Drug Resistance of Enteric Bacteria

Past and Present Perspectives on β-Lactamases

β-Lactamases, the major resistance determinant for β-lactam antibiotics in Gram-negative bacteria, are ancient enzymes whose origins can be traced back millions of years ago. These well-studied enzymes, currently numbering almost 2,800 unique proteins, initially emerged from environmental sources, most likely to protect a producing bacterium from attack by naturally occurring β-lactams. Their ancestors were presumably penicillin-binding proteins that share sequence homology with β-lactamases possessing an active-site serine. Metallo-β-lactamases also exist, with one or two catalytically functional zinc ions. Although penicillinases in Gram-positive bacteria were reported shortly after penicillin was introduced clinically, transmissible β-lactamases that could hydrolyze recently approved cephalosporins, monobactams, and carbapenems later became important in Gram-negative pathogens. Nomenclature is based on one of two major systems. Originally, functional classifications were used, based on substrate and inhibitor profiles. A later scheme classifies β-lactamases according to amino acid sequences, resulting in class A, B, C, and D enzymes. A more recent nomenclature combines the molecular and biochemical classifications into 17 functional groups that describe most β-lactamases. Some of the most problematic enzymes in the clinical community include extended-spectrum β-lactamases (ESBLs) and the serine and metallo-carbapenemases, all of which are at least partially addressed with new β-lactamase inhibitor combinations. New enzyme variants continue to be described, partly because of the ease of obtaining sequence data from whole-genome sequencing studies. Often, these new enzymes are devoid of any phenotypic descriptions, making it more difficult for clinicians and antibiotic researchers to address new challenges that may be posed by unusual β-lactamases.

Evolution of complex resistance transposons from an ancestral mercury transposon

The molecular interrelationship of a transposon family which confers multiple antibiotic resistance and is assumed to have been generated from an ancestral mercury transposon was analyzed. Initially, the transposons Tn2613 (7.2 kilobases), encoding mercury resistance, and Tn2608 (13.5 kilobases), encoding mercury, streptomycin, and sulfonamide resistances, were isolated and their structures were analyzed. Next, the following transposons were compared with respect to their genetic and physical maps: Tn2613 and Tn501, encoding mercury resistance; Tn2608 and Tn21, encoding mercury, streptomycin, and sulfonamide resistance; Tn2607 and Tn4, encoding streptomycin, sulfonamide, and ampicillin resistance; and Tn2603, encoding mercury, streptomycin, sulfonamide, and ampicillin resistance. The results suggest that the transposons encoding multiple resistance were evolved from an ancestral mercury transposon.

Comparison of transcription of beta-lactamase genes specified by various ampicillin transposons

The beta-lactamase gene from four kinds of ampicillin transposons, Tn2601, Tn3, Tn2602 and Tn1, specifying the type I (or TEM type, alternatively) beta-lactamase was cloned onto plasmid pACYC184, and the level of in vivo transcription from each beta-lactamase gene was determined by DNA-RNA hybridization. Type I beta-lactamase is very uniform enzymologically, but heterogeneous in absolute levels of enzyme activity. The results demonstrated that the heterogeneity can be explained by the efficiency of transcription of each beta-lactamase gene, suggesting a difference in its promoter efficiency. A comparison of the levels of transcription of the beta-lactamase gene and the whole ampicillin transposon suggested that the beta-lactamase gene has the strongest promoter all of the genes in the ampicillin transposon.

Transposition of the oxacillin-hydrolyzing penicillinase gene

We have found that the oxacillin-hydrolyzing penicillinase gene (oxa) encoded by plasmid RGN238 transposes to various plasmids in a recA background. We call this transposable element Tn2603. Tn2603 encodes the genes for streptomycin, sulfonamide, and mercury resistance in addition to the oxa gene. Tn2603 has a molecular size of 19.6 kilobase pairs and appears to be flanked by small inverted repeat sequences of about 200 base pairs long.

Gene expression of ampicillin resistance transposons, Tn2601 and Tn2602

To establish the mode of gene expression specified by transposon, we investigated the correlation among the homology of the DNA sequence, the extent of transposon-specific transcription, the specific activity of penicillinase (PCase) per cell, and the transposition frequency by using two ampicillin resistance transposons (TnAs), Tn2601 and Tn2602. Although both the TnAs specify the so-called type I PCase, Tn2602 always conferred 10- to 20-fold higher PCase activity per cell than Tn2601 regardless of the kind of replicon carrying TnA. The transposition freuency of Tn2602 also was 8 to 50 times higher than that of Tn2601 in all combinations of donor and recipients plasmids examined. As a result, the transposability expressed by the TnA was thought to correlate with the productivity of PCase in the cell specified by the corresponding TnA. The level of TnA-specific transcription of Tn2602 was noticeably higher than that of Tn2601, whereas the two TnAs shared a high degree (more than 90%) of DNA sequence homology. These results suggest that the difference in rates of transcription of the two transposons plays a key role in determining the difference in the productivity of PCase and the transposition-protein(s) of Tn2601 and Tn2602.

Types of beta-lactamase determined by plasmids in gram-negative bacteria

Two species of beta-lactamase determined by plasmids in enteric bacteria that show some resemblance to TEM enzymes are described. Both are distinct from all other plasmid-mediated beta-lactamases and differ from the TEM beta-lactamases in ability to hydrolyze some substrates, in isoelectric point, in immunological specificity, and in susceptibility to inhibition. One of the enzyme species, mediated by plasmid p453, has been briefly described previously. We have discovered that this beta-lactamase, designated SHV-1, is unique in its response to inhibition by the sulfhydryl group reagent p-chloromercuribenzoate, because the hydrolysis of cephaloridine but not that of benzylpenicillin is affected. This enzyme is found in a variety of plasmid types which were transferred from several bacterial species collected from a wide geographic range. The other enzyme species is novel; only a single plasmid determining this kind of beta-lactamase (designated HMS-1) has been detected.

Properties of the beta-lactamase specified by the Pseudomonas plasmid RPL11

The Pseudomonas plasmid RPL11 specifies a beta-lactamase that has properties distinguishing it from other plasmid-mediated beta-lactamases so far described, although it closely resembles plasmid Rms139 beta-lactamase.

-lactamases of R factors derived from Shigella and Salmonella strains

R. factors conferring resistance to penicillins and cephalosporins were transferred by conjugation from six strains of Shigella and from four strains of Salmonella typhimurium to a standard strain of S. typhimurium LT2. The beta-lactamases produced were then characterized by using cell-free extracts. The enzymes were of two types, I and II, with respect to specific activity against benzyl penicillin, substrate profile, K(m), pH optimum, temperature optimum, inhibition by chloride and nitrate ions, and heat inactivation. The six type I enzymes were associated with R factors from Shigella strains; five of these were R(f) factors. The four type II enzymes were associated with R factors from S. typhimurium; all these were R(i) factors.

Variant of penicillinase mediated by an R factor in Escherichia coli

The penicillinase from an Escherichia coli strain harboring an R factor R(GN823) was purified and its properties were compared with those of a known type I penicillinase mediated by R factors. The molecular weight and S(20,w) of the enzyme were 22,600 and 2.42S, respectively. The isoelectric point of the enzyme was 6.9. These values are clearly different from those of type I penicillinase. The specific activity of the enzyme was 84,700 units per mg of the purified enzyme protein, which is about 20 times higher than that of the type I penicillinase. However, similarities were observed between the enzyme and the type I-penicillinase at optimal pH (6.5 to 7.0), optimal temperature (40 to 45C), substrate specificity, Michaelis constants for penicillins and cephaloridine, and effect of inhibitors. Furthermore, antiserum against type I penicillinase showed cross-reaction against this enzyme. The enzyme was named type Ib penicillinase, and the original type I penicillinase was renamed type Ia-penicillinase.

Resistance of Escherichia coli to penicillins. VII. Purification and characterization of a penicillinase mediated by the R factor R1

The penicillinase mediated by the R factor R1 in Escherichia coli has been purified and characterized. The purification procedure contained the following three steps: spheroplast formation, chromatography of the spheroplast supernatant fluid on DEAE cellulose, and preparative polyacrylamide-gel electrophoresis. The protein obtained gave only one band in analytical polyacrylamide-gel electrophoresis. To obtain milligram quantities of the enzyme, gel filtration on Sephadex G75 was run before the last step in the purification. By gel filtration on Sephadex G75, the molecular weight was estimated as 22,000. The pH optimum, tested in universal buffer, was 7.0. The turnover numbers for benzylpenicillin, d-ampicillin, and 6-aminopenicillanic acid were 4.2 x 10(4), 6.3 x 10(4), and 2.2 x 10(4) moles of substrate hydrolyzed per min by 1 mole of enzyme, whereas the Michaelis constants were 100, 160, and 440 mum, respectively. Cephalosporins were much poorer substrates for the R1 penicillinase than were the penicillins. The turnover number for cephalosporin C, cephaloridine, and 7-amino-cephalosporanic acid were 2.4 x 10(3), 5 x 10(2), and less than 2 x 10(2), respectively. These properties show that the R1 penicillinase is quite different from the chromosomally mediated penicillinase of E. coli (11). However, the R1 enzyme resembles another R-factor penicillinase previously purified by Richmond and Datta.