Plasmid-mediated mechanisms of resistance to aminoglycoside-aminocyclitol antibiotics and to chloramphenicol in group D streptococci

Genome Comparison of Erythromycin Resistant Campylobacter from Turkeys Identifies Hosts and Pathways for Horizontal Spread of erm(B) Genes

Pathogens in the genus Campylobacter are the most common cause of food-borne bacterial gastro-enteritis. Campylobacteriosis, caused principally by Campylobacter jejuni and Campylobacter coli, is transmitted to humans by food of animal origin, especially poultry. As for many pathogens, antimicrobial resistance in Campylobacter is increasing at an alarming rate. Erythromycin prescription is the treatment of choice for clinical cases requiring antimicrobial therapy but this is compromised by mobility of the erythromycin resistance gene erm(B) between strains. Here, we evaluate resistance to six antimicrobials in 170 Campylobacter isolates (133 C. coli and 37 C. jejuni) from turkeys. Erythromycin resistant isolates (n = 85; 81 C. coli and 4 C. jejuni) were screened for the presence of the erm(B) gene, that has not previously been identified in isolates from turkeys. The genomes of two positive C. coli isolates were sequenced and in both isolates the erm(B) gene clustered with resistance determinants against aminoglycosides plus tetracycline, including aad9, aadE, aph(2″)-IIIa, aph(3')-IIIa, and tet(O) genes. Comparative genomic analysis identified identical erm(B) sequences among Campylobacter from turkeys, Streptococcus suis from pigs and Enterococcus faecium and Clostridium difficile from humans. This is consistent with multiple horizontal transfer events among different bacterial species colonizing turkeys. This example highlights the potential for dissemination of antimicrobial resistance across bacterial species boundaries which may compromise their effectiveness in antimicrobial therapy.

Widespread acquisition of antimicrobial resistance among Campylobacter isolates from UK retail poultry and evidence for clonal expansion of resistant lineages

Background

Antimicrobial resistance is increasing among clinical Campylobacter cases and is common among isolates from other sources, specifically retail poultry - a major source of human infection. In this study the antimicrobial susceptibility of isolates from a UK-wide survey of Campylobacter in retail poultry in 2001 and 2004–5 was investigated. The occurrence of phenotypes resistant to tetracycline, quinolones (ciprofloxacin and naladixic acid), erythromycin, chloramphenicol and aminoglycosides was quantified. This was compared with a phylogeny for these isolates based upon Multi Locus Sequence Typing (MLST) to investigate the pattern of antimicrobial resistance acquisition.

Results

Antimicrobial resistance was present in all lineage clusters, but statistical testing showed a non-random distribution. Erythromycin resistance was associated with Campylobacter coli. For all antimicrobials tested, resistant isolates were distributed among relatively distant lineages indicative of widespread acquisition. There was also evidence of clustering of resistance phenotypes within lineages; indicative of local expansion of resistant strains.

Conclusions

These results are consistent with the widespread acquisition of antimicrobial resistance among chicken associated Campylobacter isolates, either through mutation or horizontal gene transfer, and the expansion of these lineages as a proportion of the population. As Campylobacter are not known to multiply outside of the host and long-term carriage in humans is extremely infrequent in industrialized countries, the most likely location for the proliferation of resistant lineages is in farmed chickens.

Acquired gentamicin resistance by permeability impairment in Enterococcus faecalis

Enterococci are intrinsically resistant to low levels of aminoglycosides. We previously selected in vitro and in vivo Enterococcus faecalis with intermediate-level resistance to gentamicin that did not abolish synergism with a cell-wall-active agent (E. Aslangul et al., Antimicrob. Agents Chemother. 49:4144-4148, 2005). The aim of this study was to investigate the mechanism of resistance to gentamicin in the 1688-G3 third-step mutant (MIC, 512 microg/ml) of E. faecalis JH2-2. No mutations were found in the genes for L6 ribosomal protein and the four copies of 16S rRNA. Production of a known aminoglycoside-modifying enzyme was unlikely due to the distinct resistance phenotype and absence of the corresponding genes. Efflux was also unlikely since ethidium bromide MICs were similar for JH2-2 and 1688-G3 and since the pump inhibitors reserpine and verapamil had no effect on gentamicin resistance in both strains. To study gentamicin accumulation, we developed a nonisotopic method based on a fluorescent polarization immunoassay. Impaired gentamicin accumulation was observed in 1688-G3 compared to JH2-2 and was only partially reversible by the N,N'-dicyclohexylcarbodiimide (DCCD) uncoupler agent. The lower sensitivity of 1688-G3 to DCCD suggested alteration of the FoF1-ATPase. However, no mutations were detected in the structural genes (atp) for the Fo channel and no difference in transcript levels of atpB and atpE was found between 1688-G3 and JH2-2. Our data are compatible with acquisition of intermediate-level gentamicin resistance by uptake impairment in E. faecalis.

Relationship between the level of acquired resistance to gentamicin and synergism with amoxicillin in Enterococcus faecalis

In enterococci, intrinsic low-level resistance to gentamicin does not abolish synergism with a cell wall-active antibiotic while high-level resistance due to acquired aminoglycoside-modifying enzymes does. To study the impact of intermediate levels of resistance to gentamicin (64 < MIC < 500 microg/ml), we selected in vitro three consecutive generations of mutants of Enterococcus faecalis JH2-2 with MICs of gentamicin at 128 microg/ml for G1-1477, 256 microg/ml for G2-1573, and 512 microg/ml for G3-1688. E. faecalis 102, which is highly resistant to gentamicin by enzymatic inactivation was used as control. In in vitro killing curves experiments, gentamicin concentrations allowing bactericidal activity and synergism in combination with amoxicillin increased from 4 microg/ml (1/16th the MIC), 16 microg/ml (one-eighth the MIC), 64 microg/ml (one-quarter the MIC), and 256 microg/ml (one-half the MIC) for strains JH2-2, G1-1477, G2-1573 and G3-1688, respectively. As expected, no bactericidal effect of the combination or synergism could be obtained with strain 102. In rabbits with aortic endocarditis caused by strain G1-1477 or G2-1573, combination therapy with amoxicillin and gentamicin was significantly more active than amoxicillin alone (P < 0.05) but not in those infected with the strains G3-1688 and 102. Thus, intermediate levels of resistance to gentamicin was not associated with a loss of a beneficial effect of the gentamicin-amoxicillin combination in vivo even though higher concentrations of gentamicin were necessary to achieve in vitro synergism. Therefore, the use of an MIC of 500 microg/ml as a clinical cutoff limit to predict in vivo benefit of the combination remains a simple and effective tool.

Epidemiology of multiresistant Enterococcus avium isolates in a Greek tertiary care hospital

A retrospective survey of the isolation rate of Enterococcus avium during the period March 1994-February 2000 conducted in Laikon General Hospital using the WHONET software, revealed a peak in the isolation rates of this species during March 1995-February 1996. The ten strains isolated during this time were studied further. No glycopeptide resistance was detected but resistance to ampicillin, ciprofloxacin, erythromycin, gentamicin (high-level) and streptomycin (high-level) was present in nine, ten, nine, three and seven of the isolates, respectively. The genes aac(6')-Ie+aph(2")-Ia and ant(6)-I, encoding for high-level gentamicin and streptomycin resistance, respectively, were detected only in the isolates with the corresponding phenotypes. Beta-lactamase production and haemolysis were not detected. There was evidence of ward-, floor- and building-specific distribution among the different aminoglycoside resistance phenotypes. DNA fingerprinting by PFGE grouped six of the ten isolates in a single cluster with 83% similarity, even though they expressed various resistance phenotypes. These results suggest dissemination of resistance genes among both genetically related and unrelated strains.

Detection of a streptomycin/spectinomycin adenylyltransferase gene (aadA) in Enterococcus faecalis

Genes encoding streptomycin/spectinomycin adenylyltransferases [ANT(3")(9)] have been reported to exist in gram-negative organisms and Staphylococcus aureus. During a study of high-level aminoglycoside resistance in enterococci, we encountered an isolate of Enterococcus faecalis that was streptomycin resistant but did not appear to contain the 6'-adenylyltransferase gene (aadE) when examined by PCR with specific primers. Phosphocellulose paper binding assays indicated the presence of an ANT(3")(9) enzyme. Streptomycin and spectinomycin MICs of 4,000 and 8,000 microg/ml, respectively, were observed for the isolate. PCR primers corresponding to a highly conserved region of the aadA gene were used to amplify a specific 284-bp product. The product hybridized with a digoxigenin-labeled PCR product from E. coli C600(pHP45Omega) known to contain the aadA gene. The aadA gene was transferred via filter matings from the E. faecalis donor to E. faecalis JH2-2. PCR primers designed for analysis of integrons were used to amplify a 1-kb product containing the aadA gene, which was cloned into the vector pCRII and transformed into Escherichia coli DH5-alpha competent cells. D-Rhodamine dye terminator cycle sequencing was used to determine the gene sequence, which was compared to previously reported sequences of aadA genes. We found the aadA gene in E. faecalis to be identical to the aadA genes reported by Sundstr om et al. for E. coli plasmid R6-5 (L. Sundström, P. Râdström, G. Swedberg, and O. Sköld, Mol. Gen. Genet. 213:191-201, 1988), by Fling et al. for the aadA within transposon Tn7 (M. E. Fling, J. Kopf, and C. Richards, Nucleic Acids Res. 13:7095-7106, 1985), and by Hollingshead and Vapnek for E. coli R538-1 (S. Hollingshead and D. Vapnek, Plasmid 13:17-30, 1985). Previous reports of the presence of the aadA gene in enterococci appear to be erroneous and probably describe an aadE gene, since the isolates were reported to be susceptible to spectinomycin.

In vitro susceptibility and molecular analysis of gentamicin-resistant enterococci

Enterococci with gentamicin MICs of 256 to 1,024 micrograms/mL were evaluated for susceptibility to ampicillin plus gentamicin synergism. Sixteen of eighteen enterococcal isolates were not susceptible to synergistic killing by ampicillin plus gentamicin; 11 possessed aac(6')-aph(2"), and 4 possessed aph(2")-Ic. A gentamicin MIC of 512 or 1,024 micrograms/mL predicted lack of ampicillin/gentamicin synergism, but a gentamicin MIC of 256 micrograms/mL did not. For six enterococcal strains possessing the gentamicin-resistance gene aph(2")-Ic, ampicillin plus dibekacin, ampicillin plus netilmicin, and ampicillin plus amikacin produced synergistic killing in five, three, and two strains, respectively.

Resistance to antimicrobial agents used for animal therapy in pathogenic-, zoonotic- and indicator bacteria isolated from different food animals in Denmark: a baseline study for the Danish Integrated Antimicrobial Resistance Monitoring Programme (DANMAP)

This study describes the establishment and first results of a continuous surveillance system of antimicrobial resistance among bacteria isolated from pigs, cattle and broilers in Denmark. The three categories of bacteria tested were: 1) indicator bacteria (Escherichia coli, Enterococcus faecalis, Enterococcus faecium), 2) zoonotic bacteria (Campylobacter coli/jejuni, Salmonella enterica, Yersinia enterocolitica), and 3) animal pathogens (E. coli, Staphylococcus aureus, coagulase-negative staphylococci (CNS), Staphylococcus hyicus, Actinobacillus pleuropneumoniae). A total of 3304 bacterial isolates collected from October 1995 through December 1996 were tested for susceptibility to all major classes of antimicrobial agents used for therapy in Denmark. Bacterial species intrinsically resistant to an antimicrobial were not tested towards that antimicrobial. Acquired resistance to all antimicrobials was found. The occurrence of resistance varied by animal origin and bacterial species. In general, resistance was observed more frequently among isolates from pigs than from cattle and broilers. The association between the occurrence of resistance and the consumption of the antimicrobial is discussed, as is the occurrence of resistance in other countries. The results of this study show the present level of resistance to antimicrobial agents among a number of bacterial species isolated from food animals in Denmark. Thus, the baseline for comparison with future prospective studies has been established, enabling the determination of trends over time.

High-level plasmid-mediated gentamicin resistance and pheromone response of plasmids present in clinical isolates of Enterococcus faecalis

Eleven pheromone-responding plasmids encoding erythromycin or gentamicin resistance were isolated from multiresistant clinical Enterococcus faecalis isolates. The plasmids were classified into six types with respect to their pheromone responses. The three erythromycin resistance plasmids responded to different pheromones. Of the eight gentamicin resistance plasmids, four plasmids responded to same pheromone. Southern hybridization studies showed that the genes involved in regulation of the pheromone response were conserved in the drug resistance plasmids.

The VanS-VanR two-component regulatory system controls synthesis of depsipeptide peptidoglycan precursors in Enterococcus faecium BM4147

Plasmid pIP816 of Enterococcus faecium BM4147 confers inducible resistance to vancomycin and encodes the VanH dehydrogenase and the VanA ligase for synthesis of depsipeptide-containing peptidoglycan precursors which bind the antibiotic with reduced affinity. We have characterized a cluster of five genes of pIP816 sufficient for peptidoglycan synthesis in the presence of vancomycin. The distal part of the van cluster encodes VanH, VanA, and a third enzyme, VanX, all of which are necessary for resistance. Synthesis of these enzymes was regulated at the transcriptional level by the VanS-VanR two-component regulatory system encoded by the proximal part of the cluster. VanR was a transcriptional activator related to response regulators of the OmpR subclass. VanS stimulated VanR-dependent transcription and was related to membrane-associated histidine protein kinases which control the level of phosphorylation of response regulators. Analysis of transcriptional fusions with a reporter gene and RNA mapping indicated that the VanR-VanS two-component regulatory system activates a promoter used for cotranscription of the vanH, vanA, and vanX resistance genes.

New high-content disks for determination of high-level aminoglycoside resistance in clinical isolates of Enterococcus faecalis

Disks impregnated with 500 and 1000 micrograms of streptomycin, 1000 micrograms of kanamycin and 250 and 500 micrograms of gentamicin were used for detection of high-level resistance to aminoglycosides in 120 clinical isolates of Enterococcus faecalis. Fifty-seven strains were highly resistant to streptomycin, 80 to kanamycin and 41 to gentamicin. Using disks containing 500 micrograms of streptomycin, 1000 micrograms of kanamycin and 500 micrograms of gentamicin strains resistant to high levels of these drugs (97.9%, 100% and 100%, respectively) were accurately detected. Better discrimination between high-level and low-level resistance was achieved with a 500 micrograms streptomycin or gentamicin disk. Zone-size breakpoints are proposed for detection of high-level resistance by disk diffusion.

Chromosomally mediated beta-lactamase production and gentamicin resistance in Enterococcus faecalis

We have analyzed four distinct strains of multiply resistant, beta-lactamase-producing enterococci isolated during an outbreak of colonization with these strains on an infant-toddler surgical ward at The Children's Hospital in Boston, Mass. All four strains were resistant to erythromycin, penicillin, and tetracycline and to high levels of gentamicin and streptomycin. One strain was also resistant to chloramphenicol. Plasmid profiles revealed four different plasmid patterns, with the number of identified plasmids ranging from zero to three. The gene coding for beta-lactamase production could be transferred at low frequency (less than 10(-8)) to an enterococcal recipient from one strain in conjunction with all of the other resistance determinants. Probes derived from the staphylococcal beta-lactamase gene and gentamicin resistance gene failed to hybridize with any of the detectable plasmids, but both genes were present on restriction fragments of genomic DNA in all strains. Our results indicate that the beta-lactamase genes and gentamicin resistance genes in these strains are integrated into the bacterial chromosome. The cotransmissibility of the resistance determinants raises the possibility of their incorporation into a multiresistance transposable genetic element.

Gene homogeneity for aminoglycoside-modifying enzymes in gram-positive cocci

Aminoglycoside-resistant strains of Staphylococcus and Enterococcus, approximately 500 of each, were screened by dot blot hybridization for the presence of genes encoding aminoglycoside-modifying enzymes. The MICs of various aminoglycosides for the strains were determined, and the enzyme contents of the cells were inferred from the resistance phenotypes. The agreements (in percent) of the hybridization results with the deduced enzyme contents for Staphylococcus and Enterococcus species were, respectively, 80 and 87.6 for ANT(6) (aminoglycoside nucleotidyltransferase), 99.8 and 100 for both APH(3') (aminoglycoside phosphotransferase) and APH(2")-AAC(6') (aminoglycoside acetyltransferase), and 100 and 100 for ANT(4'). The weak correlation obtained with the probe for ANT(6) was due to the fact that gram-positive cocci can also be streptomycin resistant by synthesis of APH(3") or ANT(3")(9) and by ribosomal mutation. The remaining probes appeared to be specific: they hybridized with all the resistant clinical isolates but not with the susceptible strains. These results indicate that, except for streptomycin, nucleic acid hybridization is a valid approach for the detection and characterization of aminoglycoside resistance in gram-positive cocci.

Emergence of 4',4"-aminoglycoside nucleotidyltransferase in enterococci

Enterococcus faecium BM4102 was resistant to macrolide-lincosamide-streptogramin B-type (MLS) antibiotics; tetracycline-minocycline; and high levels of kanamycin, neomycin, tobramycin, and dibekacin but not gentamicin. This aminoglycoside resistance phenotype is new in enterococci. The genes conferring resistance to aminoglycosides and MLS antibiotics in this strain were carried on a plasmid, pIP810, that was self-transferable to to other Enterococcus strains. Resistance to tobramycin and structurally related aminoglycosides, kanamycin, neomycin, and dibekacin, was due to synthesis of a 4',4"-aminoglycoside nucleotidyltransferase. Homology was detected by hybridization between pIP810 DNA and a probe specific for a gene encoding an enzyme with identical site specificity in staphylococci. The bacteriostatic activity of amikacin apparently was not affected by the presence of the enzyme, although it was modified in vitro. However, the bactericidal activity of amikacin and the synergism of this aminoglycoside with penicillin were abolished.

Dissemination of amikacin resistance gene aphA6 in Acinetobacter spp

The distribution of the aphA6 gene, encoding a 3'-aminoglycoside phosphotransferase type VI, was studied by dot blot hybridization with 115 amikacin-resistant Acinetobacter strains from various geographical areas. Nucleotide sequences related to aphA6 were found in 109 strains belonging to seven species. As inferred from results of Southern hybridization, dissemination of amikacin resistance in Acinetobacter spp. is due to a gene rather than a strain or plasmid epidemic.

Emergence of aminoglycoside resistance genes aadA and aadE in the genus Campylobacter

Resistance to streptomycin or spectinomycin or both in five Campylobacter coli strains, two Campylobacter jejuni strains, and a Campylobacter-like strain was studied by enzymatic assays and dot blot hybridization. Resistance was due to 6- or 3",9-aminoglycoside adenylyltransferases and to new types of phospho- and adenylyltransferases.

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.

Enterococci highly resistant to penicillin and ampicillin: an emerging clinical problem?

Sixteen clinical isolates of ampicillin-resistant enterococci (ARE) were recovered from the microbiology laboratory of a 450-bed rehabilitation medical center from January 1981 to September 1987. These isolates were detected when a disk diffusion test using 10 micrograms of ampicillin on a blood agar plate revealed no zones of inhibition. Tube macrodilution tests yielded an MIC of greater than or equal to 16 micrograms of ampicillin per ml. None of the isolates were penicillinase producers by the chromogenic cephalosporin disk test. Ten isolates were Enterococcus faecium, four isolates were E. raffinosus, one isolate was E. gallinarum, and one isolate was not identified (lost). There were 6 male and 10 female patients. The sources of isolates were urine (n = 7), wound (n = 5), ascitic fluid (n = 2), blood (n = 2), peritoneal catheter tip (n = 1), Bartholin's cyst abscess (n = 1), rectal swab (n = 2), and pancreatic abscess (n = 1). The organism was isolated from multiple sites in 4 patients, was a pure culture isolate in 5 patients, and was part of a polymicrobial flora in 11 patients. Six patients were diabetic, and four had liver cirrhosis. All but four patients had received at least one antibiotic within 3 weeks of ARE isolation. The MICs (micrograms per milliliter) for 50 and 90% of isolates tested, respectively, were as follows: ampicillin, 64 and 64; penicillin, 128 and greater than 128; vancomycin, 1 and 2; gentamicin, 4 and 16; ciprofloxacin, 1.6 and 3.2; imipenem, 128 and greater than 128; and daptomycin (LY146032), 1.6 and 6.4. ARE may be an emerging pathogen in the hospitalized patient population.

Transferable vancomycin and teicoplanin resistance in Enterococcus faecium

Enterococcus faecium BM4165 and BM4178, isolated from immunocompromised patients, one treated with vancomycin, were inducibly resistant to high levels of the glycopeptide antibiotics vancomycin and teicoplanin but susceptible to the new lipopeptide daptomycin (LY146032). Strain BM4165 was also resistant to macrolidelincosamide-streptogramin B-type (MLS) antibiotics. The genes conferring resistance to glycopeptides and to MLS antibiotics in strain BM4165 were carried on plasmids pIP819 and pIP821, respectively; pIP819 also carried genes that encoded resistance to MLS antibiotics. The two plasmids, which were distinct although related, were self-transferable to other E. faecium strains. Plasmid pIP819 could also conjugate to E. faecalis, Streptococcus sanguis, S. pyogenes, S. lactis, and Listeria monocytogenes, in which it conferred inducible glycopeptide resistance, but not to S. aureus. Glycopeptide-inactivating activity was not detected, and the biochemical mechanism of resistance remains unknown. Based on this first report of transferable resistance to glycopeptides, we anticipate dissemination of resistance to these antibiotics in gram-positive cocci and bacilli in which it can be phenotypically expressed.

Genetic characterization of kanamycin resistance in Campylobacter coli

The restriction map of plasmid pIP1433 from Campylobacter coli BM2509, isolated in France, was constructed and the location of the kanamycin (Km) and tetracycline (Tc) resistance markers determined using probes for aphA-3 and tetO genes, respectively. These DNA probes were used to study a second Km- and Tc-resistant strain C. coli UA696, isolated in Canada. In this strain, the Km resistance determinant was located in the chromosome. Like pIP1433, the Km resistance determinant in C. coli UA696 specified a 3'-aminoglycoside phosphotransferase of type III whereas the tetO gene was located on a non-transmissible plasmid.

Use of plasmid profiles in epidemiologic surveillance of disease outbreaks and in tracing the transmission of antibiotic resistance

Plasmids are circular deoxyribonucleic acid molecules that exist in bacteria, usually independent of the chromosome. The study of plasmids is important to medical microbiology because plasmids can encode genes for antibiotic resistance or virulence factors. Plasmids can also serve as markers of various bacterial strains when a typing system referred to as plasmid profiling, or plasmid fingerprinting is used. In these methods partially purified plasma deoxyribonucleic acid species are separated according to molecular size by agarose gel electrophoresis. In a second procedure, plasmid deoxyribonucleic acid which has been cleaved by restriction endonucleases can be separated by agarose gel electrophoresis and the resulting pattern of fragments can be used to verify the identity of bacterial isolates. Because many species of bacteria contain plasmids, plasmid profile typing has been used to investigate outbreaks of many bacterial diseases and to trace inter- and intra-species spread of antibiotic resistance.

Tn1545: a conjugative shuttle transposon

Tn1545, from Streptococcus pneumoniae BM4200, confers resistance to kanamycin (aphA-3), erythromycin (ermAM) and tetracycline (tetM). The 25.3 kb element is self-transferable to various Gram-positive bacterial genera where it transposes. Tn1545 was cloned in its entirety in the recombination deficient Escherichia coli HB101 where it was unstable. The three resistance genes aphA-3, ermAM and tetM were expressed but were not transferable to other E. coli cells. Tn1545 transposed from the hybrid plasmid to multiple sites of the chromosome of its new host. The element re-transposed, at a frequency of 5 X 10(-9), from the chromosome to various sites of a conjugative plasmid where it could be lost by apparently clean excision. The element transformed and transposed to the chromosome of Bacillus subtilis. The properties of the conjugative shuttle transposon Tn1545 may account for the recent emergence of genes from Gram-positive bacteria in Gram-negative organisms.

The enterococcus

In 1899, Thiercilin used the term “enterococcus” to describe microscopic organisms seen in pairs or short chains in feces. Later (in the 1930s) the name was used in a more restrictive sense for streptococci that had the hardy capacity to survive under extreme conditions.

Enterococci belong to a group D family of streptococci, as characterized by Lancefield in 1938. In contrast to other groups of streptococci, the group D antigen is not a wall carbohydrate but a glycerol teichoic acid containing glucose and D-alanine. This antigen appears to be related directly to the cytoplasm or plasma membrane.

Dispersal of a plasmid-borne chloramphenicol resistance gene in streptococcal and enterococcal plasmids

Plasmids coding for chloramphenicol resistance, five isolated from streptococci of groups A, B, and G, ten from enterococci (Enterococcus faecalis, Enterococcus faecium), and two from staphylococci, were tested for sequence homology with the chloramphenicol resistance gene of pIP501, a 30-kb plasmid originally isolated from a group B Streptococcus. The 6.3-kb HindIII fragment of pIP501, known to carry the chloramphenicol resistance gene, was cloned into pBR322. A 1.6-kb portion of the cloned fragment, which included most of the chloramphenicol resistance gene, was used as probe in DNA-DNA hybridization experiments. Sequence homology was detected between the probe and four of the streptococcal, seven of the enterococcal, and one of the staphylococcal plasmids. The absence of hybridization between this probe and one plasmid isolated from a group B Streptococcus, as well as three isolated from E. faecalis, indicated that there are at least two different plasmid-borne chloramphenicol resistance determinants in the streptococci and in the enterococci.

Transposable multiple antibiotic resistance in Streptococcus pneumoniae

A mobile genetic element, designated Tn1545, was detected in the chromosome of Streptococcus pneumoniae BM4200, a clinical isolate multiply resistant to antibiotics. The 25.3 kb element conferred resistance to kanamycin and structurally related aminoglycosides by synthesis of a 3'-aminoglycoside phosphotransferase type III (aphA-3), to macrolide-lincosamide-streptogramin B-type antibiotics (ermAM), and to tetracycline (tetM). Tn1545 was self-transferable to a recombination deficient S. faecalis strain where it was able to transpose to various sites, induce insertional mutations and was apparently cleanly excised. The element also conjugated to and transposed to the chromosome of S. faecalis, S. lactis, S. diacetylactis, S. cremoris, S. sanguis, Staphylococcus aureus, and Listeria monocytogenes. The properties of the conjugative transposon Tn1545 could account for the sudden emergence, rapid dissemination, and stabilisation of multiple resistance to antibiotics in S. pneumoniae in the absence of plasmids.

Heterogeneity of plasmids determining high-level resistance to gentamicin in clinical isolates of Streptococcus faecalis

Between November 1981 and October 1984, 48 of 3,458 clinical isolates of Streptococcus faecalis at the University of Michigan Hospital showed high-level (greater than 2,000 micrograms/ml) resistance to gentamicin, as well as to all other clinically available aminoglycosides. Thirteen percent of clinical isolates in the University of Michigan Hospital currently demonstrate this level of resistance. Transfer of resistance to a plasmid-free streptococcal recipient was observed in filter matings for 44 of 48 such isolates. Analysis of transconjugants by agarose gel electrophoresis showed that gentamicin resistance was transferred alone in 8 isolates and was combined with other antimicrobial resistances on the same plasmid in the other 36 isolates. There were seven isolates which transferred the property of horse blood beta-hemolysis along with gentamicin resistance. Transfer of gentamicin resistance was either by conjugative plasmids or by a nonconjugative but mobilizable plasmid. Transfer frequencies on filters ranged from 4.7 X 10(-2) to 4.1 X 10(-8). Based on donor and transconjugant antibiotic resistance markers, agarose gel electrophoresis, transfer properties, and restriction enzyme analysis, the plasmid content of transconjugants was heterogeneous and could be classified into at least seven different patterns. These findings argue against clonal dissemination as the cause of the increased frequency of resistant strains and suggest that resistance to gentamicin appears on a variety of physically distinct conjugative and nonconjugative plasmids in S. faecalis.

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.

Conjugal transfer of plasmid-mediated antibiotic resistance from streptococci to Clostridium acetobutylicum

Three broad host-range MLS plasmids, pAM beta 1 (Em), pIP501 (Em, Cm) and pJH4 (Em, Sm, Km), were transferred by a conjugation-like process from Streptococcus faecalis to Clostridium acetobutylicum. The plasmids were stably maintained and antibiotic resistances were fully expressed in C. acetobutylicum, except for chloramphenicol resistance, which was poorly expressed, if at all. The C. acetobutylicum strains harbouring every plasmid could be used as donors in intraspecific matings with frequencies of approximately 1 X 10(-5) to 1 X 10(-6).

Structural relationship between the genes encoding 3'-aminoglycoside phosphotransferases in Campylobacter and in gram-positive cocci

Campylobacter coli strain BM2509 resistant to ampicillin, chloramphenicol, erythromycin, kanamycin, spectinomycin, streptomycin and tetracycline was isolated from a patient with hospital-acquired diarrhoea. Resistance to kanamycin has not been thus far described in Campylobacter. Phosphocellulose paper-binding assay indicated that resistance to kanamycin and structurally related antibiotics in strain BM2509 was due to synthesis of a 3'-aminoglycoside phosphotransferase of type III (APH(3')-III), an enzyme so far confined to Gram-positive cocci. The kanamycin and tetracycline resistances were transferable en bloc by conjugation to C. fetus but not to Escherichia coli. Analysis by agarose gel electrophoresis of crude bacterial lysates revealed the presence, in BM2509 and in the transconjugants, of a plasmid, pIP1433, with a size of 47.2 kilobases (Kb). Strain BM2509 also harboured a 4.5-Kb cryptic plasmid. DNA annealing studies indicated a close structural relationship between the kanamycin resistance gene of C. coli BM2509 and that representative of this type of resistance determinant in Gram-positive cocci. These results indicate that emergence of resistance to kanamycin in Campylobacter is due to acquisition in vivo of a gene or a plasmid from Gram-positive bacteria.

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

The R plasmid pJH1 contains a 5.1-kilobase transposon ( Tn3871 ) that mediates inducible resistance to erythromycin. Three AvaI digestion fragments from this transposon are identical in size to and homologous with three AvaI-derived fragments from the previously described erythromycin resistance transposon Tn917 . These three DNA fragments account for greater than 90% of both transposons.

The chromosomal 3',5"-aminoglycoside phosphotransferase in Streptococcus pneumoniae is closely related to its plasmid-coded homologs in Streptococcus faecalis and Staphylococcus aureus

The apparently chromosomally encoded 3',5"-aminoglycoside phosphotransferase (type III), from the high-level aminoglycoside-resistant Streptococcus pneumoniae BM4200, was compared with homologous enzymes coded for by the plasmids pJH1 and pSH2, originally isolated from Streptococcus faecalis and Staphylococcus aureus, respectively, and also found in a wild strain of S. aureus, BM4600. The enzymes appeared to be indistinguishable, and we conclude that the gene encoding 3',5"-aminoglycoside phosphotransferase (type III) can cross generic barriers within gram-positive cocci.

Nucleotide sequence of the Streptococcus faecalis plasmid gene encoding the 3'5"-aminoglycoside phosphotransferase type III

We have cloned in Escherichia coli and sequenced a 1489-bp DNA fragment conferring resistance to kanamycin and originating from the streptococcal plasmid pJH1. The resistance gene was located by analysis of the initiation and termination codons in an open reading frame (ORF) of 792 bp. The deduced gene product, a 3'5''-aminoglycoside phosphotransferase of type III, has an Mr of 29,200. Comparison of its amino acid sequence with those of type I (Oka et al., 1981) and type II (Beck et al., 1982) 3' phosphotransferase, from transposable elements Tn903 and Tn5, respectively, indicated a statistically significant structural relationship between these enzymes from phylogenetically remote bacterial genera. The degree of homology observed indicate that phosphotransferase type III and type I genes have diverged from a common ancestor and that the phosphotransferase type II gene has emerged more recently from the type I evolutionary pathway.

Genetic, molecular, and functional analysis of Streptococcus faecalis R plasmid pJH1

Streptococcus faecalis JH1 contains two conjugative plasmids, pJH1, an R plasmid that codes for resistance to kanamycin, streptomycin, erythromycin, and tetracycline, and pJH2, a hemolysin-bacteriocin plasmid. Strain JH1 was used as an antibiotic resistance donor in conjugation experiments with two plasmid-free S. faecalis recipient strains, JH2-2 and OG1-RF1. Plasmid pJH1 was purified from one transconjugant, DL77, and subjected to restriction endonuclease analyses. Five restriction enzymes, EcoRI, XbaI, BamHI, SalI, and XhoI, yielding 10, 9, 3, 2, and 2 fragments, respectively, were used to determine the size (80.7 kilobases) of pJH1 and to construct a restriction endonuclease map of the plasmid. Twenty-eight percent of the antibiotic-resistant transconjugants examined expressed only part of the resistance pattern (Kmr Smr Emr Tcr) associated with pJH1, that is, they were resistant to kanamycin, streptomycin, and erythromycin; to erythromycin and tetracycline; or to erythromycin or to tetracycline only. Most of these strains also produced hemolysin and bacteriocin, and several contained a hybrid plasmid consisting of pJH2 and specific segments of pJH1 DNA. Several of these hybrid plasmids, as well as a deletion derivative of pJH1 that coded for resistance to tetracycline but not to kanamycin, streptomycin, or erythromycin, were purified and used to confirm the arrangement of restriction endonuclease fragments on the pJH1 map and to locate the resistance determinants on this map.

IS15, a new insertion sequence widely spread in R plasmids of gram-negative bacteria

We have shown that the IS15 element, first detected in Salmonella ordonez and previously designated IS1522 (Labigne-Roussel et al. 1981), could transpose, with an approximate frequency of 5 X 10(-5), to various sites of different replicons in an Escherichia coli host deficient for general homologous recombination. Physical mapping with restriction endonucleases of this 1,500 base pairs (bp) transposable module indicated the presence of two, possibly contiguous, directly repeated internal sequences, at least 480 bp in size. IS15 could generate in vivo, by intramolecular recombination between the two direct repeats, IS15-delta, which is 830 bp in size. The reverse transition, IS15-delta to IS15, was not observed. The two related structural forms of IS15 were detected, by Southern hybridization, on plasmids belonging to various incompatibility groups (Inc6-C, I1, 7-M, and Y) isolated from phylogenetically remote pathogenic bacterial genera (Escherichia coli, Salmonella panama, Enterobacter cloacae, and Acinetobacter calcoaceticus). Whereas IS15 could promote its own transposition and transposition of DNA fragments it flanked, IS15-delta resulting from the 670 bp 'clean' deletion and representing the most common natural deletion derivative could only induce replicon fusion. It appears, therefore, that the two structural configurations of IS15 have evolved to play, by transposition, distinct and complementary roles in bacterial evolution.

The ineffectiveness of tobramycin combination therapy in Streptococcus faecium endocarditis

A patient required mitral valve replacement following ineffective antibiotic treatment of enterococcal endocarditis caused by Streptococcus faecium. Endocarditis had relapsed despite therapy with ampicillin and tobramycin for six weeks. A second relapse had occurred following treatment with penicillin and gentamicin. Initial failure of antibiotic therapy may be related to the known lack of in vitro and in vivo synergy between penicillin and tobramycin against S. faecium. Effective therapy of enterococcal endocarditis requires considerations of bacterial speciation, determination of high-level aminoglycoside resistance, and preferably adequate antibiotic synergy studies to assure effective therapy.

Chloramphenicol acetyltransferase: enzymology and molecular biology

Naturally occurring chloramphenicol resistance in bacteria is normally due to the presence of the antibiotic inactivating enzyme chloramphenicol acetyltransferase (CAT) which catalyzes the acetyl-S-CoA-dependent acetylation of chloramphenicol at the 3-hydroxyl group. The product 3-acetoxy chloramphenicol does not bind to bacterial ribosomes and is not an inhibitor of peptidyltransferase. The synthesis of CAT is constitutive in E. coli and other Gram-negative bacteria which harbor plasmids bearing the structural gene for the enzyme, whereas Gram-positive bacteria such as staphylococci and streptococci synthesize CAT only in the presence of chloramphenicol and related compounds, especially those with the same stereochemistry of the parent compound and which lack antibiotic activity and a site of acetylation (3-deoxychloramphenicol). Studies of the primary structures of CAT variants suggest a marked degree of heterogeneity but conservation of amino acid sequence at and near the putative active site. All CAT variants are tetramers composed in each case of identical polypeptide subunits consisting of approximately 220 amino acids. The catalytic mechanism does not appear to involve an acyl-enzyme intermediate although one or more cysteine residues are protected from thiol reeagents by substrates. A highly reactive histidine residue has been implicated in the catalytic mechanism.

Characterization of two tetracycline resistance determinants in Streptococcus faecalis JH1

Streptococcus faecalis strain JH1 harbors two conjugative plasmids: pJH1, an R plasmid mediating resistance to kanamycin, streptomycin, gentamicin, erythromycin, and tetracycline, and pJH2, a hemolysin-bacteriocin plasmid. Studies of plasmid-cured derivatives of strain JH1 and of transconjugates obtained after mixed incubation of JH1 with the plasmid-free S. faecalis strain JH2-2 revealed the presence of two tetracycline resistance determinants in strain JH1. One determinant mediated constitutive resistance to 40 micrograms of tetracycline per ml and was associated with plasmid pJH1. The second determinant, either on the chromosome of strain JH1 or on an undetectable plasmid, was inducible by tetracycline and enabled the host strain, in the absence of pJH1, to grow in the presence of 80 micrograms of tetracycline per ml. One transconjugant, strain DL172, was resistant to 80 micrograms of tetracycline per ml, sensitive to kanamycin, streptomycin, and erythromycin, and hemolytic in the presence, but not in the absence, of tetracycline. A single plasmid, pDL172, from this strain consisted of plasmid pJH2 and a 17.8-kilobase segment of DNA homologous to total cell DNA from strain JH1 but did not contain plasmid pJH1. Whether the addition of heterologous DNA to plasmid pJH2 occurred by translocation of a 17.8-kilobase tetracycline resistance transposon or by classical recombination with pJH2 has not been determined.

Conjugative R plasmids in Streptococcus faecium (group D)

Ten isolates of Streptococcus faecium were found to be resistant to penicillin, tetracycline, macrolides and related drugs, streptomycin, and kanamycin, and four strains were resistant to chloramphenicol. Six of these 10 strains transferred all their resistance markers (except penicillin) by conjugation at a low frequency (10(-7) to 10(-9)). Several plasmids of different molecular weights were found in each of the wild-type strains. In 5 of 11 transconjugant strains, R plasmids were detected which had molecular weights identical to those of the plasmids found in the corresponding donor strain. Each of the six other transconjugants harbored one plasmid with a size different from those found in the corresponding donor strain, suggesting the occurrence of molecular events during or after conjugative transfer. None of the five tetracycline-resistant transconjugants contained detectable satellite DNA, HindIII restriction enzyme fingerprints of S. faecium resistance plasmids were different from the HindIII patterns of macrolide, aminoglycoside, and tetracycline resistance plasmids from other strains of streptococci.

Plasmids, drug resistance, and gene transfer in the genus Streptococcus

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Resistance to antibiotic synergism in Streptococcus faecalis: further studies with amikacin and with a new amikacin derivative, 4'-deoxy, 6'-N-methylamikacin

Streptococcus faecalis strains may resist penicillin-aminoglycoside synergy by the production of plasmid-mediated aminoglycoside-modifying enzymes. One of these enzymes, aminoglycoside 3'-phosphotransferase, has been shown to have a broad range of substrate specificity, including amikacin. We have studied a derivative of amikacin, 4'-deoxy, 6'-N-methylamikacin (BB-K311), against 11 clinical blood isolates of S. faecalis. Minimal inhibitory concentrations of BB-K311 were quite similar to those of amikacin, ranging from 125 to 1,000 micrograms/ml. In assays for antibiotic synergy, penicillin and amikacin produced enhanced killing compared with the penicillin alone only against those three strains which lacked the phosphotransferase enzyme. The other eight enzyme-positive strains actually demonstrated significant antagonism between penicillin against all 11 strains, regardless of enzyme production. Analysis of substrate profiles with crude preparations of the aminoglycoside 3'-phosphotransferase enzyme confirmed that BB-K311 was a very poor substrate for modification, as expected from the synergy studies. Use of other aminoglycoside analogs confirmed the 3'-OH site of modification. These findings suggest that removing the 4'-OH group in amikacin effectively blocks 3'-phosphorylation by S. faecalis enzyme.

Identification of tetracycline-resistant R-plasmids in Streptococcus agalactiae (group B)

In this report, 30 tetracycline-resistant clinical isolates of group B Streptococcus were examined to assess the extent to which tetracycline resistance is plasmid mediated. Of these, 27 showed no physical or genetic evidence of plasmid-mediated resistance; however, one conjugative and two small (3.5 X 10(6)-dalton) multicopy non-self-transmissible tetracycline resistance plasmids were identified. The conjugative plasmid was transmissible to Streptococcus faecalis as well as to Streptococcus agalactiae (group B). The two nonconjugative plasmids were readily mobilized by a number of sex factors into these same two backgrounds and, in addition, readily transformed Streptococcus sanguis Challis to tetracycline resistance. Due to readily available sites for several site-specific endonuycleases, these small, multicopy plasmids should prove useful as cloning vehicles in this host system.

Plasmid-mediated resistance to aminocyclitol antibiotics in group D streptococci

Streptococcus faecalis BM4100 was resistant to high levels of gentamicin, kanamycin, and structurally related antibiotics. The genes conferring resistance to aminocyclitols in this strain were carried by a plasmid, pIP800, self-transferable to other S. faecalis strains. The aminocyclitol resistance was mediated by constitutively synthesized phosphotransferase and acetyltransferase activities. It was inferred that phosphorylation occurred at the 2"-hydroxyl group and that acetylation occurred at the 6'-hydroxyl group of the aminocyclitols. The enzyme activities were not separable by gel filtration or by isoelectric focusing. Their apparent molecular weight was 31,000, and their isoelectric point was 5.3. With respect to substrate profile, size, and charge, the transferases from strain BM4100 resembled closely those with identical site specificity described in staphylococci. These results suggest that plasmid gene transfer may occur between the two pathogenic bacterial genera.

Conjugative transfer of multiple antibiotic resistance markers in Streptococcus pneumoniae

Two antibiotic-resistant isolates of Streptococcus pneumoniae were investigated for conjugative transfer of their drug resistance markers into streptococcal (groups B and D) and pneumococcal (encapsulated and non-encapsulataed) recipients. Of these, 7 wild-type donor pneumococci transferred all their resistance markers (except Pc [penicillin], Su [sulfonamide], and Tp [trimethoprim]) into group D Streptococcus and non-encapsulated S. pneumoniae recipients at a low frequency (10(-5) to 10(-6)). The resistance markers transferred were Tc (tetracycline); Tc and Cm (chloramphenicol); Tc and MLS (macrolides, lincosamides, and streptogramin B); Tc, MLS, Km (kanamycin), and Cm. The transconjugants obtained retransferred their resistance markers into appropriate streptococcal or pneumococcal recipients or both. The resistance markers of streptococcal transconjugants could not be cured by chemical agents. All attempts to detect extra-chromosomal deoxyribonucleic acid from pneumococcal or streptococcal transconjugants were unsuccessful. The molecular weight of a streptococcal conjugative R plasmid (pIP501) was investigated after transfer into the non-encapsulated S. pneumoniae recipient and was found to be similar to that of the wild-type group B Streptococcus host (20 x 10(6)).