Streptococcus faecalis R plasmid pJH1 contains an erythromycin resistance transposon (Tn3871) similar to transposon Tn917

Enterococci as a One Health indicator of antimicrobial resistance

The rapid increase of antimicrobial-resistant bacteria in humans and livestock is concerning. Antimicrobials are essential for the treatment of disease in modern day medicine, and their misuse in humans and food animals has contributed to an increase in the prevalence of antimicrobial-resistant bacteria. Globally, antimicrobial resistance is recognized as a One Health problem affecting humans, animals, and environment. Enterococcal species are Gram-positive bacteria that are widely distributed in nature. Their occurrence, prevalence, and persistence across the One Health continuum make them an ideal candidate to study antimicrobial resistance from a One Health perspective. The objective of this review was to summarize the role of enterococci as an indicator of antimicrobial resistance across One Health sectors. We also briefly address the prevalence of enterococci in human, animal, and environmental settings. In addition, a 16S RNA gene-based phylogenetic tree was constructed to visualize the evolutionary relationship among enterococcal species and whether they segregate based on host environment. We also review the genomic basis of antimicrobial resistance in enterococcal species across the One Health continuum.

Antibiotic resistant enterococci-tales of a drug resistance gene trafficker

Enterococci have been recognized as important hospital-acquired pathogens in recent years, and isolates of E. faecalis and E. faecium are the third- to fourth-most prevalent nosocomial pathogen worldwide. Acquired resistances, especially against penicilin/ampicillin, aminoglycosides (high-level) and glycopeptides are therapeutically important and reported in increasing numbers. On the other hand, isolates of E. faecalis and E. faecium are commensals of the intestines of humans, many vertebrate and invertebrate animals and may also constitute an active part of the plant flora. Certain enterococcal isolates are used as starter cultures or supplements in food fermentation and food preservation. Due to their preferred intestinal habitat, their wide occurrence, robustness and ease of cultivation, enterococci are used as indicators for fecal pollution assessing hygiene standards for fresh- and bathing water and they serve as important key indicator bacteria for various veterinary and human resistance surveillance systems. Enterococci are widely prevalent and genetically capable of acquiring, conserving and disseminating genetic traits including resistance determinants among enterococci and related Gram-positive bacteria. In the present review we aimed at summarizing recent advances in the current understanding of the population biology of enterococci, the role mobile genetic elements including plasmids play in shaping the population structure and spreading resistance. We explain how these elements could be classified and discuss mechanisms of plasmid transfer and regulation and the role and cross-talk of enterococcal isolates from food and food animals to humans.

Mobile genetic elements and their contribution to the emergence of antimicrobial resistant Enterococcus faecalis and Enterococcus faecium

Mobile genetic elements (MGEs) including plasmids and transposons are pivotal in the dissemination and persistence of antimicrobial resistance in Enterococcus faecalis and Enterococcus faecium. Enterococcal MGEs have also been shown to be able to transfer resistance determinants to more pathogenic bacteria such as Staphylococcus aureus. Despite their importance, we have a limited knowledge about the prevalence, distribution and genetic content of specific MGEs in enterococcal populations. Molecular epidemiological studies of enterococcal MGEs have been hampered by the lack of standardized molecular typing methods and relevant genome information. This review focuses on recent developments in the detection of MGEs and their contribution to the spread of antimicrobial resistance in clinically relevant enterococci.

Parameters associated with cloning in Actinobacillus actinomycetemcomitans

Characterization of virulence traits in Actinobacillus actinomycetemcomitans requires the application of recombinant DNA techniques. To develop appropriate genetic tools it is necessary to identify suitable host-vector systems. The current study assessed cloning parameters in A. actinomycetemcomitans for two previously described vectors, pDMG4 and pMMB67. It was determined that the maximum size of recombinant molecules that could be transferred to A. actinomycetemcomitans strain ATCC29522 via electroporation was 33 kb. The size limit for transformation of the same strain with ligation mixtures (direct cloning), however, was limited to 23-24 kb. Additional experiments included electroporation of various A. actinomycetemcomitans strains with plasmid DNA isolated from Escherichia coli and different A. actinomycetemcomitans sources. Differences in transformation efficiencies suggested the presence of a restriction modification system for pDMG4 in some strains of A. actinomycetemcomitans. Cloning of portions of the enterococcal plasmid pJH1 into A. actinomycetemcomitans resulted in the insertion of the intact vector into the chromosome.

Detection of Tn917-like sequences within a Tn916-like conjugative transposon (Tn3872) in erythromycin-resistant isolates of Streptococcus pneumoniae

A series of macrolide-lincosamide-streptogramin B (MLS)-resistant pneumococcal isolates of a variety of serotypes was examined and was found to contain Tn917-like elements by DNA-DNA hybridization. Like Tn1545, Tn917 also encodes an ermAM gene but does not mediate resistance to other antimicrobial agents. Furthermore, nucleotide sequence analyses of the DNAs flanking three of the Tn917-like elements revealed that they were inserted into orf9 of a Tn916-like element in a composite transposon-like structure (Tn3872). Other MLS-resistant strains appeared to contain Tn1545-like elements that had suffered a deletion of sequences including the aphA-3 sequences responsible for kanamycin resistance. Thus, the MLS resistance phenotype in pneumococci appears to be mediated by the ermAM present on a much wider variety of genetic elements than was previously appreciated.

Novel mechanism of macrolide resistance in Streptococcus pneumoniae

The mechanism of macrolide resistance was examined in 73 clinical isolates of Streptococcus pneumoniae. Two distinct resistance phenotypes were observed: high-level macrolides-lincosamides-streptogramin B (MLS) resistance and low-level macrolide resistance with lincosamide susceptibility. High-level MLS resistance was associated with the presence of ermAM. Strains with the low-level resistant phenotype (novel) were negative for ermA, ermC, ermAM, ereA, ereB and msrA by polymerase chain reaction (PCR) amplification with gene-specific primers. Ribosomes isolated from novel strains bound the same amount of [14C]-erythromycin as ribosomes from sensitive strains. These novel strains also did not inactivate the macrolide. The novel mechanism was found in 41% of the erythromycin resistant S. pneumoniae examined.

Characterization of antibiotic resistance in Streptococcus suis

Twenty one isolates of Streptococcus suis were screened for antibiotic resistance by growth on antibiotic-containing media, by measuring minimum inhibitory concentrations, by hybridization to specific DNA and oligonucleotide probes for antibiotic resistance genes, and by PCR. The isolates were from a slaughter house survey of respiratory pathogens in Norwegian pigs in 1986. Fifteen isolates were resistant to tetracycline, with MICs ranging from 4-128 micrograms/ml. Genes coding for the Tet O and Tet M determinants were detected in eight and five isolates, respectively. Genes coding for other Gram positive Tet determinants, Tet K, Tet L, and Tet P, were not detected. One isolate was constitutive resistant to erythromycin with MIC of 128 micrograms/ml. Five other isolates carried inducible erythromycin resistance. All these isolates, and five others, were positive in a PCR assay for erythromycin resistance, and hybridized with the Erm C and/or Erm B probes. No resistance against chloramphenicol (5 micrograms/ml) or rifampin (10 micrograms/ml) could be could be detected, but five isolates were resistant to streptomycin (250 micrograms/ml), four isolates were resistant to kanamycin (10 micrograms/ml), and one isolate was resistant to fusicic acid (10 micrograms/ml). In mating experiments with Enterococcus faecalis JH2-2 as recipient, tetracycline, erythromycin, and kanamycin genes were transferred separately to the recipient strain at a rate of 10(-7) transconjugants/recipient cell.

Bacterial resistance to macrolide, lincosamide, and streptogramin antibiotics by target modification

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Movable genetic elements and antibiotic resistance in enterococci

The enterococci possess genetic elements able to move from one strain to another via conjugation. Certain enterococcal plasmids exhibit a broad host range among gram-positive bacteria, but only when matings are performed on solid surfaces. Other plasmids are more specific to enterococci, transfer efficiently in broth, and encode a response to recipient-produced sex pheromones. Transmissible non-plasmid elements, the conjugative transposons, are widespread among the enterococci and determine their own fertility properties. Drug resistance, hemolysin, and bacteriocin determinants are commonly found on the various transmissible enterococcal elements. Examples of the different systems are discussed in this review.

Location of antibiotic resistance markers in clinical isolates of Enterococcus faecalis with similar antibiotypes

Eight wild-type strains of Enterococcus faecalis, resistant to chloramphenicol (Cmr), erythromycin (Emr), tetracycline (Tcr), and minocycline (Mnr), were examined for the genetic basis of their antibiotic resistance, Five of the strains transferred all of their antibiotic resistance markers by conjugation, while the other three strains transferred only Tcr and Mnr. Cmr and Emr determinants were localized by DNA-DNA hybridization experiments, in which the Cmr gene of plasmid pIP501, of group B Streptococcus origin, and the Emr gene of transposon Tn917, of E. faecalis origin, served as probes. A chromosomal location was found for the nonconjugative Cmr and Emr markers of one wild-type strain. In two strains these markers were carried by nonconjugative plasmids, and in the other strains they were carried by plasmids that transferred by conjugation. Plasmids isolated from three transconjugants resistant to tetracycline but susceptible to minocycline bore nucleotide sequences homologous to the tetL gene. Nucleotide sequences homologous to conjugative transposon Tn916, of E. faecalis origin, were detected by hybridization in the tetracycline-minocycline-resistant transconjugants. Three of these transconjugants were plasmid free, while four harbored conjugative cryptic plasmids. Sequences homologous to Tn916 were also found on two conjugative plasmids, one of which appeared to be a conjugative cryptic plasmid that had acquired chromosomal Tcr Mnr markers during transfer.

Transfer of the conjugal tetracycline resistance transposon Tn916 from Streptococcus faecalis to Staphylococcus aureus and identification of some insertion sites in the staphylococcal chromosome

The Streptococcus faecalis pheromone-dependent conjugative plasmid pAD1::Tn916 and the membrane filter-dependent conjugative plasmid pPD5::Tn916 were used to introduce Tn916 into Staphylococcus aureus by intergeneric protoplast fusions and intergeneric membrane-filter matings. In recombinants obtained by protoplast fusion where no plasmid DNA could be detected, tetracycline resistance resulted from transposition of Tn916 from pAD1 to the S. aureus chromosome. Transformation analyses showed that S. aureus Tn916 chromosomal insertions occurred near pig, ilv, uraA, tyrB, fus, ala, and the trp operon. DNA hybridization analyses of EcoRI- and HindIII-digested chromosomal DNAs confirmed the diversity of chromosomal sites involved and demonstrated that the inserts were Tn916 insertions rather than integrations of all or part of pAD1::Tn916. Both pAD1::Tn916 and pPD5::Tn916 were transferred to S. aureus by membrane-filter matings. These plasmids remained intact and expressed tetracycline resistance in S. aureus. S. aureus strains carrying pAD1::Tn916, but not a chromosomal insert of Tn916, and any one of several conjugal gentamicin-resistance plasmids lost their ability to serve as conjugal donors of the gentamicin-resistance plasmids.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

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Broad geographical distribution of homologous erythromycin, kanamycin, and streptomycin resistance determinants among group D streptococci of human and animal origin

The Emr, Kmr, and Smr determinants of the Streptococcus faecalis R plasmid pJH1 were cloned in Streptococcus sanguis with a streptococcal plasmid vector, pVA380-1. Each cloned determinant was used as a probe in hybridization reactions with dot blots containing plasmid-enriched DNA from 91 group D streptococcal isolates resistant to erythromycin, kanamycin, and streptomycin; the isolates were obtained from animal and human sources in a variety of geographical locations. Nearly 70% of the strains contained DNA that hybridized to each of the three resistance determinants from pJH1. Five plasmids mediating resistance to erythromycin, kanamycin, and streptomycin were examined in more detail. These plasmids varied in size between 26 and 105 kilobase pairs (kbp) and exhibited very different EcoRI restriction patterns. However, each plasmid contained the resistance determinants on a single 13- to 20-kbp EcoRI fragment. Southern blot hybridizations and additional restriction endonuclease digests revealed extensive DNA sequence homology and virtually indistinguishable restriction endonuclease maps within a 9- to 11-kbp region of each plasmid which included the resistance determinants.

Streptococcus faecalis R plasmid pJH1 contains a pAM alpha 1 delta 1-like replicon

Streptococcus faecalis R plasmid pJH1 did not transform competent strains of Streptococcus sanguis. A hybrid plasmid, pDL310, consisting of virtually all of the S. faecalis hemolysin-bacteriocin plasmid pJH2 and a segment of pJH1 DNA that included the tetracycline resistance determinant, yielded tetracycline-resistant transformants at a frequency of less than 10(-8) transformants per CFU, when it was added to a competent culture of S. sanguis Wicky. Four of the transformants contained a 4.7-kilobase plasmid (pDL316) that transformed strain Wicky at a frequency of 8.6 X 10(-8). Restriction endonuclease digests, agarose gel electrophoresis, and Southern blot hybridizations indicated that pDL316 consisted entirely of pJH1-derived DNA. Additional restriction studies, Southern blot hybridizations, and heteroduplex analyses indicated that pDL316 was very closely related to 4.6-kilobase tetracycline resistance plasmid pAM alpha 1 delta 1, a derivative of 9.0-kilobase S. faecalis plasmid pAM alpha 1.

Complete nucleotide sequence of macrolide-lincosamide-streptogramin B-resistance transposon Tn917 in Streptococcus faecalis

Streptococcus faecalis transposon Tn917 was cloned in Escherichia coli on plasmid vector pBR325. The erythromycin resistance determinant of Tn917 was not expressed in the E. coli background. The nucleotide sequence of Tn917 was determined and found to be 5,257 base pairs in length. Six open reading frames (ORFs) were identified and designated 1 through 6 (5' to 3'); all were on the same DNA strand. A region exhibiting strong homology with known promoters was identified upstream from ORF1. ORFs 1 to 3 were virtually identical to the previously sequenced erythromycin resistance determinant on Streptococcus sanguis plasmid pAM77. At the 3' point, where the homology between Tn917 and pAM77 ends, was a 20-base-pair region about 80% homologous with a component of the res site of Tn3. The amino acid sequence of ORF4 showed homology with other site-specific recombination enzymes, including approximately 30% homology with the resolvase of Tn3. Contained within Tn917 was a directly oriented 73-base-pair duplication of the left terminus. The Tn917 sequence revealed that antibiotic-enhanced transposition might be due to extension of transcription from the resistance-related genes (in ORFs 1 to 3) into transposition genes (in ORFs 4 to 6). Transcription analyses resulted in data consistent with this interpretation.

Evidence for a disseminated erythromycin resistance determinant mediated by Tn917-like sequences among group D streptococci isolated from pigs, chickens, and humans

A total of 199 streptococci isolated from feces of healthy chickens, pigs, and beef cattle and 26 human clinical isolates were tested for resistance to kanamycin, streptomycin, tetracycline, erythromycin, and lincomycin. Of 66 isolates resistant to these antibiotics, 12 transferred one or more resistance traits by conjugation in broth. Erythromycin resistance (Emr) was transferred from 10 of the 12 successful donors. AvaI digests of plasmids isolated from Emr transconjugants derived from two human, two chicken, and one pig isolate contained three fragments similar in size to those produced from Tn3871, an Emr transposon. The three fragments from each of the five digests on Southern blots hybridized to radiolabeled Tn3871. Plasmid DNA from a transconjugant derived from a second pig isolate contained two of the three Tn3871-associated AvaI fragments. One of the AvaI fragments from each of the six plasmids hybridized with a radiolabeled probe containing a cloned AvaI fragment from Tn3871 that contained the Emr determinant. Transposition of the Emr trait was demonstrated for the plasmids derived from one human and one pig isolate. We concluded that extensive DNA homology existed between plasmids from streptococcal strains obtained from two human patients, two chickens, and two pigs and the Emr transposon Tn3871, which is very similar or identical to the well-characterized Emr transposon Tn917. The detection of Tn3871-like sequences in streptococcal isolates from Arkansas, Illinois, North Carolina, and Washington, D.C. indicates wide dissemination of Emr mediated by the same or closely related transposons.