Yasuyoshi Ike

Novel Plasmid-Mediated 16S rRNA m1A1408 Methyltransferase, NpmA, Found in a Clinically Isolated Escherichia coli Strain Resistant to Structurally Diverse Aminoglycosides

ABSTRACT We have isolated a multiple-aminoglycoside-resistant Escherichia coli strain, strain ARS3, and have been the first to identify a novel plasmid-mediated 16S rRNA methyltransferase, NpmA. This new enzyme shared a relatively low level of identity (30%) to the chromosomally encoded 16S rRNA methyltransferase (KamA) of Streptomyces tenjimariensis, an actinomycete aminoglycoside producer. The introduction of a recombinant plasmid carrying npmA could confer on E. coli consistent resistance to both 4,6-disubstituted 2-deoxystreptamines, such as amikacin and gentamicin, and 4,5-disubstituted 2-deoxystreptamines, including neomycin and ribostamycin. The histidine-tagged NpmA elucidated methyltransferase activity against 30S ribosomal subunits but not against 50S subunits and the naked 16S rRNA molecule in vitro. We further confirmed that NpmA is an adenine N-1 methyltransferase specific for the A1408 position at the A site of 16S rRNA. Drug footprinting data indicated that binding of aminoglycosides to the target site was apparently interrupted by methylation at the A1408 position. These observations demonstrate that NpmA is a novel plasmid-mediated 16S rRNA methyltransferase that provides a panaminoglycoside-resistant nature through interference with the binding of aminoglycosides toward the A site of 16S rRNA through N-1 methylation at position A1408.

Novel Plasmid-Mediated 16S rRNA Methylase, RmtC, Found in a Proteus mirabilis Isolate Demonstrating Extraordinary High-Level Resistance against Various Aminoglycosides

ABSTRACT Proteus mirabilis ARS68, which demonstrated a very high level of resistance to various aminoglycosides, was isolated in 2003 from an inpatient in Japan. The aminoglycoside resistance of this strain could not be transferred to recipient strains Escherichia coli CSH-2 and E. coli HB101 by a general conjugation experiment, but E. coli DH5α was successfully transformed by electroporation with the plasmid of the parent strain, ARS68, and acquired an unusually high degree of resistance against aminoglycosides. Cloning and sequencing analyses revealed that the presence of a novel 16S rRNA methylase gene, designated rmtC, was responsible for resistance in strain ARS68 and its transformant. The G+C content of rmtC was 41.1%, and the deduced amino acid sequences of the newly identified 16S rRNA methylase, RmtC, shared a relatively low level of identity (≤29%) to other plasmid-mediated 16S rRNA methylases, RmtA, RmtB, and ArmA, which have also been identified in pathogenic gram-negative bacilli. Also, RmtC shared a low level of identity (≤28%) with the other 16S rRNA methylases found in aminoglycoside-producing actinomycetes. The purified histidine-tagged RmtC clearly showed methyltransferase activity against E. coli 16S rRNA in vitro. rmtC was located downstream of an ISEcp1-like element containing tnpA. Several plasmid-mediated 16S rRNA methylases have been identified in pathogenic gram-negative bacilli belonging to the family Enterobacteriaceae, and some of them are dispersing worldwide. The acceleration of aminoglycoside resistance among gram-negative bacilli by producing plasmid-mediated 16S rRNA methylases, such as RmtC, RmtB, and RmtA, may indeed become an actual clinical hazard in the near future.

Potency of carbapenems for the prevention of carbapenem- resistant mutants of Pseudomonas aeruginosa : The high potency of a new carbapenem doripenem

The potencies of the carbapenems; doripenem (DRPM), meropenem (MEPM) and imipenem (IPM) in preventing the emergence of carbapenem-resistant mutants were examined in Pseudomonas aeruginosa strains. The carbapenems predominantly selected carbapenem-resistant mutants or carbapenem mutants with reduced susceptibilities that were specifically resistant to carbapenems and had arisen as a result of the reduced level of expression of the outer membrane protein with a molecular weight of about 48,000 (OprD). The potency of carbapenems in preventing the growth of the mutants differed for DRPM, MEPM and IPM. The isolation frequency of the mutant was examined on agar plates containing each of the carbapenems at a concentration of 1/2 or 1/4 MIC of each carbapenem for that mutant. Mutants were not selected on agar containing DRPM at a frequency of greater than 10−9 per cell per generation, whereas mutants of each strain were selected on agar containing MEPM or IPM at frequencies of 10−7 to 10−9 per cell per generation. The drug concentrations and the drug concentration range for the selective increase of carbapenem resistant mutants in the broth culture containing each carbapenem differed for each carbapenem. DRPM exhibited both the lowest drug concentration and the narrowest range of drug concentration for selection of the carbapenem-resistant mutants. The results shown in this report indicated that DRPM exhibited the greatest ability to prevent the emergence of the mutant.

Possible Connection between a Widely Disseminated Conjugative Gentamicin Resistance (pMG1-Like) Plasmid and the Emergence of Vancomycin Resistance in Enterococcus faecium

ABSTRACT A total of 640 vancomycin-resistant Enterococcus faecium (VRE) isolates, which were obtained between 1994 and 1999 from the Medical School Hospital of the University of Michigan, Ann Arbor, were used in this study. Of the 640 strains, 611 and 29 were VanA and VanB VRE, respectively, based on PCR analysis. Four hundred ninety-two (77%) of the strains exhibited resistance to concentrations of gentamicin from 64 μg/ml (MIC) to more than 1,024 μg/ml (MIC). The gentamicin resistance of each of 261 (53%) of the 492 gentamicin-resistant strains was transferred to E. faecium at a frequency of about 10−5 to 10−6 per donor cell in broth mating. More than 90% of vancomycin resistances of the 261 strains cotransferred with the gentamicin resistances to E. faecium strains by filter mating. The conjugative gentamicin resistance plasmids were identified and were classified into five types (A through E) with respect to their EcoRI restriction profiles. The transfer frequencies of each type of plasmid between E. faecium strains or Enterococcus faecalis strains were around 10−3 to 10−5 per donor cell or around 10−6 to 10−7 per donor cell, respectively, in broth mating. Type A and type B were the most frequently isolated, at an isolation frequency of about 40% per VRE isolate harboring the gentamicin resistance conjugative plasmid. The plasmids did not show any homology in Southern hybridization with the pheromone-responsive plasmids and broad-host-range plasmids pAMβ1 and pIP501. The EcoRI or NdeI restriction fragments of each type of plasmids hybridized to the conjugative gentamicin resistance plasmid pMG1 (65.1 kb), which was originally isolated from an E. faecium clinical isolate, and transfer efficiently in broth mating.

Fluoroquinolone enhances the mutation frequency for meropenem-selected carbapenem resistance in Pseudomonas aeruginosa, but use of the high-potency drug doripenem inhibits mutant formation.

ABSTRACT The mutation frequency for carbapenem resistance in Pseudomonas aeruginosa strains that were selected with carbapenems was enhanced in the presence of subinhibitory concentrations of fluoroquinolones. The mutants showed either a loss of OprD activity or increased mexAB-oprM expression. The highest mutant isolation frequency was obtained by selection with meropenem, while doripenem inhibited mutant growth.

pAM401-Based Shuttle Vectors That Enable Overexpression of Promoterless Genes and One-Step Purification of Tag Fusion Proteins Directly from Enterococcus faecalis

ABSTRACT Two novel Enterococcus faecalis-Escherichia colishuttle vectors that utilize the promoter and ribosome binding site ofbacA on the E. faecalis plasmid pPD1 were constructed. The vectors were named pMGS100 and pMGS101. pMGS100 was designed to overexpress cloned genes in E. coli andE. faecalis and encodes the bacA promoter followed by a cloning site and stop codon. pMGS101 was designed for the overexpression and purification of a cloned protein fused to a Strep-tag consisting of 9 amino acids at the carboxyl terminus. The Strep-tag provides the cloned protein with an affinity to immobilized streptavidin that facilitates protein purification. We cloned a promoterless β-galactosidase gene from E. coli and cloned the traA gene of the E. faecalis plasmid pAD1 into the vectors to test gene expression and protein purification, respectively. β-Galactosidase was expressed in E. coliand E. faecalis at levels of 103 and 10 Miller units, respectively. By cloning the pAD1 traA into pMGS101, the protein could be purified directly from a crude lysate of E. faecalis or E. coli with an immobilized streptavidin matrix by one-step affinity chromatography. The ability of TraA to bind DNA was demonstrated by the DNA-associated protein tag affinity chromatography method using lysates prepared from both E. coli and E. faecalis that overexpress TraA. The results demonstrated the usefulness of the vectors for the overexpression and cis/trans analysis of regulatory genes, purification and copurification of proteins from E. faecalis, DNA binding analysis, determination of translation initiation site, and other applications that require proteins purified from E. faecalis.

Molecular Characterization of Vancomycin-Resistant Enterococcus faecium Isolates from Mainland China

ABSTRACT Little is known about vancomycin-resistant enterococci in China. Thirteen pulsed-field gel electrophoresis-confirmed heterogeneous VanA-type vancomycin-resistant Enterococcus faecium (VRE) isolates were obtained from five Chinese hospitals from 2001 to 2005. The isolates were typed by multilocus sequence typing into nine different sequence types (STs), including five new STs (ST18, ST25, ST78, ST203, ST320, ST321, ST322, ST323, and ST335). Vancomycin resistance in each isolate was encoded on conjugative plasmids; two of the plasmids, pZB18 (67 kbp) and pZB22 (200 kbp), were highly conjugative and were able to transfer at high frequencies of around 10−4 and 10−7 per donor cell in broth mating, respectively. None of the plasmids identified in these isolates carried traA, which is usually conserved in the pMG1-like highly conjugative plasmid for E. faecium, implying that pZB18 and pZB22 were novel types of a highly conjugative plasmid in enterococci. Thirteen Tn1546-like elements encoding VanA-type VRE on the conjugative plasmids were classified into six types (types I to VI), and most of them contained both IS1216V and IS1542 insertions. The isolates carrying the type II element were predominant. The six type elements were different from that of a VanA-type Enterococcus faecalis strain isolated from Chinese chicken meat. The results suggested that the disseminations of VRE in these areas were by Tn1546-like elements being acquired by the conjugative plasmids and transferred among E. faecium strains.

Nationwide survey shows that methicillin-resistant Staphylococcus aureus strains heterogeneously and intermediately resistant to vancomycin are not disseminated throughout Japanese hospitals

ABSTRACT A total of 6,625 methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates obtained from 278 hospitals throughout Japan were obtained between November and December 1997 and were examined for their sensitivities to vancomycin using Mueller Hinton (MH), brain heart infusion (BHI), agar plates, or the broth microdilution method. A concentrated inoculum of an MRSA strain or the use of highly enriched medium, such as BHI medium, allows an individual cell to grow on agar plates containing a vancomycin concentration greater than the MIC for the parent strain. However, cells of the colonies which grew on BHI agar plates containing the higher vancomycin concentrations did not acquire a level of vancomycin resistance greater than that of the parent strain and were not subpopulations of heterogeneously vancomycin-resistant MRSA. There was no significance in the fact that these colonies grew on the higher concentration of vancomycin: none showed stable resistance to vancomycin at a concentration above the MIC for the parent strain, and no cell from these colonies showed a relationship between the MIC and the ability of these colonies to grow on higher concentrations of vancomycin. The vancomycin MIC was not above 2 μg/ml for any of the cells originating from these colonies. No Mu3-type heterogeneously resistant MRSA strains, which constitutively produce subpopulations from MRSA clinical isolates with intermediate vancomycin resistance at a high frequency, were detected. There was a unipolar distribution of the MICs ranging from 0.25 to 2 μg of vancomycin/ml among the 6,625 MRSA clinical isolates, indicating that there was no Mu50-type intermediately vancomycin-resistant MRSA (MIC, 8 μg/ml by National Committee for Clinical Laboratory Standards criteria) among the clinical isolates, and there was no evidence of dissemination of Mu3-type MRSA heteroresistant to vancomycin.

Isolation of VanB-Type Enterococcus faecalis Strains from Nosocomial Infections: First Report of the Isolation and Identification of the Pheromone-Responsive Plasmids pMG2200, Encoding VanB-Type Vancomycin Resistance and a Bac41-Type Bacteriocin, and pMG2201, Encoding Erythromycin Resistance and Cytolysin (Hly/Bac)

ABSTRACT Eighteen identical VanB-type Enterococcus faecalis isolates that were obtained from different hospitalized patients were examined for their drug resistance and plasmid DNAs. Of the 18 strains, 12 strains exhibited resistance to erythromycin (Em), gentamicin (Gm), kanamycin (Km), tetracycline (Tc), and vancomycin (Van) and produced cytolysin (Hly/Bac) and a bacteriocin (Bac) active against E. faecalis strains. Another six of the strains exhibited resistance to Gm, Km, Tc, and Van and produced a bacteriocin. Em and Van resistance was transferred individually to E. faecalis FA2-2 strains at a frequency of about 10−4 per donor cell by broth mating. The Em-resistant transconjugants and the Van-resistant transconjugants harbored a 65.7-kbp plasmid and a 106-kbp plasmid, respectively. The 106-kbp and 65.7-kbp plasmids isolated from the representative E. faecalis NKH15 strains were designated pMG2200 and pMG2201, respectively. pMG2200 conferred vancomycin resistance and bacteriocin activity on the host strain and responded to the synthetic pheromone cCF10 for pCF10, while pMG2201 conferred erythromycin resistance and cytolysin activity on its host strain and responded to the synthetic pheromone cAD1 for pAD1. The complete DNA sequence of pMG2200 (106,527 bp) showed that the plasmid carried a Tn1549-like element encoding vanB2-type resistance and the Bac41-like bacteriocin genes of pheromone-responsive plasmid pYI14. The plasmid contained the regulatory region found in pheromone-responsive plasmids and encoded the genes prgX and prgQ, which are the key negative regulatory elements for plasmid pCF10. pMG2200 also encoded TraE1, a key positive regulator of plasmid pAD1, indicating that pMG2200 is a naturally occurring chimeric plasmid that has a resulting prgX-prgQ-traE1 genetic organization in the regulatory region of the pheromone response. The functional oriT region and the putative relaxase gene of pMG2200 were identified and found to differ from those of pCF10 and pAD1. The putative relaxase of pMG2200 was classified as a member of the MOBMG family, which is found in pheromone-independent plasmid pHTβ of the pMG1-like plasmids. This is the first report of the isolation and characterization of a pheromone-responsive highly conjugative plasmid encoding vanB resistance.

Horizontal Transfer of blaCMY-Bearing Plasmids among Clinical Escherichia coli and Klebsiella pneumoniae Isolates and Emergence of Cefepime-Hydrolyzing CMY-19

ABSTRACT Nine Escherichia coli and 5 Klebsiella pneumoniae clinical isolates resistant to various cephalosporins and cephamycins were identified in a Japanese general hospital between 1995 and 1997. All nine E. coli isolates and one K. pneumoniae isolate carried blaCMY-9, while the other four K. pneumoniae isolates harbored a variant of blaCMY-9, namely, blaCMY-19. The pulsed-field gel electrophoresis patterns of the nine CMY-9-producing E. coli isolates were almost identical, suggesting their clonal relatedness, while those of the five K. pneumoniae isolates were divergent. Plasmid profiles, Southern hybridization, and conjugation assays revealed that the genes for the CMY-9 and the CMY-19 β-lactamases were located on very similar conjugative plasmids in E. coli and K. pneumoniae. The genetic environment of blaCMY-19 was identical to that of blaCMY-9. A single amino acid substitution, I292S, adjacent to the H-10 helix region was observed between CMY-9 and CMY-19. This substitution was suggested to be responsible for the expansion of the hydrolyzing activity against several broad-spectrum cephalosporins, and this finding was consistent with the kinetic parameters determined with purified enzymes. These findings suggest that the blaCMY-19 genes found in the four K. pneumoniae isolates might have originated from blaCMY-9 gene following a point mutation and dispersed among genetically different K. pneumoniae isolates via a large transferable plasmid.