Antibiotic resistance in pathogenic and producing bacteria, with special reference to beta-lactam antibiotics

Penicillin-Binding Proteins, β-Lactamases, and β-Lactamase Inhibitors in β-Lactam-Producing Actinobacteria: Self-Resistance Mechanisms

The human society faces a serious problem due to the widespread resistance to antibiotics in clinical practice. Most antibiotic biosynthesis gene clusters in actinobacteria contain genes for intrinsic self-resistance to the produced antibiotics, and it has been proposed that the antibiotic resistance genes in pathogenic bacteria originated in antibiotic-producing microorganisms. The model actinobacteria Streptomyces clavuligerus produces the β-lactam antibiotic cephamycin C, a class A β-lactamase, and the β lactamases inhibitor clavulanic acid, all of which are encoded in a gene supercluster; in addition, it synthesizes the β-lactamase inhibitory protein BLIP. The secreted clavulanic acid has a synergistic effect with the cephamycin produced by the same strain in the fight against competing microorganisms in its natural habitat. High levels of resistance to cephamycin/cephalosporin in actinobacteria are due to the presence (in their β-lactam clusters) of genes encoding PBPs which bind penicillins but not cephalosporins. We have revised the previously reported cephamycin C and clavulanic acid gene clusters and, in addition, we have searched for novel β-lactam gene clusters in protein databases. Notably, in S. clavuligerus and Nocardia lactamdurans, the β-lactamases are retained in the cell wall and do not affect the intracellular formation of isopenicillin N/penicillin N. The activity of the β-lactamase in S. clavuligerus may be modulated by the β-lactamase inhibitory protein BLIP at the cell-wall level. Analysis of the β-lactam cluster in actinobacteria suggests that these clusters have been moved by horizontal gene transfer between different actinobacteria and have culminated in S. clavuligerus with the organization of an elaborated set of genes designed for fine tuning of antibiotic resistance and cell wall remodeling for the survival of this Streptomyces species. This article is focused specifically on the enigmatic connection between β-lactam biosynthesis and β-lactam resistance mechanisms in the producer actinobacteria.

The UvrA-like protein Ecm16 requires ATPase activity to render resistance against echinomycin

Bacteria use various strategies to become antibiotic resistant. The molecular details of these strategies are not fully understood. We can increase our understanding by investigating the same strategies found in antibiotic‐producing bacteria. In this work, we characterize the self‐resistance protein Ecm16 encoded by echinomycin‐producing bacteria. Ecm16 is a structural homolog of the nucleotide excision repair protein UvrA. Expression of ecm16 in the heterologous system Escherichia coli was sufficient to render resistance against echinomycin. Ecm16 binds DNA (double‐stranded and single‐stranded) using a nucleotide‐independent binding mode. Ecm16’s binding affinity for DNA increased by 1.7‐fold when the DNA is intercalated with echinomycin. Ecm16 can render resistance against echinomycin toxicity independently of the nucleotide excision repair system. Similar to UvrA, Ecm16 has ATPase activity, and this activity is essential for Ecm16’s ability to render echinomycin resistance. Notably, UvrA and Ecm16 were unable to complement each other's function. Together, our findings identify new mechanistic details of how a refurbished DNA repair protein Ecm16 can specifically render resistance to the DNA intercalator echinomycin.

Comparison of Antibiotic Resistance Mechanisms in Antibiotic-Producing and Pathogenic Bacteria

Antibiotic resistance poses a tremendous threat to human health. To overcome this problem, it is essential to know the mechanism of antibiotic resistance in antibiotic-producing and pathogenic bacteria. This paper deals with this problem from four points of view. First, the antibiotic resistance genes in producers are discussed related to their biosynthesis. Most resistance genes are present within the biosynthetic gene clusters, but some genes such as paromomycin acetyltransferases are located far outside the gene cluster. Second, when the antibiotic resistance genes in pathogens are compared with those in the producers, resistance mechanisms have dependency on antibiotic classes, and, in addition, new types of resistance mechanisms such as Eis aminoglycoside acetyltransferase and self-sacrifice proteins in enediyne antibiotics emerge in pathogens. Third, the relationships of the resistance genes between producers and pathogens are reevaluated at their amino acid sequence as well as nucleotide sequence levels. Pathogenic bacteria possess other resistance mechanisms than those in antibiotic producers. In addition, resistance mechanisms are little different between early stage of antibiotic use and the present time, e.g., β-lactam resistance in Staphylococcus aureus. Lastly, guanine + cytosine (GC) barrier in gene transfer to pathogenic bacteria is considered. Now, the resistance genes constitute resistome composed of complicated mixture from divergent environments.

Cell Wall Deficiency as a Coping Strategy for Stress

The cell wall is a surface layer located outside the cell membrane of almost all bacteria; it protects cells from environmental stresses and gives them their typical shape. The cell wall is highly conserved in bacteria and is the target for some of our best antibiotics. Surprisingly, some bacteria are able to shed their wall under the influence of stress, yielding cells that are cell-wall-deficient. Notably, wall-deficient cells are flexible and are able to maneuver through narrow spaces, insensitive to wall-targeting antibiotics, and capable of taking up and exchanging DNA. Moreover, given that wall-associated epitopes are often recognized by host defense systems, wall deficiency provides a plausible explanation for how some bacteria may hide in their host. In this review we focus on this paradoxical stress response, which provides cells with unique opportunities that are unavailable to walled cells.

Antibiotic resistance genes in the Actinobacteria phylum

The Actinobacteria phylum is one of the oldest bacterial phyla that have a significant role in medicine and biotechnology. There are a lot of genera in this phylum that are causing various types of infections in humans, animals, and plants. As well as antimicrobial agents that are used in medicine for infections treatment or prevention of infections, they have been discovered of various genera in this phylum. To date, resistance to antibiotics is rising in different regions of the world and this is a global health threat. The main purpose of this review is the molecular evolution of antibiotic resistance in the Actinobacteria phylum.

Self-resistance in Streptomyces, with Special Reference to β-Lactam Antibiotics

Antibiotic resistance is one of the most serious public health problems. Among bacterial resistance, β-lactam antibiotic resistance is the most prevailing and threatening area. Antibiotic resistance is thought to originate in antibiotic-producing bacteria such as Streptomyces. In this review, β-lactamases and penicillin-binding proteins (PBPs) in Streptomyces are explored mainly by phylogenetic analyses from the viewpoint of self-resistance. Although PBPs are more important than β-lactamases in self-resistance, phylogenetically diverse β-lactamases exist in Streptomyces. While class A β-lactamases are mostly detected in their enzyme activity, over two to five times more classes B and C β-lactamase genes are identified at the whole genomic level. These genes can subsequently be transferred to pathogenic bacteria. As for PBPs, two pairs of low affinity PBPs protect Streptomyces from the attack of self-producing and other environmental β-lactam antibiotics. PBPs with PASTA domains are detectable only in class A PBPs in Actinobacteria with the exception of Streptomyces. None of the Streptomyces has PBPs with PASTA domains. However, one of class B PBPs without PASTA domain and a serine/threonine protein kinase with four PASTA domains are located in adjacent positions in most Streptomyces. These class B type PBPs are involved in the spore wall synthesizing complex and probably in self-resistance. Lastly, this paper emphasizes that the resistance mechanisms in Streptomyces are very hard to deal with, despite great efforts in finding new antibiotics.

Beta-lactamase induction and cell wall metabolism in Gram-negative bacteria

Production of beta-lactamases, the enzymes that degrade beta-lactam antibiotics, is the most widespread and threatening mechanism of antibiotic resistance. In the past, extensive research has focused on the structure, function, and ecology of beta-lactamases while limited efforts were placed on the regulatory mechanisms of beta-lactamases. Recently, increasing evidence demonstrate a direct link between beta-lactamase induction and cell wall metabolism in Gram-negative bacteria. Specifically, expression of beta-lactamase could be induced by the liberated murein fragments, such as muropeptides. This article summarizes current knowledge on cell wall metabolism, beta-lactam antibiotics, and beta-lactamases. In particular, we comprehensively reviewed recent studies on the beta-lactamase induction by muropeptides via two major molecular mechanisms (the AmpG-AmpR-AmpC pathway and BlrAB-like two-component regulatory system) in Gram-negative bacteria. The signaling pathways for beta-lactamase induction offer a broad array of promising targets for the discovery of new antibacterial drugs used for combination therapies. Therefore, to develop effective mitigation strategies against the widespread beta-lactam resistance, examination of the molecular basis of beta-lactamase induction by cell wall fragment is highly warranted.

Avoidance of suicide in antibiotic-producing microbes

Many microbes synthesize potentially autotoxic antibiotics, mainly as secondary metabolites, against which they need to protect themselves. This is done in various ways, ranging from target-based strategies (i.e. modification of normal drug receptors or de novo synthesis of the latter in drug-resistant form) to the adoption of metabolic shielding and/or efflux strategies that prevent drug-target interactions. These self-defence mechanisms have been studied most intensively in antibiotic-producing prokaryotes, of which the most prolific are the actinomycetes. Only a few documented examples pertain to lower eukaryotes while higher organisms have hardly been addressed in this context. Thus, many plant alkaloids, variously described as herbivore repellents or nitrogen excretion devices, are truly antibiotics-even if toxic to humans. As just one example, bulbs of Narcissus spp. (including the King Alfred daffodil) accumulate narciclasine that binds to the larger subunit of the eukaryotic ribosome and inhibits peptide bond formation. However, ribosomes in the Amaryllidaceae have not been tested for possible resistance to narciclasine and other alkaloids. Clearly, the prevalence of suicide avoidance is likely to extend well beyond the remit of the present article.

Functional diversity among metallo-beta-lactamases: characterization of the CAR-1 enzyme of Erwinia carotovora

Metallo-beta-lactamases (MBLs) are zinc-dependent bacterial enzymes characterized by an efficient hydrolysis of carbapenems and a lack of sensitivity to commercially available beta-lactamase inactivators. Apart from the acquired subclass B1 enzymes, which exhibit increasing clinical importance and whose evolutionary origin remains unclear, most MBLs are encoded by resident genes found in the genomes of organisms belonging to at least three distinct phyla. Using genome database mining, we identified an open reading frame (ORF) (ECA2849) encoding an MBL-like protein in the sequenced genome of Erwinia carotovora, an important plant pathogen. Although no detectable beta-lactamase activity could be found in E. carotovora, a recombinant Escherichia coli strain in which the ECA2849 ORF was cloned showed decreased susceptibility to several beta-lactams, while carbapenem MICs were surprisingly poorly affected. The enzyme, named CAR-1, was purified by means of ion-exchange chromatography steps, and its characterization revealed unique structural and functional features. This new MBL was able to efficiently hydrolyze cephalothin, cefuroxime, and cefotaxime and, to a lesser extent, penicillins and the other cephalosporins but only poorly hydrolyzed meropenem, while imipenem was not recognized. CAR-1 is the first example of a functional naturally occurring MBL in the family Enterobacteriaceae (order Enterobacteriales) and highlights the extraordinary structural and functional diversity exhibited by MBLs.

Distribution of beta-lactamases in actinomycetes

The distribution of beta-lactamase activities in a collection of actinomycete strains was surveyed. Six of 127 strains were found to produce beta-lactamase. This low frequency was in contrast to the case with Streptomyces species. The producing strains were not related phylogenetically. MICs of benzylpenicillin did not correlate with beta-lactamase production.

Membrane-bound and extracellular beta-lactamase production with developmental regulation in Streptomyces griseus NRRL B-2682

A new type of beta-lactamase has been isolated and characterized in Streptomyces griseus NRRL B-2682. The enzyme has membrane-bound and extracellular forms. Biochemical characterization of some of the properties of the enzyme showed that it belongs to the class A group of penicillinases. Comparison of the membrane-bound and extracellular forms of the beta-lactamases suggests that they seem to be differently processed forms of the same enzyme. The N-terminal amino acid sequence of the extracellular form of the beta-lactamase showed a high degree of similarity to a D-aminopeptidase of another Streptomyces griseus strain. Secretion of the beta-lactamase was affected by the differentiation state of the strain since in spontaneous non-sporulating mutants only the membrane-bound form was present. In accordance with this when sporulation of the wild-type strain was inhibited it failed to secrete extracellular beta-lactamase. Addition of globomycin to the non-sporulating cells liberated the enzyme from the membrane, indicating that the protein is processed normally by signal peptidase II and a glyceride-thioether group, together with a fatty acid amide-linkage, is responsible for the attachment of the enzyme to the cellular membrane. Under sporulation-repressed conditions addition of peptidoglycan fragments and analogues or inhibition of cell wall biosynthesis by penicillin-G induced beta-lactamase secretion and also restored sporulation both in solid and submerged cultures. These results confirm that beta-lactamase secretion is tightly coupled to the sporulation process in S. griseus.

Molecular analysis of a beta-lactam resistance gene encoded within the cephamycin gene cluster of Streptomyces clavuligerus

A Streptomyces clavuligerus gene (designated pcbR) which is located immediately downstream from the gene encoding isopenicillin N synthase in the cephamycin gene cluster was characterized. Nucleotide sequence analysis and database searching of PcbR identified a significant similarity between PcbR and proteins belonging to the family of high-molecular-weight group B penicillin-binding proteins (PBPs). Eight of nine boxes (motifs) conserved within this family of proteins are present in the PcbR protein sequence in the same order and with approximately the same spacing between them. When a mutant disrupted in pcbR was constructed by gene replacement, the resulting pcbR mutant exhibited a significant decrease in its resistance to benzylpenicillin and cephalosporins, indicating that pcbR is involved in beta-lactam resistance in this organism. Western blot (immunoblot) analysis of S. clavuligerus cell membranes using PcbR-specific antibodies suggested that PcbR is a membrane protein. PcbR was also present in cell membranes when expressed in Escherichia coli and was able to bind radioactive penicillin in a PBP assay, suggesting that PcbR is a PBP. When genomic DNAs from several actinomycetes were probed with pcbR, hybridization was observed to some but not all beta-lactam-producing actinomycetes.

Cloning, sequencing, and site-directed mutagenesis of beta-lactamase gene from Streptomyces fradiae Y59

The beta-lactamase gene from Streptomyces fradiae Y59 was cloned and sequenced. To determine which amino acid residues are critical in binding activity to blue dextran, chimera beta-lactamases were constructed and their binding abilities were determined. The results suggested that blue dextran binding may depend more on overall conformation of about two-thirds of the beta-lactamase molecule from the N terminus than on the primary structure.

Nucleotide sequence and transcriptional analysis of activator-regulator proteins for beta-lactamase in Streptomyces cacaoi

The nucleotide sequence of the 2.7-kb DNA fragment upstream of the structural gene of beta-lactamase in Streptomyces cacaoi was determined. Computer-aided "FRAME" analysis revealed four possible open reading frames (ORFs), three in one direction and one in the opposite direction. One of them (ORF1, BlaA) encoded an activator-regulator protein whose deduced amino acid sequence was similar to that of other activator-regulator proteins in bacteria. Insertion of an 8-bp BamHI linker into the BlaA region decreased the beta-lactamase activity sharply, from 50 U to 1 U/ml. This protein (BlaA) was found to bind to the nucleotide sequence between the bla (beta-lactamase structural gene) and blaA genes. Another ORF (ORF2, BlaB) in the same orientation had a couple of amino acid sequences similar to that of pBR322 beta-lactamase. However, insertion of the 8-bp BamHI linker indicated that this ORF was functional as an activator-regulator but not as a beta-lactamase. Therefore, there were two activator-regulator proteins in the upstream region of the structural gene of the beta-lactamase. Nuclease S1 mapping predicted that transcription for the activator proteins commenced at the translational initiation codon or within a few nucleotides from the translational start site. Transcription was in the opposite direction to that of the beta-lactamase structural gene.

Beta-lactamase expression in Streptomyces cacaoi

Plasmids were prepared by inserting genomic DNA fragments from Streptomyces cacaoi within the mel gene of plasmid pIJ702. The inserted DNA fragments contain the beta-lactamase-encoding bla gene and upstream nucleotide sequences of various lengths. The transcription start point of bla was identified by nuclease S1 mapping. Upstream nucleotide sequences of sufficient lengths had an enhancing effect on beta-lactamase production by the Streptomyces host. The dot blot hybridization assay revealed that this effect was exerted at the transcriptional level. Experimental evidence strongly suggests that the underlying mechanism involves, at least in part, one or several trans-acting elements. In one of the constructs, in which the upstream nucleotide sequence was reduced to 0.3 kb, the bla promoter was present but the bla gene was expressed by readthrough from a promoter, possibly the mel promoter, of the pIJ702 vector.

Isolation and characterization of a beta-lactamase-inhibitory protein from Streptomyces clavuligerus and cloning and analysis of the corresponding gene

Culture filtrates of Streptomyces clavuligerus contain a proteinaceous beta-lactamase inhibitor (BLIP) in addition to a variety of beta-lactam compounds. BLIP was first detected by its ability to inhibit Bactopenase, a penicillinase derived from Bacillus cereus, but it has also been shown to inhibit the plasmid pUC- and chromosomally mediated beta-lactamases of Escherichia coli. BLIP showed no inhibitory effect against Enterobacter cloacae beta-lactamase, and it also showed no activity against an alternative source of B. cereus penicillinase. BLIP was purified to homogeneity, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave a size estimate for BLIP of 16,900 to 18,000. The interaction between purified BLIP and the E. coli(pUC) beta-lactamase was investigated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and determined to be noncovalent, with an estimated 1:1 molar stoichiometry. The BLIP gene was isolated on a 13.5-kilobase fragment of S. clavuligerus chromosomal DNA which did not overlap a 40-kilobase region of DNA known to contain genes for beta-lactam antibiotic biosynthesis. The gene encoded a mature protein with a deduced amino acid sequence of 165 residues (calculated molecular weight of 17,523) and also encoded a 36-amino-acid signal sequence. No significant sequence similarity to BLIP was found by pairwise comparisons using various protein and nucleotide sequence data banks or by hybridization experiments, and no BLIP activity was detected in the culture supernatants of other Streptomyces spp.

In vitro evaluation of BRL 42715, a novel beta-lactamase inhibitor

The penem BRL 42715, C6-(N1-methyl-1,2,3-triazolylmethylene)penem, is a potent inhibitor of a broad range of bacterial beta-lactamases, including the plasmid-mediated TEM, SHV, OXA, and staphylococcal enzymes, as well as the chromosomally mediated enzymes of Bacteroides, Enterobacter, Citrobacter, Serratia, Morganella, Escherichia, Klebsiella, and Proteus species. The concentration of BRL 42715 needed to reduce the initial rate of hydrolysis of most beta-lactamase enzymes by 50% was less than 0.01 micrograms/ml, which was 10- to 100-fold lower than for other beta-lactamase inhibitors. These potent inhibitory activities were reflected in the low concentrations of BRL 42715 needed to potentiate the antibacterial activity of beta-lactamase-susceptible beta-lactams. Concentrations of 0.25 micrograms/ml or less considerably enhanced the activity of amoxicillin against many beta-lactamase-producing strains. The MIC50 (MIC for 50% of strains tested) of amoxicillin for 412 beta-lactamase-producing members of the family Enterobacteriaceae fell from greater than 128 to 2 micrograms/ml in the presence of 1 microgram of BRL 42715 per ml, whereas 5 micrograms of clavulanic acid per ml brought the MIC50 down to 8 micrograms/ml. Among these 412 strains were 73 Citrobacter and Enterobacter strains, and 1 microgram of BRL 42715 per ml reduced the MIC50 of amoxicillin from greater than 128 to 2 micrograms/ml for the 48 cefotaxime-susceptible strains and from greater than 128 to 8 micrograms/ml for the 25 cefotaxime-resistant strains.

Clinical and pathological evaluation of sulbactam/ampicillin for treatment of experimental bovine pneumonic pasteurellosis

An experiment was conducted to evaluate the efficacy of sulbactam/ampicillin for treatment of bovine pneumonic pasteurellosis. Twenty-one Hereford calves were experimentally infected with bovine herpesvirus-1 and an ampicillin-resistant strain of Pasteurella haemolytica, then treated for three days with either sulbactam/ampicillin, chloramphenicol, or a placebo. The treatments were evaluated by comparing clinical illness scores, total sick days, weight changes, mortality rates, and postmortem lung scores between treatment groups. Both antibiotics were highly effective in reducing respiratory disease in the experimentally infected calves. The clinical response to sulbactam/ampicillin treatment was comparable with that of chloramphenicol and was significantly improved compared with the response to the placebo treatment. These findings suggest that the efficacy of sulbactam/ampicillin may be comparable to that of chloramphenicol for treatment of pneumonic pasteurellosis involving ampicillin-resistant strains of P. haemolytica.

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.

Frequency of antibiotic and heavy metal resistance, pigmentation, and plasmids in bacteria of the marine air-water interface

A field investigation of marine coastal waters revealed that the frequency of pigmented bacteria and the occurrence of bacterial antibiotic resistance were higher at the air-water interface than in the bulk water. The differences in the frequency of pigmented colonies at the surface and in the bulk-water samples could not be explained by the degree of cell surface hydrophobicity or by bacterial adhesion to air-water interfaces. Pigmented strains exhibited a higher degree of multiple drug resistance than did nonpigmented strains. However, the frequency of multiple drug resistance in nonpigmented strains was also substantial. An average of 91% of all strains were resistant to more than one antibiotic, and 21% of the bacteria isolated were resistant to five of the eight antibiotics tested. High numbers of plasmid-carrying strains were found among selected surface isolates, but the presence of detectable plasmids could not be correlated with either pigmentation or multiple drug resistance. Furthermore, selected surface isolates were significantly more resistant to mercury than were bulk-water bacteria. The higher frequency of pigmented, antibiotic-resistant, and mercury resistant strains at the air-water interface than in the bulk water are discussed in terms of various forms of selective pressure and genetic exchange at the surface.

Synergistic effects of dicloxacillin or clavulanic acid in combination with penicillin G or cephalothin against Yersinia enterocolitica

Cultures of Yersinia enterocolitica grown at 22 degrees C produced beta-lactamases, whereas cultures grown at 37 degrees C produced these enzymes much less effectively. Both dicloxacillin and clavulanic acid inhibited the beta-lactamase activity of bacterial crude extracts and potentiated the activity of penicillin G or cephalothin against 14 Y. enterocolitica strains. It appeared that the beta-lactamase activity present in Y. enterocolitica cells grown at 37 degrees C was great enough to play a role in bacterial resistance to beta-lactam antibiotics, since combining penicillin G or cephalothin with clavulanic acid or dicloxacillin resulted in synergistic activity against cultures grown at 37 degrees C that was equal to or greater than the activity against cultures grown at 22 degrees C.

Rapid, inexpensive method for specific detection of microbial beta-lactamases by detection of fluorescent end products

A rapid method was developed for specific detection of microbial beta-lactamases which uses ampicillin and cephalexin as substrates. The end products (open beta-lactam ring forms) generated after separately incubating either substrate with beta-lactamase-producing organisms initially were separated from the unhydrolyzed substrates by high-voltage electrophoresis at pH 2.1. The end products of both antibiotics were highly fluorescent and could be analyzed visually and semiquantitatively under a long-wave UV lamp. Application of 5 microliters of the same incubation mixture onto filter paper without subsequent electrophoretic separation also resulted in development of fluorescence after brief heating at 120 degrees C for 5 min. This spot test differentiates penicillinase activity from cephalosporinase activity and distinguishes between beta-lactamase and acylase activities, since the end products of acylase [the common side chain, D(-)-alpha-aminophenylacetic acid, and the intact beta-lactam nuclei, 6-aminopenicillanic acid and 7-aminodeacetoxycephalosporanic acid] are not fluorescent. This method was relatively rapid, inexpensive, and more sensitive than the chromogenic cephalosporin (nitrocefin) method when 21 strains of 7 gram-positive species and 77 strains of 29 gram-negative species of bacteria were tested.

beta-Lactamase inhibitors

In summary, Table XVI shows the inhibition profiles of representative beta-lactamases from each major class of Richmond and Sykes. Either resistance (R) or sensitivity (S) is given as a general guide to the type of compounds likely to inhibit each class. Thus the (qualitative) statements regarding the effectiveness of clavulanic acid can be taken to represent those for the penam sulfones and similarly for MM4550 and the other olivanic acids, carpetimycins, PS series, and asparenomycins. This can also be said of cloxacillin and the other aromatic carboxamido penicillins. Compounds are also included which are specifically or particularly inhibitory to certain beta-lactamases.

Occurrence and expression of imipemide (N-formimidoyl thienamycin) resistance in clinical isolates of coagulase-negative staphylococci

More than 500 clinical isolates were screened for resistance to a number of antibiotics, including imipemide (N-formimidoyl thienamycin [MK0787]). Of the 25 coagulase-negative staphylococcal isolates present in the screening sample, almost one-third showed one of two patterns of imipemide resistance. One pattern apparently involves constitutive expression of drug resistance, whereas the other pattern seems to result from an inducible resistance having an apparent induction threshold higher than the minimal inhibitory concentration of imipemide. The mechanism(s) responsible for this imipemide resistance is unclear, but may be distinct from the more common staphylococcal mechanisms of resistance to beta-lactam antibiotics. Only two of the patients from whom imipemide-resistant staphylococci were cultured had actually been treated with the antibiotic.

Chromosomal beta-lactamases of Enterobacter cloacae are responsible for resistance to third-generation cephalosporins

About 70% of all Enterobacter cloacae strains tested possessed one of two species-specific beta-lactamases. These enzymes, E. cloacae beta-lactamase A and E. cloacae beta-lactamase B, with isoelectric points of 8.8 and 7.8, respectively, had the same pH and temperature optima. Both showed similar enzyme kinetics and were inhibited by cloxacillin but not by p-chloromercuribenzoate. E. cloacae beta-lactamase B appeared to be identical with the enzyme of E. cloacae P99. By a mutation in a regulatory gene, inducible enzyme production could be converted into constitutive expression. In E. cloacae, both enzymes did not hydrolyze third-generation cephalosporins, but they were solely responsible for resistance toward these drugs. This was demonstrated by the characterization of Escherichia coli strains expressing an identical resistance pattern after transfer of the corresponding Enterobacter gene.