Plasmid-mediated AmpC-type beta-lactamase isolated from Klebsiella pneumoniae confers resistance to broad-spectrum beta-lactams, including moxalactam

Characterization of SFO-1, a plasmid-mediated inducible class A beta-lactamase from Enterobacter cloacae

Enterobacter cloacae 8009 produced an inducible class A beta-lactamase which hydrolyzed cefotaxime efficiently. It also hydrolyzed other beta-lactams except cephamycins and carbapenems. The activity was inhibited by clavulanic acid and imipenem. The bla gene was transferable to Escherichia coli by electroporation of plasmid DNA. The molecular mass of the beta-lactamase was 29 kDa and its pI was 7.3. All of these phenotypic characteristics of the enzyme except for inducible production resemble those of some extended-spectrum class A beta-lactamases like FEC-1. The gene encoding this beta-lactamase was cloned and sequenced. The deduced amino acid sequence of the beta-lactamase was homologous to the AmpA sequences of the Serratia fonticola chromosomal enzyme (96%), MEN-1 (78%), Klebsiella oxytoca chromosomal enzymes (77%), TOHO-1 (75%), and FEC-1 (72%). The conserved sequences of class A beta-lactamases, including the S-X(T)-X(S)-K motif, in the active site were all conserved in this enzyme. On the basis of the high degree of homology to the beta-lactamase of S. fonticola, the enzyme was named SFO-1. The ampR gene was located upstream of the ampA gene, and the AmpR sequence of SFO-1 had homology with the AmpR sequences of the chromosomal beta-lactamases from Citrobacter diversus (80%), Proteus vulgaris (68%), and Pseudomonas aeruginosa (60%). SFO-1 was also inducible in E. coli. However, a transformant harboring plasmid without intact ampR produced a small amount of beta-lactamase constitutively, suggesting that AmpR works as an activator of ampA of SFO-1. This is the first report from Japan describing an inducible plasmid-mediated class A beta-lactamase in gram-negative bacteria.

Salmonella enteritidis: AmpC plasmid-mediated inducible beta-lactamase (DHA-1) with an ampR gene from Morganella morganii

DHA-1, a plasmid-mediated cephalosporinase from a single clinical Salmonella enteritidis isolate, conferred resistance to oxyimino-cephalosporins (cefotaxime and ceftazidime) and cephamycins (cefoxitin and moxalactam), and this resistance was transferable to Escherichia coli HB101. An antagonism was observed between cefoxitin and aztreonam by the diffusion method. Transformation of the transconjugant E. coli strain with plasmid pNH5 carrying the ampD gene (whose product decreases the level of expression of ampC) resulted in an eightfold decrease in the MIC of cefoxitin. A clone with the same AmpC susceptibility pattern with antagonism was obtained, clone E. coli JM101(pSAL2-ind), and its nucleotide sequence was determined. It contained an open reading frame with 98. 7% DNA sequence identity with the ampC gene of Morganella morganii. DNA sequence analysis also identified a gene upstream of ampC whose sequence was 97% identical to the partial sequence of the ampR gene (435 bp) from M. morganii. The gene encoded a protein with an amino-terminal DNA-binding domain typical of transcriptional activators of the LysR family. Moreover, the intercistronic region between the ampC and ampR genes was 98% identical to the corresponding region from M. morganii DNA. AmpR was shown to be functional by enzyme induction and a gel mobility-shift assay. An ampG gene was also detected in a Southern blot of DNA from the S. enteritidis isolate. These findings suggest that this inducible plasmid-mediated AmpC type beta-lactamase, DHA-1, probably originated from M. morganii.

Chromosomally encoded ampC-type beta-lactamase in a clinical isolate of Proteus mirabilis

A clinical strain of Proteus mirabilis (CF09) isolated from urine specimens of a patient displayed resistance to amoxicillin (MIC >4,096 microg/ml), ticarcillin (4,096 microg/ml), cefoxitin (64 microg/ml), cefotaxime (256 microg/ml), and ceftazidime (128 microg/ml) and required an elevated MIC of aztreonam (4 microg/ml). Clavulanic acid did not act synergistically with cephalosporins. Two beta-lactamases with apparent pIs of 5.6 and 9.0 were identified by isoelectric focusing on a gel. Substrate and inhibition profiles were characteristic of an AmpC-type beta-lactamase with a pI of 9.0. Amplification by PCR with primers for ampC genes (Escherichia coli, Enterobacter cloacae, and Citrobacter freundii) of a 756-bp DNA fragment from strain CF09 was obtained only with C. freundii-specific primers. Hybridization results showed that the ampC gene is only chromosomally located while the TEM gene is plasmid located. After cloning of the gene, analysis of the complete nucleotide sequence (1,146 bp) showed that this ampC gene is close to blaCMY-2, from which it differs by three point mutations leading to amino acid substitutions Glu --> Gly at position 22, Trp --> Arg at position 201, and Ser --> Asn at position 343. AmpC beta-lactamases derived from that of C. freundii (LAT-1, LAT-2, BIL-1, and CMY-2) have been found in Klebsiella pneumoniae, E. coli, and Enterobacter aerogenes and have been reported to be plasmid borne. This is the first example of a chromosomally encoded AmpC-type beta-lactamase observed in P. mirabilis. We suggest that it be designated CMY-3.

Characterization of FOX-3, an AmpC-type plasmid-mediated beta-lactamase from an Italian isolate of Klebsiella oxytoca

Klebsiella oxytoca 1731, which showed a wide spectrum of resistance to beta-lactams, including cefoxitin, was isolated in 1994 from a patient in Genoa, Italy. This strain contained a plasmid-mediated AmpC beta-lactamase with a pI of 7.25. Sequencing of the corresponding DNA of K. oxytoca 1731 revealed 96 and 97% identities of the deduced amino acid sequence with FOX-1 and FOX-2, respectively.

Sequences of homologous beta-lactamases from clinical isolates of Serratia marcescens with different substrate specificities

Genes for two group 1 beta-lactamases, SRT-1 and SST-1, were sequenced. These beta-lactamases were produced by clinical isolates of Serratia marcescens, isolates GN16694 and GN19450, respectively. The resulting enzymes were 96% identical. SRT-1 hydrolyzed oxyimino cephalosporins, but SST-1 hardly hydrolyzed them. At residue 213 in the third motif, which is conserved among group 1 beta-lactamases, SRT-1 and SST-1 had Lys and Glu, respectively. By site-directed mutagenesis, the substitution of Glu by Lys at residue 213 in SST-1 resulted in an enzyme that hydrolyzed oxyimino cephalosporins.

Imipenem resistance in Klebsiella pneumoniae is associated with the combination of ACT-1, a plasmid-mediated AmpC beta-lactamase, and the foss of an outer membrane protein

Six Escherichia coli and 12 Klebsiella pneumoniae isolates from a single hospital expressed a common beta-lactamase with a pI of approximately 9.0 and were resistant to cefoxitin and cefotetan (MIC ranges, 64 to > 128 and 16 to > 128 micrograms/ml, respectively). Seventeen of the 18 strains produced multiple beta-lactamases. Most significantly, three K. pneumoniae strains were also resistant to imipenem (MICs, 8 to 32 micrograms/ml). Spectrophotometric beta-lactamase assays with purified enzyme indicated hydrolysis of cephamycins, in addition to cephaloridine and benzylpenicillin. The 4ene encoding the pI 9.0 beta-lactamase (designated ACT-1 for AmpC type) was cloned and sequenced, which revealed an ampC-type beta-lactamase gene that originated from Enterobacter cloacae and that had 86% sequence homology to the P99 beta-lactamase and 94% homology to the partial sequence of MIR-1. Southern blotting revealed that the gene encoding ACT-1 was on a large plasmid in some of the K. pneumoniae strains as well as on the chromosomes of all of the strains, suggesting that the gene is located on an easily mobilized element. Outer membrane protein profiles of the K. pneumoniae strains revealed that the three imipenem-resistant strains were lacking a major outer membrane protein of approximately 42 kDa which was present in the imipenem-susceptible strains. ACT-1 is the first plasmid-mediated AmpC-type beta-lactamase derived from Enterobacter which has been completely sequenced. This work demonstrates that in addition to resistance to cephamycins, imipenem resistance can occur in K. pneumoniae when a high level of the ACT-1 beta-lactamase is produced in combination with the loss of a major outer membrane protein.

Molecular aspects of high-level resistance to sulbactam-cefoperazone in Klebsiella oxytoca clinical isolates

Nine Klebsiella oxytoca strains which demonstrated resistance to the combination of sulbactam and cefoperazone were isolated from geographically separate hospitals in Japan in 1995. Among them, K. oxytoca SB23 showed high-level resistance to sulbactam-cefoperazone (MIC > 128 micrograms/ml) and aztreonam (MIC, 128 micrograms/ml). The sulbactam-cefoperazone resistance was not transferred from strain SB23 to Escherichia coli CSH2 by conjugation, beta-Lactamase RbiA, produced by strain SB23, was purified, and the molecular mass was estimated to be 29 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Kinetic parameters for RbiA revealed that cefoperazone and aztreonam were hydrolyzed efficiently by this enzyme. Moreover, ceftazidime and imipenem were also hydrolyzed weakly by RbiA, although strain SB23 did not show any resistance to these agents. Clavulanate, sulbactam, and tazobactam failed to block the hydrolysis of cefoperazone by RbiA. The structural gene of RbiA (blaRBI) was cloned and sequenced, and the deduced amino acid sequence of RbiA demonstrated high-level similarities to those of the beta-lactamases found in K. oxytoca D488, E23004, and plasmid-mediated MEN-1, which have been classified into Bush functional group 2be. Although RbiA demonstrates high-level molecular similarity to the enzymes in group 2be, from an enzymological point of view, this enzyme might be differentiated from the enzymes in that group. Hybridization analysis revealed that beta-lactamase genes highly similar to blaRBI were generally encoded on the chromosome of the sulbactam-cefoperazone-resistant clinical isolates of K. oxytoca tested in the study, despite their different derivations. This observation suggests that sulbactam-cefoperazone-resistant A. oxytoca strains which produce RbiA-type beta-lactamases have been proliferating in many hospitals in Japan.

The roles of residues Tyr150, Glu272, and His314 in class C beta-lactamases

Serine beta-lactamases contribute widely to the beta-lactam resistance phenomena. Unfortunately, the intimate details of their catalytic mechanism remain elusive and subject to some controversy even though many "natural" and "artificial" mutants of these different enzymes have been isolated. This paper is essentially focused on class C beta-lactamases, which contain a Tyr (Tyr150) as the first residue of the second conserved element, in contrast to their class A counterparts, in which a Ser is found in the corresponding position. We have modified this Tyr residue by site-directed mutagenesis. On the basis of the three-dimensional structure of the Enterobacter cloacae P99 enzyme, it seemed that residues Glu272 and His314 might also be important. They were similarly substituted. The modified enzymes were isolated and their catalytic properties determined. Our results indicated that His314 was not required for catalysis and that Glu272 did not play an important role in acylation but was involved to a small extent in the deacylation process. Conversely, Tyr150 was confirmed to be central for catalysis, at least with the best substrates. On the basis of a comparison of data obtained for several class C enzyme mutants and in agreement with recent structural data, we propose that the phenolate anion of Tyr150, in conjunction with the alkyl ammonium of Lys315, acts as the general base responsible for the activation of the active-site Ser64 during the acylation step and for the subsequent activation of a water molecule in the deacylation process. The evolution of the important superfamily of penicillin-recognizing enzymes is further discussed in the light of this proposed mechanism.

Comparative characterization of the cephamycinase blaCMY-1 gene and its relationship with other beta-lactamase genes

A plasmidic beta-lactamase which hydrolyzed cephamycins was first detected and reported in 1989. At that time its description was restricted to phenotypic characteristics. We analyzed nucleotide sequence of its gene and explored it genetic relationship with other bla genes. The deduced amino acid sequence of the blaCMY-1 product was compared with those of other known plasmidic cephamycinases and of chromosomal AmpC beta-lactamases. The results indicate that the relationship of CMY-1 is closest to MOX-1 among the plasmidic cephamycinases and to AmpC of Pseudomonas aeruginosa among the chromosomal cephalosporinases. We conclude that the plasmidic cephamycinases described up to now may be classified into three families, as follows: CMY-1, MOX-1, and FOX-1 with AmpC of P. aeruginosa; CMY-2, BIL-1 and LAT-1 with AmpC of Citrobacter freundii; and MIR-1 with AmpC of Enterobacter cloacae. Plasmidic cephamycinases are now recognized as clinically relevant class C beta-lactamases.

beta-Lactamases in laboratory and clinical resistance

beta-Lactamases are the commonest single cause of bacterial resistance to beta-lactam antibiotics. Numerous chromosomal and plasmid-mediated types are known and may be classified by their sequences or phenotypic properties. The ability of a beta-lactamase to cause resistance varies with its activity, quantity, and cellular location and, for gram-negative organisms, the permeability of the producer strain. beta-Lactamases sometimes cause obvious resistance to substrate drugs in routine tests; often, however, these enzymes reduce susceptibility without causing resistance at current, pharmacologically chosen breakpoints. This review considers the ability of the prevalent beta-lactamases to cause resistance to widely used beta-lactams, whether resistance is accurately reflected in routine tests, and the extent to which the antibiogram for an organism can be used to predict the type of beta-lactamase that it produces.

Effect of pH on activities of novel beta-lactamases and beta-lactamase inhibitors against these beta-lactamases

The effects of acidic conditions on activities of seven beta-lactamases--TEM-1 (class A), KOXY (class A), IMP-1 (class B), AmpC (class C), MOX-1 (class C), OXA-5 (class D), and PSE-2 (class D)--and their inhibitors were measured. The enzymatic activities of KOXY, IMP-1, and MOX-1 at pH 5.8 were slightly lower than those at pH 7.5. However, the activities of PSE-2 and OXA-5 were greatly reduced at pH 5.8. All of the beta-lactamase inhibitors tested had poorer inhibitory activities at pH 5.8 than at pH 7.5 except clavulanic acid for TEM-1.

Plasmid-mediated dissemination of the metallo-beta-lactamase gene blaIMP among clinically isolated strains of Serratia marcescens

The distribution of strains producing metallo-beta-lactamase among 105 strains of Serratia marcescens was investigated. All of these strains were isolated in seven general hospitals located in Aichi Prefecture, Japan, from April to May 1993. Southern hybridization analysis suggested that four S. marcescens strains, AK9373, AK9374, AK9385, and AK9391, had a metallo-beta-lactamase genes similar to the blaIMP gene found by our laboratory (E. Osano, Y. Arakawa, R. Wacharotayankun, M. Ohta, T. Horii, H. Ito, F. Yoshimura, and N. Kato, Antimicrob. Agents Chemother. 38:71-78, 1994), and these four strains showed resistance to carbapenems as well as to the other broad-spectrum beta-lactams. In particular, strains AK9373, AK9374, and AK9391 showed an extraordinarily high-level resistance to imipenem (MICs, > or = 64 micrograms/ml), whereas strain AK9385 demonstrated moderate imipenem resistance (MIC, 8 micrograms/ml). The imipenem resistance of AK9373 was transferred to Escherichia coli CSH2 by conjugation with a frequency of 10(-5). The DNA probe of the blaIMP gene hybridized to a large plasmid (approximately 120 kb) transferred into the E. coli transconjugant as well as to the large plasmids harbored by AK9373. On the other hand, although we failed in the conjugational transfer of imipenem resistance from strains AK9374, AK9385, and AK9391 to E. coli CSH2, imipenem resistance was transferred from these strains to E. coli HB101 by transformation. A plasmid (approximately 25 kb) was observed in each transformant which acquired imipenem resistance. The amino acid sequence at the N terminus of the enzyme purified from strain AK9373 was identical to that of the metallo-beta-lactamase IMP-1. In contrast, strains ES9348, AK9386, and AK93101, which were moderately resistant to imipenem (MICs, > or = 4 to < or = 8 micrograms/ml), had no detectable blaIMP gene. As a conclusion, 19% of clinically isolated S. marcescens strains in Aichi Prefecture, Japan, in 1993 were resistant to imipenem (MICs, > or = 2 micrograms/ml), and strains which showed high-level imipenem resistance because of acquisition of a plasmid-mediated blaIMP-like metallo-beta-lactamase gene had already proliferated as nosocomial infections, at least in a general hospital.

Nucleotide sequence of a plasmid-mediated cephalosporinase gene (blaLAT-1) found in Klebsiella pneumoniae

The nucleotide sequence of the gene encoding a novel cephalosporinase (LAT-1), carried by a non-self-transferable plasmid from Klebsiella pneumoniae, has been determined. It was found that the sequence shares a high degree of homology with the Citrobacter freundii OS60 ampC structural gene.

Gene sequence and biochemical characterization of FOX-1 from Klebsiella pneumoniae, a new AmpC-type plasmid-mediated beta-lactamase with two molecular variants

Klebsiella pneumoniae BA32, a clinical isolate from Buenos Aires, Argentina, was found to produce a plasmid-encoded beta-lactamase (FOX-1) which conferred resistance to broad-spectrum cephalosporins and cephamycins. Resistance could be transferred by conjugation or transformation into Escherichia coli K-12 via a 48.5-kb plasmid (pGLK1) that produced two FOX-1 molecular variants with isoelectric points of 6.8 and 7.2 and apparent molecular sizes of 37 and 35 kDa, respectively. The kinetic study revealed that the two variants had very similar substrate and inhibition profiles. These values resemble those of chromosomally mediated class C (group 1) cephalosporinases. The structural gene of FOX-1 (blaFOX-1) was cloned into a 2,270-bp PstI-PstI fragment and was expressed in E. coli TG1. The deduced 382-amino-acid sequence of FOX-1 exhibited a high degree of similarity with chromosomally encoded AmpC beta-lactamases of Pseudomonas aeruginosa, Serratia marcescens, Enterobacter cloacae, E. coli, and Citrobacter freundii. These findings suggest that FOX-1 is a plasmid-mediated AmpC-type beta-lactamase that is encoded by a single gene and that has two molecular variants.

Characterization of a plasmid-borne and constitutively expressed blaMOX-1 gene encoding AmpC-type beta-lactamase

A 1954-bp DNA fragment containing the blaMOX-1 gene, identified on a large resident plasmid (pRMOX-1) of Klebsiella pneumoniae NU2936, was sequenced and an open reading frame (ORF) coding for a 390-amino-acid (aa) MOX-1 was found. The total deduced aa sequence of MOX-1 shared considerable homology with that of AmpC-type class C beta-lactamases of Gram- bacteria, especially of Pseudomonas aeruginosa PAO1 [51.3%; 63.8% at the nucleotide (nt) level]. However, the regulatory gene ampR and a 38-bp AmpR-binding region were not present upstream from blaMOX-1, although the expression of P. aeruginosa ampC is directly regulated by AmpR. Possible -35 and -10 regions, a Shine-Dalgarno (SD) sequence and terminators were identified which are peculiar to blaMOX-1. On the other hand, a sequence highly homologous (91.6%) to the region upstream from dhfrX in the In7 integron carried by plasmid pDGO100 was found upstream from blaMOX-1 at nt 1 to 488. No significant difference was detected between the promoter activities of blaMOX-1 in ampD- and ampD+ strains of Enterobacter cloacae, as measured by the chloramphenicol acetyltransferase (CAT) assay. These results clearly show that blaMOX-1 belongs to the group of ampC-related bla genes and that it is expressed constitutively, independently of transcriptional regulators such as AmpR, AmpG and AmpD. Homology analysis among AmpC enzymes or ampC genes implied that integration of the chromosomal ampC gene into a large resident plasmid, followed by transconjugation, was involved in the evolution of blaMOX-1.

Origin and impact of plasmid-mediated extended-spectrum beta-lactamases

Resistance to oxyimino cephalosporins was originally highlighted by the emergence of plasmid-encoded extended-spectrum beta-lactamases deriving by mutation from TEM-1, TEM-2 and SHV type enzymes (class A). The broader spectrum of resistance produced by these enzymes is related to more amino acid substitutions, but susceptibility to seven alpha-methoxyimino cephalosporins and carbapenems was preserved until recently. Clavulanate-sensitive extended-spectrum beta-lactamases are distributed worldwide, mainly among Klebsiella pneumoniae isolates. Novel clavulanate-sensitive extended-spectrum beta-lactamases deriving from other class A enzymes (e.g. MEN-1 from beta la OXY, OXA-11 in Pseudomonas aeruginosa from PSE-2) have been reported. Recently, clavulanate-resistant extended-spectrum beta-lactamases (class C) were encountered amongst single isolates, mostly Klebsiella pneumoniae. These cephalosporinases or cefamycinases (usually chromosomally mediated) have expanded the spectrum of plasmid-encoded resistance to include seven alpha-methoxyimino cephalosporins. Thus far, only two isolates (1 Pseudomonas aeruginosa, 1 Bacteroides fragilis), both recovered in Japan, with plasmid-mediated resistance to carbapenems have been found.

Genetics of extended-spectrum beta-lactamases

Bacteria have adapted to the introduction of aztreonam, cefotaxime, ceftazidime, ceftriaxone and other oxyimino-beta-lactams by altering existing plasmid-mediated class A and class D beta-lactamases so as to expand their spectrum of activity. In the TEM and SHV families of extended-spectrum beta-lactamases, relative activity toward oxyimino-substrates increases with the number of amino acid substitutions but at the price of lowered intrinsic efficiency, so that compensatory up-promoter events are often associated with increased enzyme expression. Another new mechanism of resistance is the capture on plasmids of normally chromosomal genes from Enterobacter cloacae, Citrobacter freundii or Pseudomonas aeruginosa, which upon transfer can provide Klebsiella pneumoniae or Escherichia coli with resistance to alpha-methoxy-beta-lactams, such as cefoxitin or cefotetan, as well as to oxyimino-beta-lactams.

Molecular characterization of an enterobacterial metallo beta-lactamase found in a clinical isolate of Serratia marcescens that shows imipenem resistance

A clinical isolate of Serratia marcescens (TN9106) produced a metallo beta-lactamase (IMP-1) which conferred resistance to imipenem and broad-spectrum beta-lactams. The blaIMP gene providing imipenem resistance was cloned and expressed in Escherichia coli HB101. The IMP-1 was purified from E. coli HB101 that harbors pSMBNU24 carrying blaIMP, and its apparent molecular mass was calculated to be about 30 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Kinetic studies of IMP-1 against various beta-lactams revealed that this enzyme hydrolyzes not only various broad-spectrum beta-lactams but also carbapenems. However, aztreonam was relatively stable against IMP-1. Although clavulanate or cloxacillin failed to inhibit IMP-1, Hg2+, Fe2+, or Cu2+ blocked the enzyme's activity. Moreover, the presence of EDTA in the reaction buffer resulted in a decrease in the enzyme's activity. Carbapenem resistance was not transferred from S. marcescens TN9106 to E. coli CSH2 by conjugation. A hybridization study confirmed that blaIMP was encoded on the chromosome of S. marcescens TN9106. By nucleotide sequencing analysis, blaIMP was found to encode a protein of 246 amino acid residues and was shown to have considerable homology to the metallo beta-lactamase genes of Bacillus cereus, Bacteroides fragilis, and Aeromonas hydrophila. The G+C content of blaIMP was 39.4%. Four consensus amino acid residues, His-95, His-97, Cys-176, and His-215, which form putative zinc ligands, were conserved in the deduced amino acid sequence of IMP-1. By determination of the amino acid sequence at the N terminus of purified mature IMP-1, 18 amino acid residues were found to be processed from the N terminus of the premature enzyme as a signal peptide. These results clearly show that IMP-1 is an enterobacterial metallo beta-lactamase, of which the primary structure has been completely determined, that confers resistance to carbapenems and other broad-spectrum beta-lactams.