The trimethoprim resistance transposon Tn7 contains a cryptic streptothricin resistance gene

Detection of Tn7-Like Transposons and Antibiotic Resistance in Enterobacterales From Animals Used for Food Production With Identification of Three Novel Transposons Tn6813, Tn6814, and Tn6765

Enterobacterales are widely distributed in the gastro-intestinal system of animals and may cause opportunistic infections. Worse still, multidrug-resistant Enterobacterales also poses a serious threat to public health. Tn7-like transposons have been found in several species of the Enterobacterales order and play an important role in dissemination of antibiotic resistance. This study aimed to investigate the distribution and genetic characterization of Tn7-like transposons in Enterobacterales isolates from food animals and their association with antibiotic resistance. Enterobacterales isolated from the samples were identified and classified according to the 16S rDNA sequence. Tn7-like transposons and associated integrons were detected by polymerase chain reaction (PCR) and sequencing. The antibiotic resistance of each Tn7-like transposon positive isolate was detected according to the Kirby-Bauer disk diffusion method. Then, six representative strains were selected to study the genetic environment by whole-genome sequencing (WGS). In total, we isolated 377 Tn7-like transposons positive strains of Enterobacterales. Class 2 integrons were detected in 99.5% of the isolates, and there were high frequency mutation sites especially in base 535, a stop mutation. Many isolates (54.9%) were multidrug-resistant and observed high resistance rates to trimethoprim/sulfamethoxazole and streptomycin. Among these strains, we found three new types of Tn7-like transposons, named Tn6813, Tn6814, and Tn6765. This is the first comprehensive survey that shows Tn7-like transposons in Enterobacterales from animals used for food production in different regions of China. This study also provides an insight into the horizontal transfer of resistance genes associated with Tn7-like transposons.

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.

Multiple-drug resistance in D-tartrate-positive Salmonella enterica serovar paratyphi B isolates from poultry is mediated by class 2 integrons inserted into the bacterial chromosome

The presence of integrons in 85 multiresistant German isolates of the predominating Salmonella enterica subsp. enterica serovar Paratyphi B dT(+) clone was investigated. All isolates possessed a chromosomally located Tn7-like class 2 integron carrying the same dfrA1-sat1-aadA1 array of gene cassettes. Only four isolates (4.7%) revealed an additional class 1 integron with two strains each containing the aadA1 or dfrA1-aadA1 gene cassettes.

Characterization of aminoglycoside resistance genes and class 1 integrons in porcine and bovine gentamicin-resistant Escherichia coli

A total of 78 gentamicin-resistant Escherichia coli strains from swine (27) and cattle (51) were characterized by phenotypic resistance, presence of selected aminoglycoside resistance genes, class 1 integrons and gene cassettes, and pulsed-field gel electrophoresis. Gentamicin resistance was mainly encoded by the ant(2'')-I gene that was found in 76% of all the strains investigated, whereas the aac(3)-IIa gene was found in 14%. The ant(2'')-I gene was predominant in strains from cattle, whereas the porcine strains contained both ant(2'')-I, aac(3)-IIa, and the aac(3)-IVa genes. The pulsed-field gel electrophoresis (PFGE) investigation indicated a close clonal relationship in half of the bovine strains whereas the remaining E. coli were unrelated. Among the E. coli investigated, 20% contained class 1 integrons. Genes encoding resistance to trimethoprim (dhfrI, dhfrIb, and dhfrVII), gentamicin, tobramycin, and kanamycin (ant(2'')-Ia streptomycin and spectinomycin (ant(3'')-Ia) and streptothricin (sat1) were identified as gene cassettes. The most prevalent gene cassettes were ant(3'')-Ia (11 isolates) and the dhfrI (nine 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.

Nucleotide sequence of the bacterial streptothricin resistance gene sat3

The nucleotide sequence of the sat3 gene which encodes resistance of enteric bacteria to the antibiotic streptothricin is reported. A protein with a molecular mass of about 23 kDa is expressed from this gene. The sat3 gene is not obviously related to any one of the streptothricin resistance determinants identified so far among Gram-negative or Gram-positive bacteria.