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

The Plasmidome of Firmicutes: Impact on the Emergence and the Spread of Resistance to Antimicrobials

The phylum Firmicutes is one of the most abundant groups of prokaryotes in the microbiota of humans and animals and includes genera of outstanding relevance in biomedicine, health care, and industry. Antimicrobial drug resistance is now considered a global health security challenge of the 21st century, and this heterogeneous group of microorganisms represents a significant part of this public health issue.The presence of the same resistant genes in unrelated bacterial genera indicates a complex history of genetic interactions. Plasmids have largely contributed to the spread of resistance genes among Staphylococcus, Enterococcus, and Streptococcus species, also influencing the selection and ecological variation of specific populations. However, this information is fragmented and often omits species outside these genera. To date, the antimicrobial resistance problem has been analyzed under a "single centric" perspective ("gene tracking" or "vehicle centric" in "single host-single pathogen" systems) that has greatly delayed the understanding of gene and plasmid dynamics and their role in the evolution of bacterial communities.This work analyzes the dynamics of antimicrobial resistance genes using gene exchange networks; the role of plasmids in the emergence, dissemination, and maintenance of genes encoding resistance to antimicrobials (antibiotics, heavy metals, and biocides); and their influence on the genomic diversity of the main Gram-positive opportunistic pathogens under the light of evolutionary ecology. A revision of the approaches to categorize plasmids in this group of microorganisms is given using the 1,326 fully sequenced plasmids of Gram-positive bacteria available in the GenBank database at the time the article was written.

Axe-Txe, a broad-spectrum proteic toxin-antitoxin system specified by a multidrug-resistant, clinical isolate of Enterococcus faecium

Enterococcal species of bacteria are now acknowledged as leading causes of bacteraemia and other serious nosocomial infections. However, surprisingly little is known about the molecular mechanisms that promote the segregational stability of antibiotic resistance and other plasmids in these bacteria. Plasmid pRUM (24 873 bp) is a multidrug resistance plasmid identified in a clinical isolate of Enterococcus faecium. A novel proteic-based toxin-antitoxin cassette identified on pRUM was demonstrated to be a functional segregational stability module in both its native host and evolutionarily diverse bacterial species. Induced expression of the toxin protein (Txe) of this system resulted in growth inhibition in Escherichia coli. The toxic effect of Txe was alleviated by co-expression of the antitoxin protein, Axe. Homologues of the axe and txe genes are present in the genomes of a diversity of Eubacteria. These homologues (yefM-yoeB) present in the E. coli chromosome function as a toxin-antitoxin mechanism, although the Axe and YefM antitoxin components demonstrate specificity for their cognate toxin proteins in vivo. Axe-Txe is one of the first functional proteic toxin-antitoxin systems to be accurately described for Gram-positive bacteria.

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.

Molecular detection of antimicrobial resistance

The determination of antimicrobial susceptibility of a clinical isolate, especially with increasing resistance, is often crucial for the optimal antimicrobial therapy of infected patients. Nucleic acid-based assays for the detection of resistance may offer advantages over phenotypic assays. Examples are the detection of the methicillin resistance-encoding mecA gene in staphylococci, rifampin resistance in Mycobacterium tuberculosis, and the spread of resistance determinants across the globe. However, molecular assays for the detection of resistance have a number of limitations. New resistance mechanisms may be missed, and in some cases the number of different genes makes generating an assay too costly to compete with phenotypic assays. In addition, proper quality control for molecular assays poses a problem for many laboratories, and this results in questionable results at best. The development of new molecular techniques, e.g., PCR using molecular beacons and DNA chips, expands the possibilities for monitoring resistance. Although molecular techniques for the detection of antimicrobial resistance clearly are winning a place in routine diagnostics, phenotypic assays are still the method of choice for most resistance determinations. In this review, we describe the applications of molecular techniques for the detection of antimicrobial resistance and the current state of the art.

Nucleotide sequence and analysis of conjugative plasmid pVT745

The complete nucleotide sequence and genetic map of pVT745 are presented. The 25-kb plasmid was isolated from Actinobacillus actinomycetemcomitans, a periodontal pathogen. Two-thirds of the plasmid encode functions related to conjugation, replication, and replicon stability. Among potential gene products with a high degree of similarity to known proteins are those associated with plasmid conjugation. It was shown that pVT745 derivatives not only mobilized a coresident nontransmissible plasmid, pMMB67, but also mediated their own conjugative transfer to different A. actinomycetemcomitans strains. However, transfer of pVT745 derivatives from A. actinomycetemcomitans to Escherichia coli JM109 by conjugation was successful only when an E. coli origin of replication was present on the pVT745 construct. Surprisingly, 16 open reading frames encode products of unknown function. The plasmid contains a conserved replication region which belongs to the HAP (Haemophilus-Actinobacillus-Pasteurella) theta replicon family. However, its host range appears to be rather narrow compared to other members of this family. Sequences homologous to pVT745 have previously been detected in the chromosomes of numerous A. actinomycetemcomitans strains. The nature and origin of these homologs are discussed based on information derived from the nucleotide sequence.

Improved vectors for nisin-controlled expression in gram-positive bacteria

A set of shuttle vectors, able to replicate in Escherichia coli and in gram-positive bacteria, containing a nisin-inducible promoter (PnisA) and genes encoding NisR and NisK, the two-component signaling mechanism for activating transcription from PnisA in the presence of nisin, was constructed. To test these vectors, Enterococcus faecalis pCF10 plasmid genes prgX, prgY, and prgZ, which respectively encode cytosolic, integral membrane, and cell surface proteins, were cloned downstream of PnisA. Increased protein expression, in the presence of nisin, was demonstrated by Western blot analysis.

Comparison of antimicrobial resistance phenotypes and resistance genes in Enterococcus faecalis and Enterococcus faecium from humans in the community, broilers, and pigs in Denmark

Enterococcus faecalis and E. faecium isolated from humans in the community (98 and 65 isolates), broilers (126 and 122), and pigs (102 and 88) during 1998 were tested for susceptibility to 12 different antimicrobial agents and for the presence of selected genes encoding resistance using PCR. Furthermore, the presence of vancomycin resistant enterococci was examined in 38 human stool samples using selective enrichment. Widespread resistance to chloramphenicol, macrolides, kanamycin, streptomycin, and tetracycline was found among isolates from all three sources. All E. faecium isolates from humans and pigs were susceptible to avilamycin, whereas 35% of isolates from broilers were resistant. All E. faecium isolates from humans were susceptible to vancomycin, whereas 10% and 17% of isolates from broilers and pigs, respectively, were resistant. A vancomycin resistant E. faecium isolate was found in one of the 38 human fecal samples examined using selective enrichment. All vancomycin resistant isolates contained the vanA gene, all chloramphenicol resistant isolates the cat(pIP501) gene, and all five gentamicin resistant isolates the aac6-aph2 gene. Sixty-one (85%) of 72 erythromycin resistant E. faecalis examined and 57 (90%) of 63 erythromycin resistant E. faecium isolates examined contained ermB. Forty (91%) of the kanamycin resistant E. faecalis and 18 (72%) of the kanamycin resistant E. faecium isolates contained aphA3. The tet(M) gene was found in 95% of the tetracycline resistant E. faecalis and E. faecium isolates of human and animal origin, examined. tet(K) was not observed, whereas tet(L) was detected in 17% of tetracycline resistant E. faecalis isolates and in 16% of the E. faecium isolates. tet(O) was not detected in any of the isolates from pigs, but was observed in 38% of E. faecalis isolates from broilers, in two E. faecalis isolates from humans and in three E. faecium isolates from broilers. tet(S) was not detected among isolates from animals, but was observed in 31% of E. faecalis and one E. faecium isolate from humans. This study showed a frequent occurrence of antimicrobial resistance and the presence of selected resistance genes in E. faecalis and E. faecium isolated from humans, broilers and pigs. Differences in the occurrence of resistance and tetracycline resistance genes were observed among isolates from the different sources. However, similar resistance patterns and resistance genes were detected frequently indicating that transmission of resistant enterococci or resistance genes takes place between humans, broilers, and pigs.

Analysis of expression of prgX, a key negative regulator of the transfer of the Enterococcus faecalis pheromone-inducible plasmid pCF10

Conjugative transfer of pCF10, a plasmid found in Enterococcus faecalis, is induced by the peptide pheromone cCF10 and the donor-recipient aggregation is mediated by PrgB. Expression of prgB in pCF10 is negatively regulated by PrgX. The prgX gene is adjacent to prgQ which provides the promoter for prgB expression. The prgX and prgQ genes are transcribed in opposite directions. A deletion spanning nucleotides+259 to +398 from the prgQ transcription initiation site abolished the prgX gene products at both RNA and protein levels. An RNA, named Qa, was found to be transcribed in the antisense direction in the prgQ region. The transcription start site and the promoter were found to be almost identical with those of the traD determinant in pAD1, another pheromone-responsive plasmid. The first inverted repeat sequence in prgQ, IRS1, was required for the full activity of the Qa promoter. Although the size of the major Qa RNA detected by Northern blot analysis was too short (ca 120 nt) to be an mRNA for PrgX protein, the transcription from the Qa promoter was shown to proceed through to prgX. Transcriptional fusion analyses showed that the transcription of prgX requires one or more trans elements. Moreover, deletion of prgX greatly reduced the level of the Qa RNA and abolished transcription of prgX. Although transcription initiation from the Qa promoter was not increased by PrgX, transcriptional readthrough to prgX was increased by the protein. Based on these data, we conclude that transcription of prgX is initiated from the Qa promoter in prgQ, and PrgX autoregulates its transcription either by mediating transcriptional readthrough or increasing mRNA stability. The PrgX protein, prgX RNA, and the Qa RNA were downregulated by cCF10 induction; however, prgX gene products were still detected for at least 40 minutes after induction.

Growth-phase-dependent expression of virulence factors in an M1T1 clinical isolate of Streptococcus pyogenes

The effect of growth phase on expression of virulence-associated factors was studied by Northern hybridization in an M1T1 clinical isolate of Streptococcus pyogenes. Expression of M protein, C5a peptidase, and capsule was maximal in the exponential phase of growth, while streptococcal pyrogenic exotoxins A and B and mitogenic factor were maximally expressed in later phases of growth.

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.

Surveillance of antimicrobial resistance in bacteria isolated from food animals to antimicrobial growth promoters and related therapeutic agents in Denmark

This study was conducted to describe the occurrence of acquired resistance to antimicrobials used for growth promotion among bacteria isolated from swine, cattle and poultry in Denmark. Resistance to structurally related therapeutic agents was also examined. Three categories of bacteria were tested: 1) indicator bacteria (Escherichia coli, Enterococcus faecalis, Enterococcus faecium), 2) zoonotic bacteria (Campylobacter, Salmonella, Yersinia enterocolitica), and 3) animal pathogens (E. coli, Staphylococcus aureus, coagulase-negative staphylococci (CNS), Staphylococcus hyicus, Actinobacillus pleuropneumoniae). All antimicrobials used as growth promoters in Denmark and some structurally related therapeutic agents (in brackets) were included: Avilamycin, avoparcin (vancomycin), bacitracin, carbadox, flavomycin, monensin, olaquindox, salinomycin, spiramycin (erythromycin, lincomycin), tylosin (erythromycin, lincomycin), and virginiamycin (pristinamycin). Bacterial species intrinsically resistant to an antimicrobial were not tested towards that antimicrobial. Breakpoints for growth promoters were established by population distribution of the bacteria tested. A total of 2,372 bacterial isolates collected during October 1995 to September 1996 were included in the study. Acquired resistance to all currently used growth promoting antimicrobials was found. A frequent occurrence of resistance were observed to avilamycin, avoparcin, bacitracin, flavomycin, spiramycin, tylosin and virginiamycin, whereas resistance to carbadox, monensin, olaquindox and salinomycin was less frequent. The occurrence of resistance varied by animal origin and bacterial species. The highest levels of resistance was observed among enterococci, whereas less resistance was observed among zoonotic bacteria and bacteria pathogenic to animals. The association between the occurrence of resistance and the consumption of the antimicrobial is discussed. The results show the present level of resistance to growth promoters in bacteria from food animals in Denmark. They will form the baseline for comparison with future prospective studies, thereby enabling the determination of trends over time.

Forespore expression and processing of the SigE transcription factor in wild-type and mutant Bacillus subtilis

SigmaE is a mother cell-specific transcription factor of sporulating Bacillus subtilis that is derived from an inactive precursor protein (pro-sigmaE). To examine the process that prevents sigmaE activity from developing in the forespore, we fused the sigmaE structural gene (sigE) to forespore-specific promoters (PdacF and PspoIIIG), placed these fusions at sites on the B. subtilis chromosome which translocate into the forespore either early or late, and used Western blot analysis to monitor SigE accumulation and pro-sigmaE processing. sigE alleles, placed at sites which entered the forespore early, were found to generate more protein product than the same fusion placed at a late entering site. SigE accumulation and processing in the forespore were enhanced by null mutations in spoIIIE, a gene whose product is essential for translocation of the distal portion of the B. subtilis chromosome into the forespore. In other experiments, a chimera of pro-sigmaE and green fluorescence protein, previously shown to be unprocessed if it is synthesized within the forespore, was found to be processed in this compartment if coexpressed with the gene for the pro-sigmaE-processing enzyme, SpoIIGA. The need for spoIIGA coexpression is obviated in the absence of SpoIIIE. We interpret these results as evidence that selective degradation of both SigE and SpoIIGA prevent mature sigmaE from accumulating in the forespore compartment of wild-type B. subtilis. Presumably, a gene(s) located at a site that is distal to the origin of chromosome transfer is responsible for this phenomenon when it is translocated and expressed in the forespore.

Antimicrobial resistance in enterococci isolated from Turkey flocks fed virginiamycin

From 125 separate cloacal cultures from three turkey flocks fed virginiamycin, 104 Enterococcus faecium and 186 Enterococcus faecalis isolates were obtained. As the turkeys aged, there was a higher percentage of quinupristin-dalfopristin-resistant E. faecium isolates, with isolates from the oldest flock being 100% resistant. There were no vancomycin-resistant enterococci. Results of pulsed-field gel electrophoresis (PFGE) indicated there were 11 PFGE types of E. faecalis and 7 PFGE types of E. faecium that were in more than one group of flock cultures.

Transfer of Tn5385, a composite, multiresistance chromosomal element from Enterococcus faecalis

Tn5385 is a ca. 65-kb element integrated into the chromosomes of clinical Enterococcus faecalis strains CH19 and CH116. It confers resistance to erythromycin, gentamicin, mercuric chloride, streptomycin, tetracycline-minocycline, and penicillin via beta-lactamase production. Tn5385 is a composite structure containing regions previously found in staphylococcal and enterococcal plasmids. Several transposons and transposon-like elements within Tn5385 have been identified, including conjugative transposon Tn5381, composite transposon Tn5384, and elements indistinguishable from staphylococcal transposons Tn4001 and Tn552. The divergent regions of Tn5385 are linked by a series of insertion sequence (IS) elements (IS256, IS257, and IS1216) of staphylococcal and enterococcal origin. The ends of Tn5385 consist of directly repeated copies of enterococcal IS1216. Within the chromosomes of strains CH19 and CH116, Tn5385 has interrupted an open reading frame with substantial homology to previously described alkyl hydrogen peroxide reductase genes. Segments of this open reading frame in both CH19 and CH116 have been deleted, but the amount of deleted DNA differs for the two insertions. Transfer of Tn5385 from both donors into E. faecalis recipients occurs at a low frequency. Two types of transconjugants have been identified. In one type, the target alkyl hydrogen peroxide reductase open reading frame has been deleted, and sequences flanking Tn5385 in the respective donors are carried over to the transconjugants. These data suggest that the mechanism of Tn5385 insertion into the recipient chromosome in these transconjugants was recombination across flanking regions in the donors and homologous sequences in the recipients. The second type of transconjugant appears to have resulted from excision of Tn5385 from the CH19 chromosome by recombination across the terminal IS1216 elements and insertion into the recipient chromosome by recombination across Tn5381 (within Tn5385) and a previously transferred Tn5381 copy in the recipient chromosome. These data confirm that Tn5385 is a composite structure with genetic material from diverse genera and suggest that it is a functional transposon. They also suggest that chromosomal recombination is a mechanism of genetic exchange in enterococci.

Antibiotic resistance among enterococci isolated from clinical specimens between 1953 and 1954

Two hundred twenty group D streptococci isolated from 1953 to 1954 from patients in the Washington, D.C., area were characterized. All were susceptible to ampicillin, vancomycin, and gentamicin; none produced beta-lactamase activity. High-level resistance to streptomycin was expressed by 117 strains, and 2 strains were resistant to >8 microg of chloramphenicol per ml. Three isolates were resistant to both erythromycin and lincomycin, and DNA from these hybridized to an ermAM probe. Of 118 strains resistant to tetracycline and minocycline, genomic DNA from 63 was examined for homology to tet(M), tet(O), and tet(S). DNA from 20 strains hybridized to tet(M), DNA from 37 strains hybridized to tet(S), and DNA from none of the strains hybridized to tet(O).

Adjacent and divergently oriented operons under the control of the sporulation regulatory protein GerE in Bacillus subtilis

The DNA-binding protein GerE is the latest-acting regulatory protein in the mother cell line of gene expression during sporulation in Bacillus subtilis. GerE directs the transcription of several genes that encode structural components of the protein coat that encases the mature spore. We report on the identification and characterization of a cluster of additional genes whose transcription is dependent on GerE. These genes, which are located in the replication terminus region of the chromosome (181 degrees on the genetic map), are arranged in adjacent and divergently oriented operons called cgeAB and cgeCDE, which consist of two and at least three genes, respectively. CgeD, the product of the second member of the cgeCDE operon, is strikingly similar to the product of a B. subtilis gene (ipa-63d) of unknown function and is similar at its amino terminus to certain glycosyl transferases involved in polysaccharide biosynthesis. Strains with mutations in the cgeAB and cgeCDE operons produce spores with altered surface properties, on which basis we propose that proteins encoded by these operons influence maturation of the outermost layer of the spore, perhaps by glycosylation of coat proteins at the spore surface.

Use of a novel mobilizable vector to inactivate the scrA gene of Streptococcus sobrinus by allelic replacement

The virulence factors of the cariogenic bacterium Streptococcus sobrinus have been difficult to assess because of a lack of tools for the genetic manipulation of this organism. The construction of an Escherichia coli-Streptococcus shuttle vector, pDL289, that can be mobilized into S. sobrinus by the conjugative plasmid pAM beta 1 was described in a previous report. The vector contains pVA380-1 for replication and mobilization in streptococci, the pSC101 replicon for maintenance in E. coli, a kanamycin resistance marker that functions in both hosts, and the multiple cloning site and lacZ from pGEM7Zf(-). pDL289 is stable with or without selection in several species of Streptococcus. In this study, a derivative with a deletion in the minus origin of the pVA380-1 component of pDL289 was constructed. This derivative, pDL289 delta 202, was less stable than pDL289 in Streptococcus gordonii Challis, Streptococcus mutans, and S. sobrinus. Both pDL289 and pDL289 delta 202 were mobilizable by pAM beta 1 into S. sobrinus, with frequencies of 3 x 10(-6) and 1 x 10(-7) transconjugants per recipient CFU, respectively. The cloned scrA gene of S. sobrinus 6715-10 coding for the EIISuc of the sucrose-specific phosphoenolpyruvate phosphotransferase system was interrupted by the insertion of a streptococcal spectinomycin resistance gene active in E. coli and streptococci. The interrupted scrA gene was subcloned into both pDL289 and pDL289 delta 202. Each recombinant plasmid was introduced into the DL1 strain of S. gordonii Challis, which was then used as a recipient for the conjugative transfer of pAM beta 1. The latter plasmid was used to mobilize each recombinant plasmid from S. gordonii Challis DL1 to S. sobrinus 6715-10RF. Subsequently, recombinants derived from a double-crossover event were isolated on the basis of resistance to spectinomycin and susceptibility to kanamycin. Recombinational events were confirmed by Southern hybridization, and the inactivation of the EII Suc in double crossovers was confirmed by phosphotransferase system assays. This is the first report of allelic replacement in S. sobrinus.

Tn5384, a composite enterococcal mobile element conferring resistance to erythromycin and gentamicin whose ends are directly repeated copies of IS256

We have identified a 26-kb mobile element from Enterococcus faecalis CH116, designated Tn5384, which confers resistance to erythromycin and to high levels of gentamicin. Tn5384 is a composite element containing three copies of insertion element IS256. Two of the IS256 copies flank the aac6'-aph2" bifunctional aminoglycoside-modifying-enzyme gene in the inverted orientation, forming a structure similar to staphylococcal gentamicin resistance transposon Tn4001. One of the IS256 elements involved in the Tn4001-like structure also forms the left end of Tn5384, the right end of which is a directly repeated insertion of IS256 approximately 23 kb downstream of the leftmost insertion. Insertions of Tn5384 into enterococcal plasmid pLRM1 have been found associated with 8- and 9-bp duplications of the target sequence.

Genetic analysis of a region of the Enterococcus faecalis plasmid pCF10 involved in positive regulation of conjugative transfer functions

The prgB gene encodes the surface protein Asc10, which mediates cell aggregation resulting in high-frequency conjugative transfer of the pheromone-inducible tetracycline resistance plasmid pCF10 in Enterococcus faecalis. Previous Tn5 insertional mutagenesis and sequencing analysis of a 12-kb fragment of pCF10 indicated that a region containing prgX, -Q, -R, -S, and -T, located 3 to 6 kb upstream of prgB, is required to activate the expression of prgB. Complementation studies showed that the positive regulatory region functions in cis in an orientation-dependent manner (J. W. Chung and G. M. Dunny, Proc. Natl. Acad. Sci. USA 89:9020-9024, 1992). In order to determine the involvement of each gene in the activation of prgB, Tn5 insertional mutagenesis and exonuclease III deletion analyses of the regulatory region were carried out. The results indicate that prgQ and -S are required for the expression of prgB, while prgX, -R, and -T are not required. Western blot (immunoblot) analysis of these mutants shows that prgQ is also essential for the expression of prgA (encoding the surface exclusion protein Sec10), which is located between prgB and the positive-control region. Complementation analysis demonstrates that a cis-acting regulatory element is located in the prgQ region and that pCF10 sequences in an untranslated region 3' from prgQ are an essential component of the positive-control system. Analyses of various Tn5 insertions in pCF10 genes suggest that transcription reading into this transposon is terminated in E. faecalis but that outward-reading transcripts may initiate from within the ends of Tn5 or from the junction sequences.

Transcriptional analysis of a region of the Enterococcus faecalis plasmid pCF10 involved in positive regulation of conjugative transfer functions

The prgB gene encodes aggregation substance (Asc10) which is essential for transfer of the pheromone-inducible conjugative plasmid pCF10 in Enterococcus faecalis. The prgQ and prgS regions, located 4 kb upstream of prgB, are required for the expression of prgB. Complementation studies indicated that the prgQ region functions in cis and in an orientation-dependent manner relative to the prgB gene (J. W. Chung and G. M. Dunny, Proc. Natl. Acad. Sci. USA 89:9020-9024, 1992). Analysis of transcriptional fusions in this study, using a promoterless lacZ gene in several locations between prgQ and prgB, confirmed that the prgQ region does not carry a promoter for the expression of prgB and that prgB does not comprise an operon with prgA (which encodes the surface exclusion protein Sec10), the gene immediately upstream from prgB. Northern (RNA) blot analysis demonstrated that two distinct transcripts (Qs RNA and QL RNA), much larger than the prgQ gene, were expressed in the prgQ region. QS RNA was produced constitutively, whereas QL RNA was produced inducibly by pheromone. The lack of any other open reading frame in QL RNA and significant sequence complementarity between the 3' end of QL RNA and the promoter region of prgB suggested that the functional products of the prgQ region might be RNA molecules rather than proteins. A mutation in prgS completely abolished the production of QL RNA. A model for transcriptional activation of prgB is presented.

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.

Cis-acting, orientation-dependent, positive control system activates pheromone-inducible conjugation functions at distances greater than 10 kilobases upstream from its target in Enterococcus faecalis

The prgB gene encodes the surface protein, Asc10, which mediates cell aggregation, resulting in high-frequency conjugative transfer of the pheromone-inducible tetracycline-resistance plasmid pCF10 in Enterococcus faecalis. Messenger RNA analysis by Northern blot hybridization and primer extension indicates that prgB transcription is pheromone-inducible and monocistronic. Previous transposon mutagenesis and sequencing analysis of a 12-kilobase (kb) region of pCF10 indicated that several genes including prgR and prgS are required to activate expression of prgB. The distance (3-4 kb) between these regulatory genes and prgB suggested that the activation might function in trans. To test this, a promoterless lacZ gene fusion to prgB was constructed and cloned without some or all of the regulatory genes. Several restriction fragments of the regulatory region were cloned in a higher copy-number plasmid, and numerous complementation studies were carried out in E. faecalis. Complementation in trans was not observed in any of these experiments. However, when the regulatory region and target genes were cloned in different sites of the same plasmid, separated by as much as 12 kb, activation of prgB was observed. Interestingly, this activation occurred only when the regions were cloned in the same relative orientation in which they exist on wild-type pCF10. These results suggest that one or more regulatory molecules may bind to an upstream cis-acting site and track along the DNA to reach a target site to activate prgB transcription.

Molecular, genetic, and functional analysis of the basic replicon of pVA380-1, a plasmid of oral streptococcal origin

The 4.2-kb cryptic plasmid pVA380-1 has been used as a vector for the cloning of antibiotic resistance genes directly in streptococci, and in the construction of Escherichia coli/Streptococcus shuttle vectors. The results of subcloning experiments located the basic replicon of pVA380-1 within a 2.5-kb region. The nucleotide base sequence of this region was determined and contained a single complete open reading frame (ORF) encoding a 237-amino-acid peptide with a predicted size of 29 kDa. This peptide and a region of the DNA molecule 5' to the ORF encoding it shared homology with the replication protein and plus origin, respectively, of the Staphylococcus aureus plasmid pUB110. Data from Tn5 mutagenesis and complementation studies indicated that the protein product of the ORF was required for pVA380-1 replication in streptococci. Deletion of a region of the basic replicon distal to the plus origin and ORF produced an unstable derivative, and resulted in the accumulation of single-stranded replicative intermediates, consistent with the loss of a minus origin. All of these results suggest that pVA380-1 replicates by a rolling circle mode, and is most closely related to the pC194 family of single-stranded DNA plasmids.

Characterization of a regulatory network that controls sigma B expression in Bacillus subtilis

The sigB operon of Bacillus subtilis encodes sigma B and three additional open reading frames (orfV, orfW, and orfX). Having previously mapped several mutations that alter the induction pattern of a sigma B-dependent promoter (ctc) to regions of cloned B. subtilis DNA which contain these three open reading frames, we directly tested the regulatory potential of orfV, orfW, and orfX by creating null alleles of each of these genes and examining the effects of the mutations, either singly or in pairs, on transcription of ctc and the sigB operon. Using lacZ reporter gene fusions and Northern (RNA) blot analyses, we have determined that all three genes modulate the activation of the sigma B-dependent promoters at both the sigB operon and ctc. Our data are consistent with the three gene products participating in a single pathway of negative control. orfW and orfX single-mutant strains have high levels of sigB and ctc transcription. sigB and ctc transcription in an orfV strain is similar to that found in mutant strains which lack sigma B itself. The orfV mutation is dominant to orfX but recessive to orfW. These results suggest that OrfW is the primary inhibitor of sigma B-dependent transcription and that OrfV is capable of counteracting the negative control of OrfW but is prevented from doing this by the orfX gene product.

Cloning and nucleotide base sequence analysis of a spectinomycin adenyltransferase AAD(9) determinant from Enterococcus faecalis

Enterococcus faecalis LDR55, a human clinical isolate, is resistant to tetracycline (Tcr), erythromycin (Emr), and high levels (greater than 2,000 micrograms/ml) of spectinomycin (Spr) but not streptomycin. Filter matings between strain LDR55 and E. faecalis OG1-RF produced transconjugants with the following resistance phenotypes: Tcr Emr Spr, Tcr Emr, Tcr Spr, and Tcr only but never Emr or Spr only. The genetic determinant encoding resistance to spectinomycin was cloned in Streptococcus sanguis Challis from pDL55, a 26-kb plasmid harbored by a Tcr Spr transconjugant. Subcloning experiments yielded a 1.1-kb ClaI-NdeI fragment that encoded very high-level Spr in S. sanguis (10 mg/ml) and Escherichia coli (50 mg/ml). Cell extracts of cultures obtained from Spr strains expressed adenylating activity for spectinomycin but not for streptomycin, indicating that Spr was due to an AAD(9) activity. The nucleotide base sequence of the 1.1-kb ClaI-NdeI fragment contained a single 750-base open reading frame. The protein predicted from the open reading frame consisted of 250 amino acids and had a calculated size of approximately 28,000 daltons, similar to the size estimated from maxicell analysis (29,000 daltons). The deduced amino acid sequence of the streptococcal AAD(9) was compared with that of the AAD(9) encoded by staphylococcal transposon Tn554. The two proteins shared approximately 39% amino acid identity, which was expanded to 53% when conservative amino acid changes were included. When the streptococcal protein was compared with an AAD(3")(9) protein of E. coli, the degrees of identity were 27 and 47%, on the basis of actual amino acids and conservative replacements, respectively. The cloning and nucleotide base sequence analyses of the spectinomycin AAD(9) determinant from E. faecalis that results in high-level Spr when transferred to S. sanguis or E. coli are presented.

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

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Cariogenicity of Streptococcus mutans V403 glucosyltransferase and fructosyltransferase mutants constructed by allelic exchange

Streptococcus mutans produces several enzymes which metabolize sucrose. Three glucosyltransferase genes (gtfB, gtfC, and gtfD) and a single fructosyltransferase gene (ftf) encode enzymes which are important in formation of exopolysaccharides. Mutants of S. mutans V403 carrying single and multiple mutations of the gtfB, gtfC, gtfD, and ftf genes recently have been constructed by allelic exchange in our laboratory. Using selected strains from this panel of mutants, we examined the importance of water-insoluble glucan, water-soluble glucan, and fructan production in cariogenicity while controlling for the effects of strain and species variability. Genetic and biochemical characterization of mutants and assays of glucosyltransferase and fructosyltransferase activities were performed to ensure that the phenotypes of strains coincided with deficiencies predicted by genotype. The young gnotobiotic rat model of cariogenicity was used to assess virulence of the wild-type strain and isogenic mutants. Mutant strains were less virulent than the wild type in almost every location examined for caries on tooth surfaces and level of involvement of lesions (depth and severity). Inactivation of either gtfB and gtfC or ftf dramatically reduced virulence; the subsequent inactivation of gtfD did not enhance the effect of reduced virulence.

Improved electroporation and cloning vector system for gram-positive bacteria

A protocol for transformation of intact Enterococcus faecalis cells by electroporation was developed through a systematic examination of the effects of changes in various parameters, including (i) growth conditions; (ii) composition of the electroporation solution; (iii) electroporation conditions, such as field strength and resistance; (iv) size, concentration, and purity of DNA used for transformation; and (v) conditions used to select for transformants. Key features of this protocol include the use of exponential-phase cells grown in inhibitory concentrations of glycine and the use of an acidic sucrose electroporation solution. Frequencies of greater than 2 x 10(5) transformants per microgram of plasmid DNA were obtained for E. faecalis cells, whereas various strains of streptococci and Bacillus anthracis were transformed at frequencies of 10(3) to 10(4) transformants per microgram of plasmid DNA with the same protocol. A novel Escherichia coli-Streptococcus and Enterococcus shuttle cloning vector, pDL276, was constructed for use in conjunction with the electroporation system. This vector features a multiple cloning site region flanked by E. coli transcription termination sequences, a relatively small size (less than 7 kb), and a kanamycin resistance determinant expressed in both gram-positive and gram-negative hosts. Various enterococcal and streptococcal DNA sequences were cloned in E. coli (including sequences that could not be cloned on other vectors) and were returned to the original host by electroporation. The vector and electroporation system was also used to clone directly into E. faecalis.

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.

High-level chromosomal gentamicin resistance in Streptococcus agalactiae (group B)

This is the first report of high-level gentamicin resistance in a group B streptococcus. Strain B128 of serotype II was isolated from an infected leg wound in 1987. B128 was resistant to high levels of gentamicin as well as of all other available aminoglycosides and was also resistant to tetracyclines. No bactericidal synergism was found between ampicillin or vancomycin and any of these aminoglycosides. Gentamicin, kanamycin, streptomycin, and tetracycline resistance determinants transferred by conjugation into a plasmid-free group B streptococcus recipient at a frequency of 10(-8) to 10(-9) transconjugants per donor cell. No transconjugants were detected when streptococci of groups A, C, and G, Streptococcus sanguis, or Enterococcus faecalis was used as a recipient. No plasmids were detected in B128 or in any of the four transconjugants tested. By DNA-DNA hybridization, homology was detected between gene aac6/aph2, of E. faecalis origin, and a 2.4-kilobase HindIII chromosomal fragment of B128; homology to the genes aph3 and aadE, of E. faecalis origin, was found with HindIII chromosomal fragments of the same size (3.0 kilobases). Strains like B128, which potentially can be responsible for severe neonatal infections, are of great clinical concern, since there are to date no antibiotic combinations exhibiting bactericidal synergism against them.

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.

The life and times of the Enterococcus

Enterococci are important human pathogens that are increasingly resistant to antimicrobial agents. These organisms were previously considered part of the genus Streptococcus but have recently been reclassified into their own genus, called Enterococcus. To date, 12 species pathogenic for humans have been described, including the most common human isolates, Enterococcus faecalis and E. faecium. Enterococci cause between 5 and 15% of cases of endocarditis, which is best treated by the combination of a cell wall-active agent (such as penicillin or vancomycin, neither of which alone is usually bactericidal) and an aminoglycoside to which the organism is not highly resistant; this characteristically results in a synergistic bactericidal effect. High-level resistance (MIC, greater than or equal to 2,000 micrograms/ml) to the aminoglycoside eliminates the expected bactericidal effect, and such resistance has now been described for all aminoglycosides. Enterococci can also cause urinary tract infections; intraabdominal, pelvic, and wound infections; superinfections (particularly in patients receiving expanded-spectrum cephalosporins); and bacteremias (often together with other organisms). They are now the third most common organism seen in nosocomial infections. For most of these infections, single-drug therapy, most often with penicillin, ampicillin, or vancomycin, is adequate. Enterococci have a large number of both inherent and acquired resistance traits, including resistance to cephalosporins, clindamycin, tetracycline, and penicillinase-resistant penicillins such as oxacillin, among others. The most recent resistance traits reported are penicillinase resistance (apparently acquired from staphylococci) and vancomycin resistance, both of which can be transferred to other enterococci. It appears likely that we will soon be faced with increasing numbers of enterococci for which there is no adequate therapy.

Susceptibility of enterococci and epidemiology of enterococcal infection in the 1980s

Enterococcisensu strictoform part of the normal gut flora (1) and may be found in the mouth, vagina and anterior urethra (2). They are opportunist pathogens which can cause serious infection including endocarditis. Nosocomial enterococcal infection appears to be increasing both in the UK (Public Health Laboratory Service [PHLS] Communicable Disease Surveillance Centre [CDSC], unpublished) and the USA (3) and to correspond to usage of broad spectrum β-lactam antimicrobial agents (4−7) and invasive surgical devices (8, 9). At the same time, the incidence of enterococci resistant or tolerant to previously commonly employed antimicrobial agents or their synergistic combinations is increasing and is compromising therapy of serious enterococcal infection. Strains of enterococci with high-level resistance to streptomycin and kanamycin (minimal inhibitory concentrations [MICs] > 2000 mg/L) were first reported in 1970 (10, 11) and rapidly became widespread (8, 12−14).

Antibiotic resistance and production of inhibitory substances among bacterial strains isolated from the oral cavities of BALB/c mice

In the oral cavities of BALB/c mice, microbial population levels are regulated by multifactorial processes. Factors include the production of inhibitory substances and the exchange of genetic material. In this work, 371 isolates from different sites (saliva, tongue, teeth, and mucosa) of the oral cavities of BALB/c mice were screened for resistance to antibiotics and antimicrobial activity. Antibiotic-resistant strains represented 25% of the total flora. Among the predominant species, all the S. faecalis isolates showed multiresistance, and 23% of the Lactobacillus murinus isolates and 15% of the Staphylococcus aureus were resistant to at least one antibiotic. Resistance to aminoglycosides (neomycin, streptomycin, kanamycin, and gentamicin) was most frequently encountered. In S. faecalis, high levels of resistance were recorded to neomycin and streptomycin but not to gentamicin or kanamycin. Macrolides (M), lincosamides (L), streptogramin B (S), tetracycline (Tc), and chloramphenicol (Cm) resistance was also present in multiresistance patterns, especially among S. faecalis isolates. Hemolytic (Hly+) streptococci were less resistant to MLS, Tc, and Cm than were non-hemolytic (Hly-) isolates. Resistance to beta-lactam antibiotics was detected only among staphylococci and with a low prevalence (4%). The frequencies of strains producing antimicrobial substances against the indicator strains (S. mutans LG-1, S. sanguis Ny 101, and A. viscosus Ny 1) were high for L. murinus (76%) and S. faecalis (57% for Hly- and 90% for Hly+), but low for S. aureus (7%). These results indicate that the indigenous oral flora could interfere with colonization by allochthonous micro-organisms and that resistance patterns should be taken into account for the elimination of the oral indigenous flora by antibiotic treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative in vitro activities of new 14-, 15-, and 16-membered macrolides

The in vitro activities of several 14-, 15- and 16-membered macrolides were compared with that of erythromycin. In general, 14-membered macrolides such as erythromycin, clarithromycin, and flurithromycin were more active against streptococci and Bordetella pertussis than was the 15-membered macrolide azithromycin, which was more active than 16-membered macrolides such as miocamycin and rokitamycin. Clarithromycin was the most active compound against Streptococcus pyogenes, pneumococci, Listeria monocytogenes, and Corynebacterium species. Legionella pneumophila was most susceptible to miocamycin, clarithromycin, and rokitamycin. Branhamella catarrhalis, Neisseria gonorrhoeae, and Haemophilus influenzae were most susceptible to azithromycin. Azithromycin and dirithromycin were the most active compounds against Campylobacter jejuni. MICs of 16-membered macrolides for strains expressing inducible-type resistance to erythromycin were less than or equal to 1 microgram/ml, whereas none of the compounds had activity against strains expressing constitutive-type resistance. The MICs of roxithromycin, miocamycin, rokitamycin, and josamycin increased in the presence of human serum, whereas MICs of the other compounds either were unchanged or decreased.

Transposition of Tn4551 in Bacteroides fragilis: identification and properties of a new transposon from Bacteroides spp

Tn4551, a clindamycin resistance (Ccr) transposon from the R plasmid pBI136, was cloned onto an Escherichia coli-Bacteroides shuttle vector which could replicate normally in E. coli but was maintained unstably in Bacteroides fragilis. To aid in cloning and to ensure maintenance of Tn4551 in E. coli, a kanamycin resistance determinant (Kmr) was inserted in the transposon. The transposon-bearing shuttle vector pFD197 was transformed into B. fragilis 638, and putative insertions of Tn4551::Kmr were identified by screening for resistance to clindamycin and plasmid content. Southern hybridization analyses were used to verify integration of the transposon in the B. fragilis chromosome, and the frequency of insertion was estimated at 7.8 X 10(-5) events per generation. In 57% of the isolates tested a second integration event also occurred. This second insertion apparently involved just a single copy of the 1.2-kilobase repeat sequence which flanks the transposon. In addition, Tn4551::Kmr appeared to function as a transposon in E. coli. Evidence for this was obtained by the isolation of transposon insertions into the bacteriophage P1 genome. Finally, the transposon vector, pFD197, could be mobilized to other B. fragilis strains in which transposition was detected. Mobilization from the strain 638 background was via a conjugation like process, but occurred in the absence of known conjugative elements or other detectable plasmids. This result suggested the presence of a host-encoded transfer system in this B. fragilis strain.

Nucleic acid probes in clinical microbiology

The infectious disease applications of nucleic acid probe have been described. In addition, the basic procedures of nucleic acid probe technology have been discussed, as have the factors affecting implementation of probe technology in diagnostic laboratories. Despite the questions raised, nucleic acid probes will become part of the diagnostic laboratory in the near future. Commercial interests are developing and marketing new probes, reagents, and kits which will expedite the employment of this technology. High-volume reference laboratories will first use probes as part of a battery of tests which will include ELISA and monoclonal antibody methods. In all probability, probes will replace methods: that have proven to be ineffective, difficult, or costly such as culturing for some enteric pathogens and Legionella, that require long incubation periods, such as mycobacteria, or that have high costs and low yields, such as virology.