classified as genomovar III and were indistinguishable based on their corresponding 16S rDNA-RFLP profiles, whilst DAF further subtyped the collection. The cblA marker was identified in 47% of the isolates, many of which clustered in the genomovar III group. Class 1 integrons with recombined gene cassettes containing bla-OXA and aac(6')-1a genes were identified. SIGNIFICANCE AND IMPACT OF THE STUDY: This study demonstrates the application of molecular methods to investigate B. cepacia, a well-recognized human pathogen, cultured from Irish CF patients. Genomovar III was the most common genomic type identified. DNA fingerprinting further subtyped the latter isolates, facilitating a more detailed description of the molecular epidemiology. Drug resistance in these organisms can be explained, at least in part, by the presence of class 1 integrons. Development of targeted infection control strategies could be facilitated using these applied methods.

Amino Acid Sequence↗
PubMed2002

Characterization of antimicrobial resistance among Escherichia coli O111 isolates of animal and human origin.

Fifty isolates of Escherichia coli serogroup O111 recovered from humans and various animal species over a 24-year period (1976-1999) were examined for typical virulence-associated factors and susceptibilities to antimicrobials of human and veterinary significance. Nine H (flagellar) types were identified including nonmotile (n = 24), 32 (n = 12), negative (n = 5), and 56 (n = 3). Thirty-five (70%) isolates possessed at least one Shiga-toxin-producing E. coli (STEC)-associated virulence determinants (eae, stxl, stx2, hlyA) via PCR analysis. Of these 35 isolates, 20 possessed eae, stxl, and hlyA genes, whereas three isolates possessed eae, stxl, stx2, and hylA genes. Multiple antibiotic resistance was observed in 70% of the 50 E. coli O111 isolates. The majority of isolates displayed resistance to streptomycin, sulfamethoxazole, tetracycline, and kanamycin. Bacterial resistance to ampicillin, gentamicin, chloramphenicol, trimethoprim and apramycin was also observed. Integrons were identified in 23 (46%) of the E. coli isolates assayed, with a 1-kb amplicon being most frequently observed. DNA sequencing of these integrons revealed the presence of the aadA gene, encoding resistance to streptomycin. Two integrons of 1.5 and 2 kb contained the aadA2 and either dfrI or dfrXII genes, encoding resistance to streptomycin and trimethoprim, respectively. Integrons were also identified from isolates dating back to 1982. Isolates were further genetically characterized via ribotyping, which identified 15 distinct ribogroups, with 62% of isolates clustering into four major ribogroups. Certain riboprint patterns from different animal species, including humans, were observed in isolates spanning the 24-year collection period, suggesting the dissemination of specialized pathogenic O111 clones.

Animals↗
PubMed2001

The prevalence of trimethoprim-resistance-conferring dihydrofolate reductase genes in urinary isolates of Escherichia coli in Korea.

One-hundred and twenty-two urinary isolates of Escherichia coli were studied for trimethoprim resistance. Seventy-seven (63.1%) of the 122 isolates were found to be resistant to trimethoprim. Of the 77 trimethoprim-resistant isolates, 75 dfr genes were detected in 72 isolates as follows: the dfrA17 gene was the most prevalent, being found in 27 isolates, followed by dfrA12 in 26, dfrA1 in 15, dfrA5 in four and dfrA7 in three. Southern blot and PCR mapping analysis revealed that all of the dfrA17, dfrA12, dfrA5 and dfrA7 genes were located on class 1 integrons. The dfrA1 gene inserted as a gene cassette in class 1 integrons was found in 10 of 15 isolates, and the intI2 gene of Tn7 was detected in two out of five isolates. In conjugation experiments, the dfr genes inserted in class 1 integrons were transferred to a recipient E. coli in 32 (42.7%) of the 75 dfr genes. In conclusion, the dfrA17 and dfrA12 genes were the most prevalent genes responsible for trimethoprim resistance in urinary tract isolates of E. coli from Korea and the dfr genes inserted in integrons are more widespread than those that are not related to gene cassettes.

Anti-Bacterial Agents↗
PubMed2004

Antibiotic multiresistance plasmid pRSB101 isolated from a wastewater treatment plant is related to plasmids residing in phytopathogenic bacteria and carries eight different resistance determinants including a multidrug transport system.

Ten different antibiotic resistance plasmids conferring high-level erythromycin resistance were isolated from an activated sludge bacterial community of a wastewater treatment plant by applying a transformation-based approach. One of these plasmids, designated pRSB101, mediates resistance to tetracycline, erythromycin, roxythromycin, sulfonamides, cephalosporins, spectinomycin, streptomycin, trimethoprim, nalidixic acid and low concentrations of norfloxacin. Plasmid pRSB101 was completely sequenced and annotated. Its size is 47 829 bp. Conserved synteny exists between the pRSB101 replication/partition (rep/par) module and the pXAC33-replicon from the phytopathogen Xanthomonas axonopodis pv. citri. The second pRSB101 backbone module encodes a three-Mob-protein type mobilization (mob) system with homology to that of IncQ-like plasmids. Plasmid pRSB101 is mobilizable with the help of the IncP-1alpha plasmid RP4 providing transfer functions in trans. A 20 kb resistance region on pRSB101 is located within an integron-containing Tn402-like transposon. The variable region of the class 1 integron carries the genes dhfr1 for a dihydrofolate reductase, aadA2 for a spectinomycin/streptomycin adenylyltransferase and bla(TLA-2) for a so far unknown Ambler class A extended spectrum beta-lactamase. The integron-specific 3'-segment (qacEDelta1-sul1-orf5Delta) is connected to a macrolide resistance operon consisting of the genes mph(A) (macrolide 2'-phosphotransferase I), mrx (hydrophobic protein of unknown function) and mphR(A) (regulatory protein). Finally, a putative mobile element with the tetracycline resistance genes tetA (tetracycline efflux pump) and tetR was identified upstream of the Tn402-specific transposase gene tniA. The second 'genetic load' region on pRSB101 harbours four distinct mobile genetic elements, another integron belonging to a new class and footprints of two more transposable elements. A tripartite multidrug (MDR) transporter consisting of an ATP-binding-cassette (ABC)-type ATPase and permease, and an efflux membrane fusion protein (MFP) of the RND-family is encoded between the replication/partition and the mobilization module. Homologues of the macrolide resistance genes mph(A), mrx and mphR(A) were detected on eight other erythromycin resistance-plasmids isolated from activated sludge bacteria. Plasmid pRSB101-like repA amplicons were also obtained from plasmid-DNA preparations of the final effluents of the wastewater treatment plant indicating that pRSB101-like plasmids are released with the final effluents into the environment.

ATP Binding Cassette Transporter, Subfamily B↗
PubMed1995

Plasmid evolution by acquisition of mobile gene cassettes: plasmid pIE723 contains the aadB gene cassette precisely inserted at a secondary site in the incQ plasmid RSF1010.

Gene cassettes are mobile DNA elements which contain a specific recombination site, a 59-base element, recognized by the site-specific recombination system of integrons. Gene cassettes are normally found inserted at a unique site in an integron, downstream of a promoter which directs transcription of the cassette-associated genes. However, insertion of a gene cassette into a secondary site in a plasmid which does not contain an integron is also formally possible. Sequence analysis of the aadB gene in pIE723, a plasmid closely related to the IncQ plasmid RSF1010, revealed the presence of the complete aadB cassette inserted at a secondary site downstream of a known RSF1010 promoter. The site of insertion of the aadB cassette in RSF1010 conformed to the consensus for secondary sites recognized by the integron integrase (Int), and it is likely that the cassette was inserted via a single Int-mediated recombination event between the 59-base element of a free, circular aadB cassette and a secondary site in RSF1010. The cassette-associated recombination site was inactivated by the insertion, and Int-mediated excision of the aadB cassette from this non-specific location was not detectable, indicating that the cassette is stably inserted. The movement of gene cassettes to secondary sites is likely to play an important role in the acquisition of new genes by bacterial and plasmid genomes.

Amino Acid Sequence↗
PubMed2000

Characterization of Salmonella enterica serovar typhimurium DT104 isolated from Denmark and comparison with isolates from Europe and the United States.

A total of 136 isolates of Salmonella enterica serovar Typhimurium DT104 from Denmark (n = 93), Germany (n = 10), Italy (n = 4), Spain (n = 5), and the United Kingdom (n = 9) were characterized by antimicrobial resistance analysis, plasmid profiling, pulsed-field gel electrophoresis (PFGE) with the restriction enzymes XbaI and BlnI, and analysis for the presence of integrons and antibiotic resistance genes. The isolates from Denmark were from nine pig herds, while the isolates from other countries were both of animal and of human origin. All but 10 isolates were resistant to ampicillin, chloramphenicol, spectinomycin, streptomycin, sulfonamides, and tetracycline. Five isolates from the United Kingdom and Spain were sensitive to all antibiotics examined, whereas four isolates from the United Kingdom and the United States were also resistant to one or more of the antibiotics, namely, gentamicin, neomycin, and trimethoprim. All but two strains had the same PFGE profiles when the XbaI restriction enzyme was used, while seven different profiles were observed when the BlnI restriction enzyme was used. Different dominating BlnI types were observed among European isolates compared with the types observed among those from the United States. All the isolates harbored common 95-kb plasmids either alone or in combination with smaller plasmids, and a total of 11 different plasmid profiles were observed. Furthermore, all but one of the multidrug-resistant isolates contained two integrons, ant (3")-Ia and pse-1. Sensitive isolates contained no integrons, and isolates that were resistant to spectinomycin, streptomycin, and sulfonamides had only one integron containing ant (3")-Ia. When restriction enzyme BlnI was used, the 14 isolates from one of the nine herds in Denmark showed unique profiles, whereas isolates from the remaining herds were homogeneous. Among isolates from seven of nine herds, the same plasmid profile (95 kb) was observed, but isolates from two herds had different profiles. Thus, either PFGE (with BlnI) or plasmid profiling could distinguish isolates from three of nine pig herds in Denmark. The epidemiological markers (antimicrobial susceptibility testing, plasmid profiling, and PFGE) applied demonstrated high in vivo stability in the Danish herds. This may indicate that some different strains of multidrug-resistant S. enterica serovar Typhimurium DT104 have been introduced into Danish food animal herds. The presence of isolates from six different countries with similar profiles by PFGE with XbaI and highly homogeneous profiles by PFGE with BlnI indicate that multidrug-resistant S. enterica serovar Typhimurium DT104 has probably been spread clonally in these countries. However, some minor variation could be observed by using plasmid profiling and profiling by PFGE with BlnI. Thus, a more sensitive technique for subtyping of strains of DT104 and a broader investigation may help in elucidating the epidemiological spread of DT104 in different parts of the world.

Animals↗
PubMed2003

Characterization of Salmonella enterica serotype newport isolated from humans and food animals.

Salmonella enterica serotype Newport isolates resistant to at least nine antimicrobials (including extended-spectrum cephalosporins), known as serotype Newport MDR-AmpC isolates, have been rapidly emerging as pathogens in both animals and humans throughout the United States. Resistance to extended-spectrum cephalosporins is associated with clinical failures, including death, in patients with systemic infections. In this study, 87 Salmonella serotype Newport strains were characterized by pulsed-field gel electrophoresis (PFGE) and antimicrobial susceptibility testing and examined for the presence of class 1 integrons and bla(CMY) genes. Thirty-five PFGE patterns were observed with XbaI, and three of these patterns were indistinguishable among isolates from humans and animals. Fifty-three (60%) Salmonella serotype Newport isolates were identified as serotype Newport MDR-AmpC, including 16 (53%) of 30 human isolates, 27 (93%) of 29 cattle isolates, 7 (70%) of 10 swine isolates, and 3 (30%) of 10 chicken isolates. However, 28 (32%) Salmonella serotype Newport isolates were susceptible to all 16 antimicrobials tested. The bla(CMY) gene was present in all serotype Newport MDR-AmpC isolates. Furthermore, the plasmid-mediated bla(CMY) gene was transferable via conjugation to an Escherichia coli strain. The transconjugant showed the MDR-AmpC resistance profile. Thirty-five (40%) of the isolates possessed class 1 integrons. Sequence analyses of the integrons showed that they contained aadA, which confers resistance to streptomycin, or aadA and dhfr, which confer resistance to trimethoprim-sulfamethoxazole. One integron from a swine isolate contained the sat-1 gene, which encodes resistance to streptothricin, an antimicrobial agent that has never been approved for use in the United States. In conclusion, Salmonella serotype Newport MDR-AmpC was commonly identified among Salmonella serotype Newport isolates recovered from humans and food animals. These findings support the possibility of transmission of this organism to humans through the food chain.

Animals↗
PubMed2003

Antimicrobial resistance and genetic diversity of Shigella sonnei isolates from western Ireland, an area of low incidence of infection.

Shigella sonnei is a significant cause of gastroenteritis in both developing and industrialized countries. Definition of the diversity and antimicrobial susceptibility of S. sonnei isolates may be helpful in the management of individual cases and outbreaks. Antimicrobial susceptibility testing and pulsed-field gel electrophoresis (PFGE) were performed with 67 isolates of S. sonnei predominantly (n = 59) from three counties in the west of Ireland. Phage typing (n = 17), plasmid profiling (n = 28), and integron analysis (n = 24) were performed with subsets of strains. PFGE typing permitted recognition of two major clusters: PFGE type A (n = 53) and PFGE type B (n = 14). PFGE type A was associated with resistance to ampicillin, streptomycin, and sulfonamides (51 of 53 isolates), and those that were phage typed (n = 6) were phage type 3. PFGE type B was associated with resistance to streptomycin, sulfonamides, tetracycline, and trimethoprim (11 of 14 isolates) and phage type 6 (9 of 11 isolates). Fifteen different plasmid profiles were identified among the 28 isolates analyzed. A class 2 integron was present in all 14 PFGE type B isolates. One of these isolates also contained a class 1 integron and showed a unique variant of the PFGE type B pattern. Sequence analysis of the gene cassette structures contained within these integrons identified distinct open reading frames that encoded determinants of resistance to trimethoprim, streptomycin, and streptothricin. Our data demonstrate two predominant PFGE types among S. sonnei isolates circulating in this region. The limited diversity of the S. sonnei isolates in this region means that detection of isolates indistinguishable by PFGE and according to their antibiograms in two or more patients is not persuasive evidence of a common-source food- or waterborne outbreak. Indistinguishable plasmid profiles in addition to indistinguishable PFGE and antibiogram types may be more suggestive of an epidemiologically relevant link between cases.

Bacterial Typing Techniques↗
PubMed2004

Molecular epidemiology of sequential outbreaks of Acinetobacter baumannii in an intensive care unit shows the emergence of carbapenem resistance.

The molecular epidemiology of multidrug-resistant Acinetobacter baumannii was investigated in the medical-surgical intensive care unit (ICU) of a university hospital in Italy during two window periods in which two sequential A. baumannii epidemics occurred. Genotype analysis by pulsed-field gel electrophoresis (PFGE) of A. baumannii isolates from 131 patients identified nine distinct PFGE patterns. Of these, PFGE clones B and I predominated and occurred sequentially during the two epidemics. A. baumannii epidemic clones showed a multidrug-resistant antibiotype, being clone B resistant to all antimicrobials tested except the carbapenems and clone I resistant to all antimicrobials except ampicillin-sulbactam and gentamicin. Type 1 integrons of 2.5 and 2.2 kb were amplified from the chromosomal DNA of epidemic PFGE clones B and I, respectively, but not from the chromosomal DNA of the nonepidemic clones. Nucleotide analysis of clone B integron identified four gene cassettes: aacC1, which confers resistance to gentamicin; two open reading frames (ORFs) coding for unknown products; and aadA1a, which confers resistance to spectinomycin and streptomycin. The integron of clone I contained three gene cassettes: aacA4, which confers resistance to amikacin, netilmicin, and tobramycin; an unknown ORF; and bla(OXA-20), which codes for a class D beta-lactamase that confers resistance to amoxicillin, ticarcillin, oxacillin, and cloxacillin. Also, the bla(IMP) allele was amplified from chromosomal DNA of A. baumannii strains of PFGE type I. Class 1 integrons carrying antimicrobial resistance genes and bla(IMP) allele in A. baumannii epidemic strains correlated with the high use rates of broad-spectrum cephalosporins, carbapenems, and aminoglycosides in the ICU during the study period.

Acinetobacter Infections↗
PubMed1999

Antibiotic resistance: genetic mechanisms and mobility.

Based on the current knowledge, resistance genes seems mainly to originate in the organisms which produce antibiotics (Davies 1994). We lack considerably in the understanding of how these genes were transferred to pathogenic bacteria, and due to the enormous diversity of e.g. the soil flora, it is doubtful that we will ever obtain more that a faint picture of this. In Gram negative bacteria, more and more resistance genes are demonstrated to be located in integrons (e.g. beta-lactamase and streptomycin resistance genes in Salmonella Typhimurium DT104 (Sandvang et al. in press)). Integrons seem primarily to act as insertion sites for resistance genes. The origin of integrons as well as the resistance gene cassettes that are the other essential element of this system, is largely unknown (Hall & Collis 1995). Integrons can be located in the chromosome, in transposons, which have the ability to copy them themselves to other DNA molecules, or on plasmids. The emergence of resistant bacteria normally happens because of selection for a resistant clone of bacteria. Several mechanisms, however, exists by which the resistance genes can be transferred from one bacteria to another. Conjugation, mediated by plasmids or conjugative transposons, is currently the most well established of these mechanisms. Still, however, the selection pressure created by the use of antibiotics determines whether bacteria that have newly acquired a resistance gene expand to dominate in the population or remains a blink in history.

Drug Resistance, Microbial↗
PubMed1997

Genetic localization of the type I trimethoprim resistance gene and its dissemination in urinary tract isolates in Taiwan.

In a total of 425 urinary isolates of E. coli, Enterobacter spp., and Klebsiella spp. selected, there were 169 (45.4%) isolates harbouring type I dihydrofolate reductase (DHFR) gene among 374 trimethoprim-resistant isolates. In these 169 isolates, only 17.2% hybridized with the Tn7 probe. According to another probe specific for the integrase gene of integron, 87.6% showed a positive reaction. Further analysis by restriction mapping proved that the type I DHFR gene was inserted into a integron-like structure. These results indicate that the type I DHFR gene that was initially observed in association with transposable element Tn7 is becoming associated with an integrase function similar to integrons in most instances. Further analysis of the distribution of Tn21-like integrase gene in clinical isolates indicated that the prevalence rates were 86.4%, 84.8%, and 76.7% respectively in E. coli, Enterobacter spp., and Klebsiella spp.. Furthermore, the integrase gene found in our clinical isolates proved to be mediated by a plasmid, demonstrated by Southern hybridization. Thus, the trimethoprim-resistant gene that developed under selective pressure from the double drug trimethoprim and sulphonamide was transmitted by insertion into integron-like structure and then mediated by plasmid transfer for dissemination.

Animals↗
PubMed2005

Characterization of Salmonella enterica serovar Typhimurium strains of veterinary origin by molecular typing methods.

Twenty-eight strains of Salmonella enterica serovar Typhimurium were characterized by three PCR-based methods. Ten strains harbored type I integrons and two different integron profiles were detected. Typing by amplified fragment length polymorphism (AFLP) resulted in observation of 10 profiles that differed by one to six bands. Salmonella strains were screened for presence of phage genes using a PCR-phage typing; five genes from P22 phage and genes encoding putative virulence factors from phages Gifsy-1, Gifsy-2 and Fels-1 were selected for testing. This set of genes was sufficient for dividing the strains into eight different PCR-phage profiles. Similar grouping of strains was observed in case of all the employed DNA techniques and they corresponded well with the phage type and antimicrobial resistance of the strains. The highest discriminating power was achieved with use of the AFLP, yet the detection of integrons and PCR-phage typing also proved to be valuable in typing the S. Typhimurium strains.

Animals↗
PubMed2005

Characterization of Salmonella Typhimurium of animal origin obtained from the National Antimicrobial Resistance Monitoring System.

Salmonella Typhimurium remains one of the most common causes of salmonellosis in animals and humans in the United States. The emergence of multi-drug resistant Salmonella reduces the therapeutic options in cases of invasive infections, and has been shown to be associated with an increased burden of illness. In this study, 588 S. Typhimurium (including var. Copenhagen) isolates obtained from either animal diagnostic specimens (n = 199) or food animals after slaughter/processing (n = 389) were examined for antimicrobial susceptibility, presence of class-1 integrons, and characterized using pulsed-field gel electrophoresis and phage typing. Seventy-six percent (448/588) of isolates were resistant to at least one antimicrobial. Salmonella isolates displayed resistance most often to streptomycin (63%), tetracycline (61%), ampicillin (61%), and to a lesser extent, chloramphenicol (36%), ceftiofur (15%), gentamicin (9%), and nalidixic acid (4%), with more resistance observed among diagnostic isolates. Salmonella recovered from turkeys (n = 38) exhibited the highest rates of resistance, with 92% of isolates resistant to least one antimicrobial, and 58% resistant to > or =10 antimicrobials. Class 1 integrons were present in 51% of all isolates. Five integron associated resistance genes (aadA, aadB, pse-1, oxa-2 and dhfr) were identified. A total of 311 PFGE patterns were generated using XbaI, indicating a genetically diverse population. The largest PFGE cluster contained 146 isolates, including DT104 isolates obtained from all seven animal species. Results demonstrated a varied spectrum of antimicrobial resistance, including several multidrug resistant clonal groups, among S. Typhimurium and S. Typhimurium var. Copenhagen isolates recovered from both diagnostic and slaughter/processing samples.

Animals↗