Prevalence and characterization of class 1 and class 2 integrons in Escherichia coli isolated from meat and meat products of Norwegian origin

OBJECTIVES: The aim of the study was to investigate the prevalence of integrons and to characterize inserted gene cassettes in Escherichia coli isolated from meat and meat products of Norwegian origin. METHODS: The strains investigated (n = 241 resistant out of 944 investigated) were collected within the frame of the Norwegian monitoring programme for antimicrobial resistance in bacteria from feed, food and animals (NORM-VET) during the years 2000-2003. PCR and DNA sequencing were used for detection of the integrase genes and gene cassettes within the integrons. RESULTS: Integrons were detected in 43 (18%) of the 241 resistant isolates. Class 1 integrons were detected in 29 (12%) strains and class 2 integrons were detected in 14 (6%) strains. Ten different gene cassettes were detected: dfrA1, dfr2a, dfrA12, aadA1, aadA2, catB2, oxa-30, sat, sat1 and orfF. The dfrA1 + aadA1 combination was the most prevalent cassette combination, detected in 12 of 29 class 1 integrons. Twelve (of 14) class 2 integrons contained a cassette area consistent with that on Tn7, the remaining two contained the cassettes sat + sat1 + aadA1. Nearly one-third of the class 1 integrons (9 of 29) lacked the sul1 gene. Ten gene cassettes (one dfr2a, two catB2 and seven aadA1) were expressed at levels below breakpoint values normally used to classify strains as resistant. CONCLUSIONS: Integrons of class 1 or 2 were present in approximately 18% of the resistant E. coli strains investigated. Certain cassette combinations in class 1 integrons seem to be more widespread than others, like the dfrA1 + aadA1. Low-level expression of antimicrobial resistance, caused by the expression of certain gene cassettes in some integrons represents an obstacle in classifying strains as susceptible or resistant.     

Distribution and content of class 1 integrons in different Vibrio cholerae O-serotype strains isolated in Thailand

In this study, 176 clinical and environmental Vibrio cholerae strains of different O serotypes isolated in Thailand from 1982 to 1995 were selected and studied for the presence of class 1 integrons, a new group of genetic elements which carry antibiotic resistance genes. Using PCR and DNA sequencing, we found that 44 isolates contained class 1 integrons harboring the aadB, aadA2, blaP1, dfrA1, and dfrA15 gene cassettes, which encode resistance to gentamicin, kanamycin, and tobramycin; streptomycin and spectinomycin; beta-lactams; and trimethoprim, respectively. Each cassette array contained only a single antibiotic resistance gene. Although resistance genes in class 1 integrons were found in strains from the same epidemic, as well as in unrelated non-O1, non-O139 strains isolated from children with diarrhea, they were found to encode only some of the antibiotic resistance expressed by the strains. Serotype O139 strains did not contain class 1 integrons. However, the appearance and disappearance of the O139 serotype in the coastal city Samutsakorn in 1992 and 1993 were associated with the emergence of a distinct V. cholerae O1 strain which contained the aadA2 resistance gene cassette. A 150-kb self-transmissible plasmid found in three O1 strains isolated in 1982 contained the aadB gene cassette. Surprisingly, several strains harbored two integrons containing different cassettes. Thus, class 1 integrons containing various resistance gene cassettes are distributed among different V. cholerae O serotypes of mainly clinical origin in Thailand.     

The prevalence of, associations between and conjugal transfer of antibiotic resistance genes in Escherichia coli isolated from Norwegian meat and meat products

OBJECTIVES: To investigate the distribution of, associations between and the transferability of antimicrobial resistance genes in resistant Escherichia coli strains isolated from Norwegian meat and meat products. METHODS: The 241 strains investigated were collected within the frame of the Norwegian monitoring programme for antimicrobial resistance in bacteria from feed, food and animals (NORM-VET) during the years 2000-2003. PCR was carried out for detection of resistance genes. Conjugation experiments were carried out with the resistant isolates from meat as donor strains and E. coli DH5alpha as the recipient strain. Statistical analyses were performed with the SAS-PC-System version 9.1 for Windows. RESULTS: Resistance genes common in pathogenic E. coli were frequently found among the isolates investigated. Strains harbouring several genes encoding resistance to the same antimicrobial agent were significantly (P < 0.0001) more frequently multiresistant than others. Strong positive associations were found between the tet(A) determinant and the genetic elements sul1, dfrA1 and aadA1. Negative associations were found between resistance genes encoding resistance to the same antimicrobial agent: tet(A)/tet(B), sul1/sul2 and strA-strB/aadA1. The resistance genes were successfully transferred from 38% of the isolates. The transfer was more frequent from resistant isolates harbouring class 1 integrons (P < 0.001). CONCLUSIONS: Acquired resistance played a major role in conferring resistance among the isolates investigated. The possibility of transferring resistance increases both by increased multiresistance and by the presence of class 1 integrons. The conjugation experiments suggest that tet(A) and class 1 integrons are often located on the same conjugative plasmid.     

Molecular characterization of class 1 integrons and antimicrobial resistance in Aeromonas strains from foodborne outbreak-suspect samples and environmental sources in Taiwan

One hundred thirty-three Aeromonas spp. isolates were examined for multiple antibiotic resistance phenotypes and prevalence of class 1 integron sequences. Twenty-four (18.0%) of these isolates contained class 1 integron. Seven different class 1 integrons were found among 24strains, with a total of 10 different gene cassettes encoding for resistance to trimethoprim (dfr12 and dfr2d), aminoglycosides (aadA1 and aadA2), beta-lactam antibiotics (oxa2), chloramphenicol (catB3 and catB8), quaternary ammonium amines (qacE2), and 2 ORFs (orfD and orfF) with unknown function. Rate of antibiotic resistance was different between integron-positive and integron-negative strains. Trimethoprim and trimethoprim-sulphamethoxazole resistances were commonly associated with integron, and all of integron-positive isolates were multiple resistant to more than 3 agents. Resistance to as many as 10 antimicrobial agents were observed in integron-positive strains. Several cassette arrays of class 1 integrons identified in this study were not previously reported in Aeromonas strains. This study demonstrates the wide distribution of class 1 integron in Aeromonas spp. isolated from foodborne outbreak-suspect samples and environmental sources in Taiwan.     

The role of horizontal gene transfer in the spread of trimethoprim-sulfamethoxazole resistance among uropathogenic Escherichia coli in Europe and Canada

OBJECTIVES: To describe the distribution of trimethoprim-sulfamethoxazole resistance genes and the role of horizontal gene transfer and clonal expansion in recent increases of antibiotic resistance rates among uropathogenic Escherichia coli in Europe and Canada. METHODS: We identified antibiotic resistance alleles sul1, sul2, sul3 and dfr along with type 1 and type 2 integrons among 350 uropathogenic E. coli isolates from a cross-sectional study of acute, uncomplicated, community-acquired urinary tract infections in 16 western European countries and Canada (ECOSENS). RESULTS: Trimethoprim resistance gene distributions showed no regional dependency (P = 0.84). The most common trimethoprim resistance gene was dfrA1, which occurred in 37.9% of dfr containing isolates. Similarly, the sulfamethoxazole resistance gene distributions did not vary significantly by region (P = 0.20). sul2, the most common sulfamethoxazole resistance gene, was found in 77.9% of sulfamethoxazole-resistant isolates. The distribution of type 1 and type 2 integrons varied slightly by region (P = 0.04) with type 1 integrons being the more common (85.9%). We observed 34 combinations of the sul genes, dfr genes and integron types; the most common combinations were broadly disseminated across every region examined. CONCLUSIONS: Horizontal gene transfer plays a larger role than clonal expansion in the increase of trimethoprim-sulfamethoxazole resistance levels in Europe and Canada.     

Class 1 integron-associated gene cassettes in Salmonella enterica subsp. enterica serovar Agona isolated from pig carcasses in Brazil

OBJECTIVES: Two multiresistant Salmonella enterica subsp. enterica serovar Agona isolates from pig carcasses were investigated for antimicrobial resistance genes and their location with particular reference to the detection of class 1 integrons. METHODS: The two S. Agona isolates were investigated for their in vitro susceptibility to antimicrobial agents and their plasmid content. The resistance genes and class 1 amplicons were identified by PCR assays. Amplicons of class 1 integrons were cloned and sequenced. Transferability of resistance plasmids was confirmed by conjugation. RESULTS: Both S. Agona isolates carried conjugative plasmids of approximately 150 kb which harboured all resistance genes detected in the respective isolates. S. Agona 231 was resistant to chloramphenicol by catA1, to tetracycline and minocycline by tet(B), and to sulphonamides by sul1. In addition, it harboured a streptomycin resistance gene strA and a class 1 integron with a new aadA variant designated aadA23, which mediates resistance to streptomycin and spectinomycin. S. Agona 242 also carried the genes catA1, tet(B), and sul1. Moreover, it harboured a second sulphonamide resistance gene, sul2, and a class 1 integron with intact gene cassettes carrying new variants of the trimethoprim resistance gene dfrA15b or the chloramphenicol resistance gene cmlA4. The third gene cassette consisted of a truncated aadA2 gene. CONCLUSIONS: The results of this study show that large conjugative multiresistance plasmids are present in S. Agona from pigs. Analysis of the class 1 integrons revealed the presence of new variants of resistance genes so far not detected in Salmonella isolates.     

Identification and characterization of class 1 integrons in bacteria from an aquatic environment.

In a survey of 3000 Gram-negative bacteria isolated from an estuarine environment over a 2 month period, the incidence of class 1 integrons was determined to be 3.6%. Of 85 integrons studied further, 11 lacked both the qacEdelta1 and sull genes usually present in the 3' conserved segment of the integron. The qacEdelta1 and sull genes were identified in the 3' conserved segment of 36 integrons. The remaining 38 integrons lacked a sull gene but contained a qacE gene. The variable region of 74 integrons was characterized by PCR and sequence analysis. Forty of the integrons were found to lack integrated gene cassettes, although 21 of these 'empty' integrons were shown to contain inserted DNA which has been tentatively identified as a novel insertion sequence (IS) element. Of the 34 integrons which contained inserted gene cassettes, the aadA1a gene was found to be the most prevalent (74%). Nineteen integrons contained additional or other gene cassettes in their variable region, including those encoding resistance to trimethoprim (dfr1a, dfrIIc, dfrV, dfrVII, dfrXII), chloramphenicol (catB3, catB5), aminoglycosides (aadA2, aacA4, aacC1), beta-lactamases (oxa2) and erythromycin (ereA). This study confirms the occurrence of integrons in bacteria from a natural habitat and suggests that in the absence of continued antibiotic selective pressures, integrons which persist appear to preferentially exist without integrated antibiotic resistance gene cassettes.     

Antibiotic resistance genes, integrons and multiple antibiotic resistance in thirty-five serotypes of Salmonella enterica isolated from humans and animals in the UK

OBJECTIVES: To examine 397 strains of Salmonella enterica of human and animal origin comprising 35 serotypes for the presence of aadB, aphAI-IAB, aadA1, aadA2, bla(Carb(2)) or pse1, bla(Tem), cat1, cat2, dhfr1, floR, strA, sul1, sul2, tetA(A), tetA(B) and tetA(G) genes, the presence of class 1 integrons and the relationship of resistance genes to integrons and antibiotic resistance. RESULTS: Some strains were resistant to ampicillin (91), chloramphenicol (85), gentamicin (2), kanamycin (14), spectinomycin (81), streptomycin (119), sulfadiazine (127), tetracycline (108) and trimethoprim (45); 219 strains were susceptible to all antibiotics. bla(Carb(2)), floR and tetA(G) genes were found in S. Typhimurium isolates and one strain of S. Emek only. Class 1 integrons were found in S. Emek, Haifa, Heidelberg, Mbandaka, Newport, Ohio, Stanley, Virchow and in Typhimurium, mainly phage types DT104 and U302. These strains were generally multi-resistant to up to seven antibiotics. Resistance to between three and six antibiotics was also associated with class 1 integron-negative strains of S. Binza, Dublin, Enteritidis, Hadar, Manhattan, Mbandaka, Montevideo, Newport, Typhimurium DT193 and Virchow. CONCLUSION: The results illustrate specificity of some resistance genes to S. Typhimurium or non- S. Typhimurium serotypes and the involvement of both class 1 integron and non-class 1 integron associated multi-resistance in several serotypes. These data also indicate that the bla(Carb(2)), floR and tetA(G) genes reported in the SG1 region of S. Typhimurium DT104, U302 and some other serotypes are still predominantly limited to S. Typhimurium strains.     

Characterization of sulphonamide resistance genes and class 1 integron gene cassettes in Enterobacteriaceae, Central African Republic (CAR)

OBJECTIVES: The aim of this study was to characterize genes encoding sulphonamide resistance and gene cassettes associated with class 1 integrons in trimethoprim-sulphamethoxazole resistant Enterobacteriaceae recovered from Bangui, Central African Republic (CAR). METHODS: We studied 78 clinical Enterobacteriaceae isolates, including 16 extended-spectrum beta-lactamases producers, 10 Salmonella and 9 Shigella, resistant to trimethoprim-sulphamethoxazole as assessed by the disc diffusion method. PCR was used to test for sul1 and sul2 genes. Class 1 integron resistance gene cassettes were characterized by directly sequencing PCR products obtained with primers recognising 5' and 3' conserved regions. RESULTS: The sul1 gene was found in 67 isolates, the sul2 gene in 72 isolates and both genes in 62 isolates, while the int1 gene was found in 74 isolates. The most prevalent dfr genes were dfrA7 (49%), dfrA1 (17%) and dfrA2d (13%). CONCLUSION: These results illustrate the wide distribution of sulphonamide and trimethoprim resistance genes among Enterobacteriaceae in Bangui (CAR).     

Molecular characterization of the integrons in Shigella strains isolated from patients with traveler's diarrhea

The prevalence and characterization of Class 1 integrons has been performed in eighty three strains of Shigella spp., isolated between 1995 and 2000 from patients with traveler's diarrhea. A low prevalence (13.25%) was recorded. Nine different integrons were found among 11 multiresistant strains, with a total of 10 different gene cassettes encoding for resistance to trimethoprim (dfrA1, dfrA5, dfrA7, dfrA12, and dfrA15), aminoglycosides (aadA1a and aadA2), beta-lactam antibiotics (oxa2) or ORF with unknown function (orfD and orfF). A high prevalence of dfr and aad gene cassettes was observed. The low incidence of Class 1 integrons observed in this study is in contrast with the known facility that the Shigella genus has to gain and transfer plasmids.     

产质粒介导AmpC酶大肠埃希菌中整合子的检测

目的探讨大肠埃希菌产质粒介导AmpC酶对抗生素的耐药情况,以及整合子的存在状况,为临床合理用药提供依据。方法收集河南省人民医院临床分离大肠埃希菌,对头孢西丁耐药株以酶提取物改良三维试验筛选高产AmpC酶株;多重PCR和DNA测序分析质粒AmpC基因型;用PCR-限制片段长度多态性（RFLP）初筛整合子并分型,PCR扩增整合子的可变区并测序。结果 126株大肠埃希菌中,检出产质粒介导AmpC酶14株（26.9%）,对筛出的耐药菌株采用琼脂对倍稀释法检测耐药性;多重PCR扩增分析示：其基因型均为CMY-2型。8株耐药菌株检出1类整合子,序列分析显示存在aadA4、aadA5、aadB、cm lA、cm lA1、和dfr17基因盒,它们分别介导对氨基糖苷类抗生素、氯霉素和甲氧苄明的耐药。结论治疗产质粒介导AmpC酶的细菌感染时,应以第四代头孢菌素和碳青霉烯类抗生素为首选。我院大肠埃希菌临床分离株产质粒介导AmpC率较高,其基因型为CMY-2型,而相关耐药基因盒并未位于整合子上。     

Occurrence of integrons and antimicrobial resistance genes among Salmonella enterica from Brazil

OBJECTIVES: To determine the occurrence of antimicrobial resistance genes and role of integrons among 135 antimicrobial-resistant Salmonella enterica from Brazil. METHODS: The presence of antimicrobial resistance genes, class 1 and 2 integrons and gene cassettes was analysed by PCR and sequencing. The genetic location of class 1 integrons was determined in 25 isolates by hybridization and plasmid transfer experiments. RESULTS: Fifty-five of the isolates were positive for class 1 integrons. Integron-positive isolates represented 17 different serovars and were mainly from human (n=28) and animal (n=13) sources. The gene cassette arrangements could be determined in 51 of the positive isolates, which harboured one [dfrA22, aadA1 or orf3 (putative trimethoprim resistance)], two [aadA1-dfrA1, aac(6')-Ib-orf1 (unknown function) or aacA4-aadA1], three [dfrA15b-cmlA4-aadA2, orf2 (unknown function)-dfrA5-orfD] or four [orf4-aacA4-blaOXA-30 (interrupted by an IS1 element)-aadA1] cassettes in their variable region. Only one isolate harboured a class 2 integron with the gene cassette array dfrA1-sat-aadA1. Several integron unrelated resistance genes were also detected in the isolates. Sulphonamide resistance was primarily mediated by sul2 and sul3, tetracycline resistance by tet(B) and tet(A), chloramphenicol resistance by catA1, streptomycin resistance by strA and ampicillin resistance by blaTEM. blaCTX and blaCMY-2 were found in cephalosporin-resistant isolates. Mating and hybridization experiments demonstrated that a high-molecular-weight plasmid mediated the gene transfer of integrons and additional resistance determinants. CONCLUSIONS: The present study revealed that integron-mediated resistance genes contributed to the multiresistance phenotype observed in the isolates, but most resistance genes were located outside the integron structure, as independent genes. However, they might be located on the same conjugative plasmid.     

New gene cassettes for trimethoprim resistance, dfr13, and Streptomycin-spectinomycin resistance, aadA4, inserted on a class 1 integron

In a previous survey of 357 trimethoprim-resistant isolates of aerobic gram-negative bacteria from commensal fecal flora, hybridization experiments showed that 25% (90 of 357) of the isolates failed to hybridize to specific oligonucleotide probes for dihydrofolate reductase types 1, 2b, 3, 5, 6, 7, 8, 9, 10, and 12. Subsequent cloning and sequencing of a plasmid-borne trimethoprim resistance gene from one of these isolates revealed a new dihydrofolate reductase gene, dfr13, which occurred as a cassette integrated in a site-specific manner in a class 1 integron. The gene product shared 84% amino acid identity with dfr12 and exhibited a trimethoprim inhibition profile similar to that of dfr12. Gene probing experiments with an oligonucleotide probe specific for this gene showed that 12.3% (44 of 357) of the isolates which did not hybridize to probes for other dihydrofolate reductases hybridized to this probe. Immediately downstream of dfr13, a new cassette, an aminoglycoside resistance gene of the class AADA ?ANT(3")(9)-I, which encodes streptomycin-spectinomycin resistance, was identified. This gene shares 57% identity with the consensus aadA1 (ant(3")-Ia) and has been called aadA4 (ant(3")-Id). The 3' end of the aadA4 cassette was truncated by IS26, which was contiguous with a truncated form of Tn3. On the same plasmid, pUK2381, a second copy of IS26 was associated with sul2, which suggests that both integrase and transposase activities have played major roles in the arrangement and dissemination of antibiotic resistance genes dfr13, aadA4, bla(TEM-1), and sul2.     

Phenotypic and genotypic characterization of antimicrobial resistance in Escherichia coli O111 isolates

OBJECTIVES: The aim of this study was to generate baseline data on the prevalence and molecular basis of antimicrobial resistance in Escherichia coli O111 isolates. METHODS: A total of 105 epidemiologically unrelated E. coli O111 isolates from humans and cattle (isolated between 1983 and 2003) were tested for susceptibility to 17 antimicrobial agents by broth microdilution. Resistant isolates were screened by molecular methods for resistance genes, class 1 and 2 integrons and mutations in the quinolone-resistance determining regions. RESULTS: Resistance was found in 76% of the isolates, with a prevalence of 72% for multiresistance. The most prevalent resistances were to streptomycin, sulfamethoxazole and tetracycline (72-68%), followed by spectinomycin, ampicillin and kanamycin/neomycin (39-25%). For each antimicrobial agent, the predominant resistance genes were ampicillin, bla(TEM) (94%); chloramphenicol, catA1 (100%); gentamicin, aac(3)-IV and aac(3)-II (50% each); kanamycin, aphA1 (100%); streptomycin, aadA1- like (66%); sulfamethoxazole, sul1 (59%); tetracycline, tet(A) (86%); and trimethoprim, dfrA1-like (83%). Class 1 integrons were found in 41% of the isolates. They carried aadA1, dfrA1-aadA1 and dfrA15-aadA1. A class 2 integron (dfrA1-sat1-aadA1) was found in one isolate. Only three isolates (3%) were resistant to nalidixic acid (reduced susceptibility to ciprofloxacin), with a single mutation in the gyrA gene. CONCLUSIONS: E. coli O111 strains exhibit a wide repertoire of genetic elements to sustain antimicrobial pressure. Two specific antimicrobial resistance pheno/genotypes, [STR-SPT]-SUL-TET/aadA1-sul1-tet(A) and STR-SUL-TET-AMP-[KAN-NEO]/strA/B-sul2-tet(A)-bla(TEM)-aphA1, are predominant.     

Dissemination of sulfonamide resistance genes (sul1, sul2, and sul3) in Portuguese Salmonella enterica strains and relation with integrons

In 200 sulfonamide-resistant Portuguese Salmonella isolates, 152 sul1, 74 sul2, and 14 sul3 genes were detected. Class 1 integrons were always associated with sul genes, including sul3 alone in some isolates. The sul3 gene has been identified in isolates from different sources and serotypes, which also carried a class 1 integron with aadA and dfrA gene cassettes.     

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.     

Molecular characterization of integrons in non-typhoid Salmonella serovars isolated in Japan: description of an unusual class 2 integron

OBJECTIVES: To characterize class 1 and class 2 integrons among non-typhoid Salmonella enterica serovars in Japan, and also to monitor the spread of the multidrug-resistant S. enterica serovar Typhimurium DT104. METHODS: A total of 105 Salmonella isolates were included in this study. The broth microdilution method was used to determine the MIC values of a range of antibiotics for these isolates. PCR and DNA sequencing were used for screening and characterization of class 1 and class 2 integrons. RESULTS: PCR sequencing analysis revealed the presence of seven profiles of class 1 integrons in addition to a new type of class 2 integron. The identified gene cassettes within class 1 integrons were as follows; aadA1, aadA2 and aadA5, which confer resistance to streptomycin and spectinomycin; aadB, which confers resistance to gentamicin, kanamycin and tobramycin; dfrA1 and dfrA17, which confer resistance to trimethoprim; bla(PSE-1), which confers resistance to ampicillin; catB3, which confers resistance to chloramphenicol; and sat1, which confers resistance to streptothricin. Two strains of the multidrug-resistant S. Typhimurium DT104 were characterized in this study. DNA sequencing of class 2 integrons identified one with an unusual array of gene cassettes, sat, sat1 and aadA1. CONCLUSIONS: In this study, we characterized the antibiotic resistance gene cassettes within class 1 integrons in different isolates of non-typhoid Salmonella serovars, and we also identified a new type of class 2 integron.     

Molecular characterization of plasmids encoding CTX-M-15 beta-lactamases from Escherichia coli strains in the United Kingdom

OBJECTIVES: The UK, like other countries worldwide, has a growing problem with CTX-M beta-lactamase-producing Escherichia coli. Five major clonally related strains have been identified among CTX-M-15 producers. We characterize here the plasmids from clonal strains A and D. METHODS: Plasmids were extracted and transformed into E. coli DH5alpha; conjugative mating was attempted on agar. MICs were determined by agar dilution. beta-Lactamases were typed by isoelectric focusing; antibiotic resistance genes and integrons were identified by PCR and sequenced. Plasmid incompatibility groups were determined by replicon PCR. RESULTS: bla(CTX-M-15) was carried by a 150 kb plasmid in strain A and a 70 kb plasmid in strain D. Conjugative transfer of cefotaxime resistance was only achieved from strain D; plasmids from both strains were transferred by transformation. The plasmid from strain A additionally carried bla(TEM-1) (variably), bla(OXA-1), aac(6')-Ib-cr and tet(A), as well as a class 1 integron with the gene cassettes aadA5 and dfr(17); the plasmid from strain D carried bla(TEM-1) consistently, also bla(OXA-1), aac(6')-Ib-cr, aac3-IIa and tet(A). Both plasmids belonged to incompatibility group FII. CONCLUSIONS: bla(CTX-M-15) was plasmid-mediated in both strains A and D and was linked to other antibiotic resistance genes including aac(6')-Ib-cr, which encodes an acetyltransferase, not previously found in Europe, acting on both aminoglycosides and some fluoroquinolones. Although the plasmids from the two strains differed in size, both were related and conferred similar multi-drug resistance phenotypes, suggesting that they may share a similar genetic scaffold. Both shared features with plasmids encoding CTX-M-15 beta-lactamases in E. coli from Canada and India.     

Antimicrobial resistance and spread of class 1 integrons among Salmonella serotypes

The resistance profiles, for 15 antimicrobial agents, of 333 Salmonella strains representing the most frequent nontyphoidal serotypes, isolated between 1989 and 1998 in a Spanish region, and 9 reference strains were analyzed. All strains were susceptible to amikacin, ceftazidime, ciprofloxacin, and imipenem, and 31% were susceptible to all antimicrobials tested. The most frequent types of resistance were to sulfadiazine, tetracycline, streptomycin, spectinomycin, ampicillin, and chloramphenicol (ranging from 46 to 22%); 13% were resistant to these six drugs. This multidrug resistance pattern was found alone or together with other resistance types within serotypes Typhimurium (45%), Panama (23%), and Virchow (4%). Each isolate was also screened for the presence of class 1 integrons and selected resistance genes therein; seven variable regions which carried one (aadA1a, aadA2, or pse-1) or two (dfrA14-aadA1a, dfrA1-aadA1a, oxa1-aadA1a, or sat1-aadA1a) resistance genes were found in integrons.