A strain of Streptococcus agalactiae displayed resistance to 14-, 15-, and 16-membered macrolides. In PCR assays, total genomic DNA from this strain contained neither erm nor mef genes. EcoRI-digested genomic DNA from this strain was cloned into lambda Zap II to construct a library of S. agalactiae genomic DNA. A clone, pAES63, expressing resistance to erythromycin, azithromycin, and spiramycin in Escherichia coli was recovered. Deletion derivatives of pAES63 which defined a functional region on this clone that encoded resistance to 14- and 15-membered, but not 16-membered, macrolides were produced. Studies that determined the levels of incorporation of radiolabelled erythromycin into E. coli were consistent with the presence of a macrolide efflux determinant. This putative efflux determinant was distinct from the recently described Mef pump in Streptococcus pyogenes and Streptococcus pneumoniae and from the multicomponent MsrA pump in Staphylococcus aureus and coagulase-negative staphylococci. Its gene has been designated mreA (for macrolide resistance efflux).
We report here the identification and characterization of mrdH, a novel chromosomal metal resistance determinant, located in the genomic island 55 of Pseudomonas putida KT2440. It encodes for MrdH, a predicted protein of ∼40 kDa with a chimeric domain organization derived from the RcnA and RND (for resistance-nodulation-cell division) metal efflux proteins. The metal resistance function of mrdH was identified by the ability to confer nickel resistance upon its complementation into rcnA mutant (a nickel- and cobalt-sensitive mutant) of Escherichia coli. However, the disruption of mrdH in P. putida resulted in an increased sensitivity to cadmium and zinc apart from nickel. Expression studies using quantitative reverse transcription-PCR showed the induction of mrdH by cadmium, nickel, zinc, and cobalt. In association with mrdH, we also identified a conserved hypothetical gene mreA whose encoded protein showed significant homology to NreA and NreA-like proteins. Expression of the mreA gene in rcnA mutant of E. coli enhanced its cadmium and nickel resistance. Transcriptional studies showed that both mrdH and mreA underwent parallel changes in gene expression. The mobile genetic elements Tn4652 and IS1246, flanking mrdH and mreA were found to be induced by cadmium, nickel, and zinc, but not by cobalt. This study is the first report of a single-component metal efflux transporter, mrdH, showing chimeric domain organization, a broad substrate spectrum, and a location amid metal-inducible mobile genetic elements.
Macrolide resistance has been demonstrated in group B streptococcus (GBS), but there is limited information regarding mechanisms of resistance and their prevalence. We determined these in GBS obtained from neonatal blood cultures and vaginal swabs from pregnant women. Of 178 isolates from cases of neonatal GBS sepsis collected from 1995 to 1998, 8 and 4.5% were resistant to erythromycin and clindamycin, respectively, and one isolate showed intermediate penicillin resistance (MIC, 0.25 μg/ml). Of 101 consecutive vaginal or rectal/vaginal isolates collected in 1999, 18 and 8% were resistant to erythromycin and clindamycin, respectively. Tetracycline resistance was high (>80%) among both groups of isolates. Of 32 erythromycin-resistant isolates, 28 possessed the erm methylase gene (7 ermB and 21 ermTR/ermA) and 4 harbored the mefA gene; one isolate harbored both genes. One isolate which was susceptible to erythromycin but resistant to clindamycin (MIC, 4 μg/ml) was found to have the linB gene, previously identified only in Enterococcus faecium. The mreA gene was found in all the erythromycin-resistant strains as well as in 10 erythromycin-susceptible strains. The rate of erythromycin resistance increased from 5% in 1995–96 to 13% in 1998–99, which coincided with an increase in macrolide usage during that time.
In the present report we describe the characteristics of 189 antimicrobial-resistant Streptococcus agalactiae isolates from bovine (38 isolates) and human (151 isolates) sources. All the strains were resistant to tetracycline (TET), and 16 (8.5%) were also resistant to erythromycin, corresponding to 23.7% of the TET-resistant bovine isolates and 4.6% of the TET-resistant human isolates. The tet(O), erm(B), and mreA resistance-related genes, as well as the bca and scpB virulence-related genes, were the most frequent among the bovine isolates, while the tet(M), erm(A), mreA, bca, lmb, and scpB genes were the most prevalent among the isolates from humans. Although a few major clusters were observed, pulsed-field gel electrophoresis results revealed a variety of profiles, reflecting the substantial genetic diversity among strains of this species isolated from either humans or bovines.
Streptococcus agalactiae (group B streptococcus [GBS]) is the leading cause of neonatal and maternal sepsis. Penicillin is recommended for intrapartum prophylaxis, but erythromycin or clindamycin is used for penicillin-allergic carriers. Antibiotic resistance (AR) has increased recently and needs to be monitored. We have developed a multiplex PCR-based reverse line blot (mPCR/RLB) hybridization assay to detect, simultaneously, seven genes encoding AR—erm(A/TR), erm(B), mef(A/E), tet(M), tet(O), aphA-3, and aad-6—and two AR-related genes, int-Tn and mreA. We tested 512 GBS isolates from Asia and Australasia and compared mPCR/RLB with antibiotic susceptibility phenotype or single-gene PCR. Phenotypic resistance to tetracycline was identified in 450 (88%) isolates, of which 442 had tet(M) (93%) and/or tet(O) (6%). Of 67 (13%) erythromycin-resistant isolates, 18 were susceptible to clindamycin, i.e., had the M phenotype, encoded by mef(A/E); 39 had constitutive (cMLSB) and 10 inducible clindamycin resistance, and of these, 34 contained erm(B) and 12 erm(A/TR). Of four additional isolates with mef(A/E), three contained erm(B) with cMLSB and one was erythromycin susceptible. Of 61 (12%) clindamycin-resistant isolates, 20 were susceptible to erythromycin and two had intermediate resistance. Based on sequencing, 21 of 22 isolates with mef had mef(E), and 8 of 353 with int-Tn had an atypical sequence. Several AR genes, erm(B), tet(O), aphA-3, aad-6, and mef(A/E), were significantly more common among Asian than Australasian isolates, and there were significant differences in distribution of AR genes between GBS serotypes. Our mPCR/RLB assay is simple, rapid, and suitable for surveillance of antibiotic resistance in GBS.
Streptococcus agalactiae UCN36 was resistant to lincomycin (MIC = 16 μg/ml) but susceptible to clindamycin (MIC = 0.12 μg/ml) and erythromycin (MIC = 0.06 μg/ml). A 4-kb HindIII fragment was cloned from S. agalactiae UCN36 total DNA on plasmid pUC18 and introduced into Escherichia coli AG100A, where it conferred resistance to lincomycin. The sequence analysis of the fragment showed the presence of a 1,724-bp element delineated by imperfect inverted repeats (22 of 25 bp) and inserted in the operon for capsular synthesis of S. agalactiae UCN36. This element carried two open reading frames (ORF). The deduced amino acid sequence of the upstream ORF displayed similarity with transposases from anaerobes and IS1. The downstream ORF, lnu(C), encoded a 164-amino-acid protein with 26% to 27% identity with the LnuAN2, LnuA, and LnuA′ lincosamide nucleotidyltransferases reported for Bacteroides and Staphylococcus, respectively. Crude lysates of E. coli AG100A containing the cloned lnu(C) gene inactivated lincomycin and clindamycin in the presence of ATP and MgCl2. Mass spectrometry experiments demonstrated that the LnuC enzyme catalyzed adenylylation of lincomycin.
Results are presented supporting a regulatory role for the product of the MA3302 gene locus (designated MreA) previously annotated as a hypothetical protein in the methanogenic species Methanosarcina acetivorans of the domain Archaea. Sequence analysis of MreA revealed identity to the TrmB family of transcription factors, albeit the sequence is lacking the sensor domain analogous to TrmBL2, abundant in nonmethanogenic species of the domain Archaea. Transcription of mreA was highly upregulated during growth on acetate versus methylotrophic substrates, and an mreA deletion (ΔmreA) strain was impaired for growth with acetate in contrast to normal growth with methylotrophic substrates. Transcriptional profiling of acetate-grown cells identified 280 genes with altered expression in the ΔmreA strain versus the wild-type strain. Expression of genes unique to the acetate pathway decreased whereas expression of genes unique to methylotrophic metabolism increased in the ΔmreA strain relative to the wild type, results indicative of a dual role for MreA in either the direct or indirect activation of acetate-specific genes and repression of methylotrophic-specific genes. Gel shift experiments revealed specific binding of MreA to promoter regions of regulated genes. Homologs of MreA were identified in M. acetivorans and other Methanosarcina species for which expression patterns indicate roles in regulating methylotrophic pathways.
Species in the domain Archaea utilize basal transcription machinery resembling that of the domain Eukarya, raising questions addressing the role of numerous putative transcription factors identified in sequenced archaeal genomes. Species in the genus Methanosarcina are ideally suited for investigating principles of archaeal transcription through analysis of the capacity to utilize a diversity of substrates for growth and methanogenesis. Methanosarcina species switch pathways in response to the most energetically favorable substrate, metabolizing methylotrophic substrates in preference to acetate marked by substantial regulation of gene expression. Although conversion of the methyl group of acetate accounts for most of the methane produced in Earth’s biosphere, no proteins involved in the regulation of genes in the acetate pathway have been reported. The results presented here establish that MreA participates in the global regulation of diverse methanogenic pathways in the genus Methanosarcina. Finally, the results contribute to a broader understanding of transcriptional regulation in the domain Archaea.
Streptomyces davawensis synthesizes the antibiotic roseoflavin, one of the few known natural riboflavin analogs, and is roseoflavin resistant. It is thought that the endogenous flavokinase (EC 220.127.116.11)/flavin adenine dinucleotide (FAD) synthetase (EC 18.104.22.168) activities of roseoflavin-sensitive organisms are responsible for the antibiotic effect of roseoflavin, producing the inactive cofactors roseoflavin-5′-monophosphate (RoFMN) and roseoflavin adenine dinucleotide (RoFAD) from roseoflavin. To confirm this, the FAD-dependent Sus scrofa d-amino acid oxidase (EC 22.214.171.124) was tested with RoFAD as a cofactor and found to be inactive. It was hypothesized that a flavokinase/FAD synthetase (RibC) highly specific for riboflavin may be present in S. davawensis, which would not allow the formation of toxic RoFMN/RoFAD. The gene ribC from S. davawensis was cloned. RibC from S. davawensis was overproduced in Escherichia coli and purified. Analysis of the flavokinase activity of RibC revealed that the S. davawensis enzyme is not riboflavin specific (roseoflavin, kcat/Km = 1.7 10−2 μM−1 s−1; riboflavin, kcat/Km = 7.5 10−3 μM−1 s−1). Similar results were obtained for RibC from the roseoflavin-sensitive bacterium Bacillus subtilis (roseoflavin, kcat/Km = 1.3 10−2 μM−1 s−1; riboflavin, kcat/Km = 1.3 10−2 μM−1 s−1). Both RibC enzymes synthesized RoFAD and RoFMN. The functional expression of S. davawensis ribC did not confer roseoflavin resistance to a ribC-defective B. subtilis strain.
Streptococcus uberis UCN 42, isolated from a case of bovine mastitis, was intermediately resistant to lincomycin (MIC = 2 μg/ml) while remaining susceptible to clindamycin (MIC = 0.06 μg/ml) and erythromycin. A 1.1-kb SacI fragment was cloned from S. uberis UCN 42 total DNA on plasmid pUC 18 and introduced into Escherichia coli AG100A, where it conferred resistance to both clindamycin and lincomycin. The sequence analysis of the fragment showed the presence of a new gene, named lnu(D), that encoded a 164-amino-acid protein with 53% identity with Lnu(C) previously reported to occur in Streptococcus agalactiae. Crude lysates of E. coli AG100A containing the cloned lnu(D) gene inactivated lincomycin and clindamycin in the presence of ATP and MgCl2. Mass spectrometry experiments demonstrated that the lnu(D) enzyme catalyzed adenylylation of clindamycin. A domain conserved in deduced sequences of lincosamide O-nucleotidyltransferases Lnu(A), Lnu(C), LinAN2, and Lin(D) and in the aminoglycoside nucleotidyltransferase ANT(2′′) was identified.
The ability of vertebrate metallothionein (MT) genes to be induced by heavy metals is controlled by metal regulatory elements (MREs) present in the promoter in multiple, non-identical copies. The binding specificity of the mouse L-cell nuclear factor(s) that interact with the element MREd of the mouse MT-I gene was analyzed by in vitro footprinting, protein blotting, and UV cross-linking assays. In vitro footprinting analyses revealed that synthetic oligodeoxynucleotides (oligomers) corresponding to the metal regulatory elements MREa, MREb, MREc, MREd and MREe of the mouse MT-I gene, as well as the MRE4 of the human MT-IIA gene and the MREa of the trout MT-B gene, all competed for the nuclear protein species binding to the MREd region of the mouse MT-I gene, the MREe oligomer being the weakest competitor. In addition, protein blotting experiments revealed that a nuclear protein of 108 kDa, termed metal element protein-1 (MEP-1), which specifically binds with high affinity to mouse MREd, binds with different affinities to the other mouse MRE elements, mimicking their relative transcriptional strength in vivo: MREd greater than or equal to MREa = MREc greater than MREb greater than MREe greater than MREf. Similarly, human MRE4 and trout MREa bind to MEP-1. A protein similar in size to MEP-1 was also detected in HeLa-cell nuclear extracts. In UV cross-linking experiments the major protein species, complexed with mouse MREd oligomers, migrated on a denaturating gel with an apparent Mr of 115,000 and was detected using each of the mouse MRE oligomers tested. These results show that a mouse nuclear factor can bind to multiple MREs in mouse, trout, and human MT genes.
The in vitro activities of the new agents linezolid, quinupristin-dalfopristin, moxifloxacin, and trovafloxacin were determined and compared with those of penicillin, clindamycin, and four macrolides against 53 erythromycin-resistant Streptococcus pneumoniae, 117 S. pyogenes (64 erythromycin-susceptible and 53 -resistant), and 101 S. agalactiae (53 erythromycin-susceptible and 48 -resistant) isolates. Differentiation of macrolide resistance phenotypes was performed by the double-disk method. The genetic basis for macrolide resistance in 52 strains was also determined. The M phenotype was found in 84.9, 6.3, and 1.9% of S. pyogenes, S. agalactiae, and S. pneumoniae isolates, respectively. These strains were susceptible to miocamycin and clindamycin. Strains with the inducible phenotype accounted for 27.1% of S. agalactiae isolates and 9.4% each of S. pyogenes and S. pneumoniae isolates. All erythromycin-resistant isolates were also resistant to the 14- and 15-membered macrolides tested. Strains with all three phenotypes were susceptible to ≤2 μg of linezolid per ml. Quinupristin-dalfopristin exhibited good in vitro activity against all strains, irrespective of their resistance to erythromycin (MICs at which 90% of the isolates tested were inhibited [MIC90s], 0.2 to 1 μg/ml). Against the erythromycin-resistant S. pyogenes and S. agalactiae strains, moxifloxacin and trovafloxacin were the most active agents (MIC90s, 0.1 μg/ml). The new antimicrobials evaluated may be alternative agents to treat infections caused by macrolide-resistant as well as macrolide-susceptible streptococci.
Streptococcus uberis UCN60 was resistant to spiramycin (MIC = 8 μg/ml) but susceptible to erythromycin (MIC = 0.06 μg/ml), azithromycin (MIC = 0.12 μg/ml), josamycin (MIC = 0.25 μg/ml), and tylosin (MIC = 0.5 μg/ml). A 2.5-kb HindIII fragment was cloned from S. uberis UCN60 DNA on plasmid pUC18 and introduced into Escherichia coli AG100A, where it conferred resistance to spiramycin by inactivation. The sequence analysis of the fragment showed the presence of an rdmC-like gene that putatively encoded a protein belonging to the alpha/beta hydrolase family and of the first 196 nucleotides of the mph(B) gene putatively encoding a phosphotransferase known to inactivate 14-, 15-, and 16-membered macrolides in E. coli. The entire mph(B) gene was then identified in S. uberis UCN60. The two genes were expressed alone or in combination in E. coli, Staphylococcus aureus, and Enterococcus faecalis. Analysis of MICs revealed that rdmC-like alone did not confer resistance to erythromycin, tylosin, and josamycin in those three hosts. It conferred resistance to spiramycin in E. coli and E. faecalis but not in S. aureus. mph(B) conferred resistance in E. coli to erythromycin, tylosin, josamycin, and spiramycin but only low levels of resistance in E. faecalis and S. aureus to spiramycin (MIC = 8 μg/ml). The combination of mph(B) and rdmC-like genes resulted in a resistance to spiramycin and tylosin in the three hosts that significantly exceeded the mere addition of the resistance levels conferred by each resistance mechanism alone.
Streptococcus pyogenes strains inducibly resistant (iMLS phenotype) to macrolide, lincosamide, and streptogramin B (MLS) antibiotics can be subdivided into three phenotypes: iMLS-A, iMLS-B, and iMLS-C. This study focused on inducibly erythromycin-resistant S. pyogenes strains of the iMLS-B and iMLS-C types, which are very similar and virtually indistinguishable in a number of phenotypic and genotypic features but differ clearly in their degree of resistance to MLS antibiotics (high in the iMLS-B type and low in the iMLS-C type). As expected, the iMLS-B and iMLS-C test strains had the erm(A) methylase gene; the iMLS-A and the constitutively resistant (cMLS) isolates had the erm(B) methylase gene; and a control M isolate had the mef(A) efflux gene. mre(A) and msr(A), i.e., other macrolide efflux genes described in gram-positive cocci, were not detected in any test strain. With a radiolabeled erythromycin method for determination of the intracellular accumulation of the drug in the absence or presence of an efflux pump inhibitor, active efflux of erythromycin was observed in the iMLS-B isolates as well as in the M isolate, whereas no efflux was demonstrated in the iMLS-C isolates. By the triple-disk (erythromycin plus clindamycin and josamycin) test, performed both in normal test medium and in the same medium supplemented with the efflux pump inhibitor, under the latter conditions iMLS-B and iMLS-C strains were no longer distinguishable, all exhibiting an iMLS-C phenotype. In conjugation experiments with an iMLS-B isolate as the donor and a Rifr Fusr derivative of an iMLS-C isolate as the recipient, transconjugants which shared the iMLS-B type of the donor under all respects, including the presence of an efflux pump, were obtained. These results indicate the existence of a novel, transferable efflux system, not associated with mef(A) or with other known macrolide efflux genes, that is peculiar to iMLS-B strains. Whereas the low-level resistance of iMLS-C strains to MLS antibiotics is apparently due to erm(A)-encoded methylase activity, the high-level resistance of iMLS-B strains appears to depend on the same methylase activity plus the new efflux system.
Recently, two related chimeric genetic elements (Tn1207.3 and Φ10394.4) were shown to carry the macrolide efflux gene mef in Streptococcus pyogenes (group A streptococci [GAS]). The dissemination of elements belonging to the Tn1207.3/Φ10394.4 family in recent isolates of GAS, Streptococcus dysgalactiae subsp. equisimilis, Streptococcus pneumoniae, and Streptococcus agalactiae recovered in Portugal was surveyed. In total, 149 GAS, 18 S. pneumoniae, 4 S. dysgalactiae subsp. equisimilis, and 5 S. agalactiae isolates from infections, presenting the M phenotype of macrolide resistance and containing the mef gene, were screened for the presence of Tn1207.3/Φ10394.4 by PCR targeting open reading frames (ORFs) specific for these related elements. All the GAS isolates tested and one of the S. dysgalactiae subsp. equisimilis isolates carried Tn1207.3. However, neither of these elements was found in the isolates of the other streptococcal species. It was also noted that the DNAs of the isolates carrying Tn1207.3 were resistant to cleavage by the endonuclease SmaI. Cloning and expression of ORF12 of Tn1207.3 in Escherichia coli showed that it encoded a methyltransferase that rendered DNA refractory to cleavage by SmaI (M.Spy10394I). Using this characteristic as a marker for the presence of the Tn1207.3/Φ10394.4 family, we reviewed the literature and concluded that these genetic elements are widely distributed among tetracycline-susceptible GAS isolates presenting the M phenotype from diverse geographic origins and may have played an important role in the dissemination of macrolide resistance in this species.
Active macrolide efflux is a major mechanism of macrolide resistance in Streptococcus pneumoniae in many parts of the world, especially North America. In Canada, this active macrolide efflux in S. pneumoniae is predominantly due to acquisition of the mef(E) gene. In the present study, we assessed the mef(E) gene sequence as well as mef(E) expression in variety of low- and high-level macrolide-resistant, clindamycin-susceptible (M-phenotype) S. pneumoniae isolates (erythromycin MICs, 1 to 32 μg/ml; clindamycin MICs, ≤0.25 μg/ml). Southern blot hybridization with mef(E) probe and EcoRI digestion and relative real-time reverse transcription-PCR were performed to study the mef(E) gene copy number and expression. Induction of mef(E) expression was analyzed by Etest susceptibility testing pre- and postincubation with subinhibitory concentrations of erythromycin, clarithromycin, azithromycin, telithromycin, and clindamycin. The macrolide efflux gene, mef(E), was shown to be a single-copy gene in all 23 clinical S. pneumoniae isolates tested, and expression post-macrolide induction increased 4-, 6-, 20-, and 200-fold in isolates with increasing macrolide resistance (erythromycin MICs 2, 4, 8, and 32 μg/ml, respectively). Sequencing analysis of the macrolide efflux genetic assembly (mega) revealed that mef(E) had a 16-bp deletion 153 bp upstream of the putative start codon in all 23 isolates. A 119-bp intergenic region between mef(E) and mel was sequenced, and a 99-bp deletion was found in 11 of the 23 M-phenotype S. pneumoniae isolates compared to the published mega sequence. However, the mef(E) gene was fully conserved among both high- and low-level macrolide-resistant isolates. In conclusion, increased expression of mef(E) is associated with higher levels of macrolide resistance in macrolide-resistant S. pneumoniae.
Molecular typing was used to investigate an outbreak of infection caused by multidrug-resistant Enterobacter aerogenes (MREA) susceptible only to gentamicin and imipenem in an intensive care unit (ICU). Over a 9-month period, ciprofloxacin-resistant E. aerogenes isolates were isolated from 34 patients, or 4.1% of ICU admissions, compared with a baseline rate of 0.1% in the previous period (P < 0.001). Infection developed in 15 (44%) patients. In vivo emergence of imipenem resistance (MIC, 32 micrograms/ml) of organisms causing deep-seated infection was observed in two (13%) of these patients following prolonged therapy with imipenem and gentamicin. Arbitrarily primed PCR (AP-PCR) analysis with ERIC1R and ERIC2 primers and pulsed-field gel electrophoresis (PFGE) analysis of XbaI macrorestriction patterns concordantly showed that outbreak-associated MREA isolates were clonally related and distinct from epidemiologically unrelated strains. AP-PCR and PFGE showed discrimination indices of 0.88 and 0.98, respectively. Space-time clustering of cases within units suggests that the epidemic-related MREA isolates were transmitted on the hands of the health care personnel. A case-control study and repeated environmental culture surveys failed to identify a common source or procedure associated with transmission. In spite of the early implementation of isolation measures, the incidence of MREA colonization remained stable until all colonized patients were discharged. This study confirms the usefulness of AP-PCR and PFGE analyses for the epidemiological study of E. aerogenes and underscores the difficulty of controlling the spread of multiresistant clones of this organism in the ICU setting. The emergence of imipenem resistance represents a threat because virtually no therapeutic option is available for such strains.
A total of 387 clinical strains of erythromycin-resistant (MIC, ≥1 μg/ml) Streptococcus pyogenes, all isolated in Italian laboratories from 1995 to 1998, were examined. By the erythromycin-clindamycin double-disk test, 203 (52.5%) strains were assigned to the recently described M phenotype, 120 (31.0%) were assigned to the inducible macrolide, lincosamide, and streptogramin B resistance (iMLS) phenotype, and 64 (16.5%) were assigned to the constitutive MLS resistance (cMLS) phenotype. The inducible character of the resistance of the iMLS strains was confirmed by comparing the clindamycin MICs determined under normal testing conditions and those determined after induction by pregrowth in 0.05 μg of erythromycin per ml. The MICs of erythromycin, clarithromycin, azithromycin, josamycin, spiramycin, and the ketolide HMR3004 were then determined and compared. Homogeneous susceptibility patterns were observed for the isolates of the cMLS phenotype (for all but one of the strains, HMR3004 MICs were 0.5 to 8 μg/ml and the MICs of the other drugs were >128 μg/ml) and those of the M phenotype (resistance only to the 14- and 15-membered macrolides was recorded, with MICs of 2 to 32 μg/ml). Conversely, heterogeneous susceptibility patterns were observed in the isolates of the iMLS phenotype, which were subdivided into three distinct subtypes designated iMLS-A, iMLS-B, and iMLS-C. The iMLS-A strains (n = 84) were highly resistant to the 14-, 15-, and 16-membered macrolides and demonstrated reduced susceptibility to low-level resistance to HMR3004. The iMLS-B strains (n = 12) were highly resistant to the 14- and 15-membered macrolides, susceptible to the 16-membered macrolides (but highly resistant to josamycin after induction), and susceptible to HMR3004 (but intermediate or resistant after induction). The iMLS-C strains (n = 24) had lower levels of resistance to the 14- and 15-membered macrolides (with erythromycin MICs increasing two to four times after induction), were susceptible to the 16-membered macrolides (but resistant to josamycin after induction), and remained susceptible to HMR3004, also after induction. The erythromycin resistance genes in 100 isolates of the different groups were investigated by PCR. All cMLS and iMLS-A isolates tested had the ermAM (ermB) gene, whereas all iMLS-B and iMLS-C isolates had the ermTR gene (neither methylase gene was found in isolates of other groups). The M isolates had only the macrolide efflux (mefA) gene, which was also found in variable proportions of cMLS, iMLS-A, iMLS-B, and iMLS-C isolates. The three iMLS subtypes were easily differentiated by a triple-disk test set up by adding a josamycin disk to the erythromycin and clindamycin disks of the conventional double-disk test. Tetracycline resistance was not detected in any isolate of the iMLS-A subtype, whereas it was observed in over 90% of both iMLS-B and iMLS-C isolates.
Culture-independent molecular characterization of infecting Leptospira human blood specimens from a 2008 outbreak of human leptospirosis in central Sri Lanka was carried out. Of 58 quantitative real-time polymerase chain reaction-positive samples analyzed for seven multi-locus sequence typing (MLST) housekeeping genes (mreA, pfkB, pntA, sucA, tpiA, fadD, and glmU), interpretable data was obtained from 12 samples. Mean bacterial load was 2.2 × 105 among specimens with complete MLST profiles compared with 1.3 × 104 among specimens without complete MLST profiles; all specimens with complete profiles had at least 4.9 × 104
Leptospira/mL (t = 5, P < 0.001). Most (11/12) identified sequence types were ST1 (L. interrogans serovar Lai) and ST44 (L. interrogans serovar Geyaweera). MLST can be used to directly identify infecting Leptospira strains in blood samples obtained during acute illness without the need for culture isolation, but it shows important limitations related to bacterial load.
This work shows that the ribC wild-type gene product has both flavokinase and flavin adenine dinucleotide synthetase (FAD-synthetase) activities. RibC plays an essential role in the flavin metabolism of Bacillus subtilis, as growth of a ribC deletion mutant strain was dependent on exogenous supply of FMN and the presence of a heterologous FAD-synthetase gene in its chromosome. Upon cultivation with growth-limiting amounts of FMN, this ribC deletion mutant strain overproduced riboflavin, while with elevated amounts of FMN in the culture medium, no riboflavin overproduction was observed. In a B. subtilis ribC820 mutant strain, the corresponding ribC820 gene product has reduced flavokinase/FAD-synthetase activity. In this strain, riboflavin overproduction was also repressed by exogenous FMN but not by riboflavin. Thus, flavin nucleotides, but not riboflavin, have an effector function for regulation of riboflavin biosynthesis in B. subtilis, and RibC seemingly is not directly involved in the riboflavin regulatory system. The mutation ribC820 leads to deregulation of riboflavin biosynthesis in B. subtilis, most likely by preventing the accumulation of the effector molecule FMN or FAD.
Erythromycin-resistant β-hemolytic streptococci (BHS) has recently emerged and quickly spread between and within countries throughout the world. In this study, we evaluate the antimicrobial susceptibility patterns and erythromycin resistance mechanisms of BHS during 2003-2004.
Materials and Methods
The MICs of seven antimicrobials were determined for 204 clinical isolates of BHS from 2003 to 2004. Resistance mechanisms of erythromycin-resistant BHS were studied by the double disk test as well as by polymerase chain reaction (PCR).
Compared with our previous study, resistance among Streptococcus pyogenes isolates to a variety of drugs decreased strikingly: from 25.7% to 4.8% in erythromycin; 15.8% to 0% in clindamycin; and 47.1% to 19.0% in tetracycline. The prevalent phenotypes and genotypes of macrolide-lincosamide-streptograminB (MLSB) resistance in Streptococcus pyogenes isolates have been changed from the constitutive MLSB phenotype carrying erm(B) to the M phenotype with mef(A) gene. In contrast with Streptococcus pyogenes, resistance rates to erythromycin (36.7%), clindamycin (43.1%), and tetracycline (95.4%) in Streptococcus agalactiae isolates did not show decreasing trends. Among the Streptococcus dysgalactiae subsp. equisimilis isolates (Lancefield group C, G), resistance rates to erythromycin, clindamycin, tetracycline and chloramphenicol were observed to be 9.4%, 3.1%, 68.8%, and 9.4%, respectively.
Continual monitoring of antimicrobial resistance among large-colony-forming BHS is needed to provide the medical community with current data regarding the resistance mechanisms that are most common to their local or regional environments.
β-hemolytic streptococci; antibiotic resistance; macrolides; erythromycin; Streptococcus agalactiae; Streptococcus pyogenes
A chromosomal region of Escherichia coli contiguous to the fabE gene at 71 min on the chromosomal map contains multiple genes that are responsible for determination of the rod shape and sensitivity to the amidinopenicillin mecillinam. The so-called mre region was cloned and analyzed by complementation of two closely related but distinct E. coli mutants characterized, respectively, by the mutations mre-129 and mre-678, that showed a rounded to irregular cell shape and altered sensitivities to mecillinam; the mre-129 mutant was supersensitive to mecillinam at 30 degrees C, but the mre-678 mutant was resistant. The mre-678 mutation also caused simultaneous overproduction of penicillin-binding proteins 1Bs and 3. A chromosomal region of the wild-type DNA containing the total mre region and the fabE gene was first cloned on a lambda phage; a 7-kilobase (kb) fragment containing the whole mre region, but not the fabE gene, was then recloned on a mini F plasmid, pLG339; and finally, a 2.8-kb fragment complementing only mre-129 was also cloned on this low-copy-number plasmid. The whole 7-kb fragment was required for complementing the mre-678 mutant phenotypes. Fragments containing fabE but not the mre-129 region could be cloned on a high-copy-number plasmid. Southern blot hybridization indicated that the mre-678 mutant had a large deletion of 5.25 kb in its DNA, covering at least part of the mre-129 gene.
Beta-hemolytic Streptococcus agalactiae is the leading cause of bacteremia and invasive infections. These diseases are treated with β-lactams or macrolides, but the emergence of less susceptible and even fully resistant strains is a cause for concern. New bacteriophage lysins could be promising alternatives against such organisms. They hydrolyze the bacterial peptidoglycan at the end of the phage cycle, in order to release the phage progeny. By using a bioinformatic approach to screen several beta-hemolytic streptococci, a gene coding for a lysin was identified on a prophage carried by Streptococcus dysgalactiae subsp. equisimilis SK1249. The gene product, named PlySK1249, harbored an original three-domain structure with a central cell wall-binding domain surrounded by an N-terminal amidase and a C-terminal CHAP domain. Purified PlySK1249 was highly lytic and bactericidal for S. dysgalactiae (2-log10 CFU/ml decrease within 15 min). Moreover, it also efficiently killed S. agalactiae (1.5-log10 CFU/ml decrease within 15 min) but not several streptococcal commensal species. We further investigated the activity of PlySK1249 in a mouse model of S. agalactiae bacteremia. Eighty percent of the animals (n = 10) challenged intraperitoneally with 106 CFU of S. agalactiae died within 72 h, whereas repeated injections of PlySK1249 (45 mg/kg 3 times within 24 h) significantly protected the mice (P < 0.01). Thus, PlySK1249, which was isolated from S. dysgalactiae, demonstrated high cross-lytic activity against S. agalactiae both in vitro and in vivo. These encouraging results indicated that PlySK1249 might represent a good candidate to be developed as a new enzybiotic for the treatment of systemic S. agalactiae infections.
The riboflavin analog roseoflavin is an antibiotic produced by Streptomyces davawensis. Riboflavin transporters are responsible for roseoflavin uptake by target cells. Roseoflavin is converted to the flavin mononucleotide (FMN) analog roseoflavin mononucleotide (RoFMN) by flavokinase and to the flavin adenine dinucleotide (FAD) analog roseoflavin adenine dinucleotide (RoFAD) by FAD synthetase. In order to study the effect of RoFMN and RoFAD in the cytoplasm of target cells, Escherichia coli was used as a model. E. coli is predicted to contain 38 different FMN- or FAD-dependent proteins (flavoproteins). These proteins were overproduced in recombinant E. coli strains grown in the presence of sublethal amounts of roseoflavin. The flavoproteins were purified and analyzed with regard to their cofactor contents. It was found that 37 out of 38 flavoproteins contained either RoFMN or RoFAD. These cofactors have different physicochemical properties than FMN and FAD and were reported to reduce or completely abolish flavoprotein function.
Macrolide-resistant bacteria can be classified as inducibly resistant or constitutively resistant. Inducibly resistant bacteria are resistant to 14-membered macrolides, such as erythromycin and clarithromycin (A-56268), but are susceptible to the 16-membered macrolides, such as tylosin and spiramycin, as well as to clindamycin. Constitutively resistant bacteria are resistant to macrolide-lincosamide-streptogramin B antibiotics. In this study, the MICs of several erythromycin and clarithromycin analogs against macrolide-susceptible and macrolide-resistant Streptococcus pyogenes strains were determined. Four 11,12-carbamate analogs of clarithromycin had lower MICs than erythromycin did against S. pyogenes with the inducible or constitutive type of macrolide-lincosamide-streptogramin B resistance. Five 11,12-carbonate analogs of erythromycin with modifications at the 4" position of cladinose had lower MICs than did erythromycin against S. pyogenes with the constitutive type of resistance, and one of these compounds, which had a naphthyl-glycyl substitution at the 4" position, had a lower MIC than erythromycin against both the inducibly resistant and constitutively resistant strains. Two analogs of erythromycin with a modification on the 4" position of cladinose had lower MICs than erythromycin did against the constitutively resistant organisms but not against the inducibly resistant organisms. Thus, 14-membered macrolides can be modified so as to confer a low MIC when tested in vitro.
Metal-responsive transcription factor-1 (MTF-1) is essential for the induction of genes encoding metallothionein by metals and hypoxia. Here, we studied the mechanism controlling the activation of MTF-1 by hypoxia. Hypoxia activation of Mt gene transcription is dependent on the presence of metal regulatory elements (MREs) in the promoter of Mt genes. We showed that MREa and MREd are the main elements controlling mouse Mt-1 gene induction by hypoxia. Transfection experiments in Mtf-1-null cells showed that MTF-1 is essential for induction by hypoxia. Chromatin immunoprecipitation analysis showed that MTF-1 DNA-binding activity was strongly enhanced in the presence of zinc but not by hypoxia. Notably, hypoxia inducible factor- (HIF) 1α was recruited to the Mt-1 promoter in response to hypoxia but not to zinc. MTF-1 activation was inhibited by PKC, JNK, and PI3K inhibitors and by the electron transport chain inhibitors rotenone and myxothiazol, but not by the antioxidant N-acetylcysteine. We showed that prolyl-hydroxylase inhibitors can activate MTF-1, but this activation requires the presence of HIF-1α. Finally, HIF-dependent transcription is enhanced in the presence of MTF-1 and induction of an MRE promoter is stimulated by HIF-1α, thus indicating cooperation between these 2 factors. However, coimmunoprecipitation experiments did not suggest direct interaction between MTF-1 and HIF-1α.
PMID: 22087877 CAMSID: cams2671
hypoxia; metal-responsive transcription factor-1 (MTF-1); hypoxia-inducible factor-1α (HIF-1α); protein kinase; signal transduction; activation