Novel plasmid-mediated beta-lactamase in members of the family Enterobacteriaceae from Ohio

Characterization and amino acid sequence analysis of a new oxyimino cephalosporin-hydrolyzing class A beta-lactamase from Serratia fonticola CUV

Serratia fonticola CUV produces two isoenzymes (forms I and II) with beta-lactamase activity which were purified by a five-step procedure. The isoenzymes had identical kinetic parameters and isoelectric point (pI = 8.12). They were characterized by a specific activity towards benzylpenicillin of 1650 U/mg. The beta-lactamase hydrolyzed benzylpenicillin, amoxycillin, ureidopenicillins, first- and second-generation cephalosporins. Carboxypenicillins and isoxazolylpenicillins were hydrolyzed to a lesser extent. Towards cefotaxime and ceftriaxone (third-generation cephalosporins), the S. fonticola enzyme exhibited catalytic efficiencies much higher than those of MEN-1 and extended-spectrum TEM derivative beta-lactamases. The beta-lactamase from S. fonticola was markedly inhibited by beta-lactamase inhibitors such as clavulanic acid, sulbactam and tazobactam. The purified isoenzymes were digested by trypsin, endoproteinase Asp-N and chymotrypsin. Amino acid sequence determinations of the resulting peptides allowed the alignment of 267 amino acid residues (Swiss-Prot, accession number P 80545) for form I beta-lactamase. Form II is five residues shorter than form I at its N-terminus. From amino acid sequence comparisons, S. fonticola CUV beta-lactamase was found to share more than 69.3% identity with the chromosomally encoded beta-lactamases of Klebsiella oxytoca, Proteus vulgaris, Citrobacter diversus and the plasmid-mediated enzymes MEN-1 and Toho-1. Therefore, the oxyimino cephalosporin-hydrolyzing beta-lactamase of S. fonticola belongs to Ambler's class A. Contribution of the serine at ABL 237 in the broad-spectrum activity of these beta-lactamases is discussed.

Outbreak of TEM-24-producing Enterobacter aerogenes in an intensive care unit and dissemination of the extended-spectrum beta-lactamase to other members of the family enterobacteriaceae

We report an outbreak of Enterobacter aerogenes in an intensive care unit (ICU) and two medicine departments that produced the extended-spectrum beta-lactamase TEM-24, which was difficult to detect by disk agar diffusion. The strains were compared by DNA restriction fragment length polymorphism after pulsed-field gel electrophoresis following cleavage with XbaI. This typing method indicated that a single strain, first isolated in the ICU, spread throughout the other medical departments as a result of patient transfer. We also observed the transfer in vivo of the plasmid encoding TEM-24 from the strain of Enterobacter aerogenes to different strains of Escherichia coli and Citrobacter freundii in the ICU. It therefore appears that the epidemic involved results from two events: dissemination of one strain of Enterobacter aerogenes and dissemination of the plasmid encoding TEM-24 among various members of the family Enterobacteriaceae.

Mutations altering substrate specificity in OHIO-1, and SHV-1 family beta-lactamase

The OHIO-1 beta-lactamase does not normally hydrolyse oxyimino-beta-lactam substrates like cefotaxime, ceftriaxone, ceftazidime or aztreonam. We were able to select spontaneous mutants of an OHIO-1-bearing strain of Escherichia coli using the antibiotic substrates listed above by enrichment methods of frequencies of 10(-8)-10(-10) for all antibiotics except ceftazidime (frequency less than 10(-10)). Most mutants with increased resistance to the other beta-lactams were also more resistant to ceftazidime. Mutations identified by DNA sequencing included a Gly238----Ser238 substitution identical with the SHV-2 mutation previously described, cysteine and valine substitutions at the identical site, and a Gly242----Cys242 substitution. The Cys238 and Cys242 mutant enzymes had less affinity for aztreonam than had the other mutant enzymes. Hydrolysis of cefotaxime, but not cephaloridine, by the cysteine-substituted enzymes was inhibited by p-chloromercuribenzoate. The mutant enzymes had, in general, greater affinity for the mechanism-based inhibitors sulbactam, clavulanic acid and tazobactam. These results suggest two non-mutually exclusive hypotheses for the structural role of substitutions in this area of the enzyme. Either potential hydrogen-bond donors, such as serine and cysteine, interact directly with the beta-lactam molecules, or the steric bulk of these substitutions distorts the beta-pleated sheet such that the beta-lactam is held in a position favourable for stable binding and catalysis. Finally, our data raise questions about a strategy relying on oligonucleotide-probe technology to detect such mutations, because of the variety of substitutions that give rise to similar phenotypes.

Discrimination of extended-spectrum beta-lactamases by a novel nitrocefin competition assay

We describe a nitrocefin competition assay for determining inhibition profiles as a useful adjunct to existing biochemical methods for the discrimination of beta-lactamases. The hydrolysis rate of nitrocefin was measured with a plate photometer as the change in A480 over 45 min in the presence of 17 inhibitors. Fourteen well-established beta-lactamases and 13 extended-spectrum beta-lactamases were tested. Correlations with data from isoelectric focusing and amino acid sequencing suggested that the inhibition profile reflects alterations in the active-site configuration of beta-lactamases. The method was especially useful in measuring the relative affinities of beta-lactamases against poorly hydrolyzed substrates and in screening large numbers of isolates for the detection of new beta-lactamase types.

Nucleotide sequence and phylogeny of SHV-2 beta-lactamase

We determined the nucleotide sequence of the blaSHV-2(pBP60-1) gene from Klebsiella ozaenae which confers resistance to broad-spectrum cephalosporins. The structural gene encodes a polypeptide product of 286 amino acids, and the estimated molecular weight of the mature protein is 28,900. Amino acid sequence comparison of the SHV-2pBP60-1 enzyme with all known class A beta-lactamases and homology studies showed that the residues were highly conserved. Furthermore, SHV-2pBP60-1 was clearly related to SHV-1, LEN-1, and OHIO-1. The SHV-2pBP60-1 enzyme differed from SHV-1 isolated from Klebsiella pneumoniae by seven amino acid substitutions. One of these substitutions, the Gly----Ser substitution at position 234, is probably a key region for the novel activity of cefotaxime hydrolysis. A phylogenetic tree was constructed by using all class A beta-lactamases of known sequences by a progressive alignment method. The data suggested that the beta-lactamases of gram-positive Streptomyces, Staphylococcus, and Bacillus species appeared early in evolution, followed by the PSE and CARB enzymes of Pseudomonas species and, more recently, by the SHV-type and TEM-type enzymes found in enteric bacteria. Larger evolutionary distances separated clusters of the gram-positive beta-lactamases than separated clusters of the gram-negative enzymes. Results of this phylogenetic study suggested that extended-spectrum enzymes are recent derivatives that are selected by the use of new cephalosporins.

OHIO-1 beta-lactamase is part of the SHV-1 family

The OHIO-1 beta-lactamase gene was subcloned in a 1.16-kilobase TaqI fragment in the 2.4-kilobase chimeric plasmid pSK04. After directional subcloning into M13, the DNA sequence of this fragment was determined. The results showed an open reading frame of 858 base pairs (bp) encoding a protein of 286 amino acids. The structural gene showed 95, 87, and 60% DNA sequence identity with SHV-1, LEN-1, and TEM-1, respectively, and 93, 85, and 62% predicted amino acid sequence identity, respectively. The 87 bp upstream of the OHIO-1 structural gene had 96% identity with the upstream flanking sequence of SHV-1, including the -35 and -10 consensus sequences and the putative ribosomal binding site. A 223-bp DNA probe derived from a PstI-HaeII fragment in the C-terminal sequence of OHIO-1 had predicted 96, 88, and 61% sequence identity with SHV-1, LEN-1, and TEM-1, respectively. This probe hybridized to SHV-1 and poorly to LEN-1, but not to TEM-1 or a variety of other plasmid-mediated beta-lactamase genes, under stringent conditions. Screening of plasmid DNA derived from 40 ampicillin-resistant clinical isolates by Southern hybridization with the 223-bp probe uncovered no strains encoding OHIO-1. Isoelectric focusing of the same collection did identify two strains producing enzymes resembling SHV-1, however. We have also performed a kinetic comparison of OHIO-1, SHV-1, and TEM-1. OHIO-1 and SHV-1 were indistinguishable from each other but could be distinguished from TEM-1. These data clearly place OHIO-1 within the SHV-1 family of beta-lactamases.

Cloning of SHV-2, OHIO-1, and OXA-6 beta-lactamases and cloning and sequencing of SHV-1 beta-lactamase

Molecular cloning of DNA fragments permitted the isolation of structural genes coding for SHV-1, SHV-2, OHIO-1, and OXA-6 beta-lactamases. DNA probes were constructed for SHV-1, and under conditions of high stringency, hybridization was observed only between SHV-1 and SHV-2. Oligonucleotide typing with a 15-mer SHV-1 probe was capable of discriminating between SHV-1 and SHV-2 but not OHIO-1. The nucleotide sequence of the SHV-1 beta-lactamase gene from plasmid R974 has been determined. The structural gene encodes a polypeptide product which differs by 9 residues from the p453 (SHV-1) PIT-2 enzyme determined by peptide sequencing. The significance of each mutation was assessed by alignment of amino acid sequences and comparisons with the Staphylococcus aureus PC1 penicillinase crystal structure. Structural similarities between SHV-1 and class A beta-lactamases are extensive, with amino acid identities of 88.9% between SHV-1 and LEN-1, 91.8% between SHV-1 and OHIO-1, and 63.7% between SHV-1 and TEM-1.

Introduction of a plasmid encoding the OHIO-1 beta-lactamase to an intermediate care ward by patient transfer

A prospective study of 147 intermediate care ward (ICW) patients for acquisition of gentamicin-resistant Enterobacteriaceae (GRE) was carried out. Fifty (34%) were colonized or infected with one or more strains of GRE. Fifteen of these patients and one nurse were colonized with 22 strains (including ten species) of GRE bearing identical 60 kb plasmids encoding a novel beta-lactamase determinant, OHIO-1 and ANT(2"). Analysis of the time course of colonized patients on the ICW revealed one probable episode of cross-transmission. Five colonized patients had been residing in the ICW from one to four months prior to study initiation. Eight patients were admitted to the ICW from other hospital areas already colonized and one additional patient acquired colonization on the ICW from an unknown source. Thus, eight of ten patients admitted to the ICW during the prospective study were already colonized on admission to the ICW. To control this level of colonization it would therefore be necessary to direct efforts at limiting admission of colonized patients or attempting to eliminate the strain from persistently colonized patients, rather than trying to limit transmission within the ward.

Detection of plasmid-mediated beta-lactamases with DNA probes

beta-Lactamase identification by colony hybridization with 32P-labeled DNA probes for TEM-1, SHV-1, OXA-1, OXA-2, PSE-1, PSE-2, and PSE-4 was compared with isoelectric focusing in 122 clinical isolates making a variety of enzyme types. All strains producing a probe-type enzyme gave a positive hybridization reaction. Cross-hybridization was observed between TEM-1 and TEM-2 or TLE-1, between SHV-1 and SHV-2, between OXA-1 and OXA-4, between OXA-2 and OXA-3 (weak), between PSE-2 and OXA-6 or OXA-5 (weak), and among PSE-1, PSE-4, and CARB-3. With allowance for such cross-hybridization, only six strains gave false-positive reactions, and the procedure was 99% specific.

Molecular epidemiology of OHIO-1 beta-lactamase

A total of 31 plasmids, all bearing a gene that encodes a novel, plasmid-mediated Richmond-Sykes class III beta-lactamase designated OHIO-1 and a gene that encodes aminoglycoside 2"-adenyltransferase, have been collected from hospitals in Ohio. By using restriction endonuclease digestion and Southern hybridization, we were able to demonstrate that all these plasmids have a common genetic origin.

Monoclonal antibodies to TEM-1 plasmid-mediated beta-lactamase

At least 28 plasmid-mediated beta-lactamases have been described in gram-negative bacteria. To assess the relationship among these enzymes, we produced and characterized 28 murine monoclonal antibodies to the TEM-1 plasmid-mediated beta-lactamase. Radial immunodiffusion identified 3 monoclonal antibodies as immunoglobulin M (IgM), 18 as subclass IgG1, 2 as IgG2a, and 5 as IgG2b. Using a newly described enzyme immunoassay, cross-reactivity of 16 of these monoclonal antibodies was tested against 24 plasmid-determined beta-lactamases. The 16 monoclonal antibodies cross-reacted with TEM-2 and TLE-1 and, to a certain extent, SHV-1. Different levels of cross-reactivity were also observed with OXA-3 (11 of 16), OXA-7 (8 of 16), OXA-1 (2 of 16), OXA-6 (2 of 16), and AER-1 (2 of 16). Six monoclonal antibodies demonstrated partial neutralization of beta-lactamase activity. This study suggests that common epitopes are shared by nine biochemically distinct plasmid-mediated beta-lactamases. On the basis of cross-reactivities with these monoclonal antibodies, we identified four epitopes on TEM-1, TEM-2, TLE-1, and SHV-1 beta-lactamases.

Prevalence of colonization with antibiotic resistant gram-negative bacilli in a nursing home care unit: the importance of cross-colonization as documented by plasmid analysis

A prevalence study was carried out on a 100-bed Veterans Administration nursing home care unit to determine the extent of colonization with gentamicin-resistant gram-negative bacilli (GRGNB). Hand cultures of 12 employees and 17 environmental cultures were negative. Twenty-six of 86 (30%) patients were colonized with 49 GRGNB. Sixteen patients (19%) had urinary colonization. Multivariate analysis revealed significant associations between rectal or perineal colonization (P less than 0.01), and the presence of a urinary device (82% condom catheters) (P less than 0.05), with urinary colonization. The most common isolates were Providencia stuartii (20), Escherichia coli (nine) and Klebsiella pneumoniae (nine). Twenty-six of 49 isolates carried plasmids. Restriction endonuclease digestion of plasmid DNA was performed for 21. Cross-colonization, as defined by the presence of the identical species with the identical restriction endonuclease digestion profile of purified plasmid DNA found in different patients, was observed for eight of 21 (38%) strains. All were geographically clustered. No strains could transfer gentamicin-resistance by conjugation and only two plasmids could transform our E coli recipient to gentamicin resistance. One E coli plasmid was identical to two Citrobacter freundii plasmids and a P stuartii plasmid isolated from three different patients. This 105 kb plasmid is conjugative and encodes resistance to ampicillin, carbenicillin, tetracycline, and sulfonamides. Thus, 57% of strains were cross-colonizing or contained identical R-plasmids. Southern hybridization using a 1 kb TEM-1 gene probe demonstrated sequences homologous to this probe in five of five nursing home plasmids examined.(ABSTRACT TRUNCATED AT 250 WORDS)