Chloramphenicol-, dihydrostreptomycin-, and kanamycin-inactivating enzymes from multiple drug-resistant Escherichia coli carrying episome 'R'

Crystal structure of a GCN5-related N-acetyltransferase from Lactobacillus curiae

Members of the GCN5-related N-acetyltransferase (GNAT) family are found in all domains of life and are involved in processes ranging from protein synthesis and gene expression to detoxification and virulence. Due to the variety of their macromolecular targets, GNATs are a highly diverse family of proteins. Currently, 3D structures of only a small number of GNAT representatives are available and thus the family remains poorly characterized. Here, the crystal structure of the guanidine riboswitch-associated GNAT from Lactobacillus curiae (LcGNAT) that acetylates canavanine, a structural analogue of arginine with antimetabolite properties, is reported. LcGNAT shares the conserved fold of the members of the GNAT superfamily, but does not contain an N-terminal β0 strand and instead contains a C-terminal β7 strand. Its P-loop, which coordinates the pyrophosphate moiety of the acetyl-coenzyme A cosubstrate, is degenerated. These features are shared with its closest homologues in the polyamine acetyltransferase subclass. Site-directed mutagenesis revealed a central role of the conserved residue Tyr142 in catalysis, as well as the semi-conserved Tyr97 and Glu92, suggesting that despite its individual substrate specificity LcGNAT performs the classical reaction mechanism of this family.

Crystal structure of prodigiosin binding protein PgbP, a GNAT family protein, in Serratia marcescens FS14

Acetylation is a conserved modification catalyzed by acetyltransferases that play prominent roles in a large number of biological processes. Members of the general control non-repressible 5 (GCN5)-N-acetyltransferase (GNAT) protein superfamily are widespread in all kingdoms of life and are characterized by highly conserved catalytic fold, and can acetylate a wide range of substrates. Although the structures and functions of numerous eukaryotic GNATs have been identified thus far, many GNATs in microorganisms remain structurally and functionally undescribed. Here, we determined the crystal structure of the putative GCN5-N-acetyltransferase PgbP in complex with CoA in Serratia marcescens FS14. Structural analysis revealed that the PgbP dimer has two cavities, each of which binds a CoA molecule via conserved motifs of the GNAT family. In addition, the biochemical studies showed that PgbP is a prodigiosin-binding protein with high thermal stability. To our knowledge, this is the first view of GNAT binding to secondary metabolites and it is also the first report of prodigiosin binding protein. Molecular docking and mutation experiments indicated that prodigiosin binds to the substrate binding site of PgbP. The structure-function analyses presented here broaden our understanding of the multifunctionality of GNAT family members and may infer the mechanism of the multiple biological activities of prodigiosin.

Post-translational Lysine Ac(et)ylation in Bacteria: A Biochemical, Structural, and Synthetic Biological Perspective

Ac(et)ylation is a post-translational modification present in all domains of life. First identified in mammals in histones to regulate RNA synthesis, today it is known that is regulates fundamental cellular processes also in bacteria: transcription, translation, metabolism, cell motility. Ac(et)ylation can occur at the ε-amino group of lysine side chains or at the α-amino group of a protein. Furthermore small molecules such as polyamines and antibiotics can be acetylated and deacetylated enzymatically at amino groups. While much research focused on N-(ε)-ac(et)ylation of lysine side chains, much less is known about the occurrence, the regulation and the physiological roles on N-(α)-ac(et)ylation of protein amino termini in bacteria. Lysine ac(et)ylation was shown to affect protein function by various mechanisms ranging from quenching of the positive charge, increasing the lysine side chains’ size affecting the protein surface complementarity, increasing the hydrophobicity and by interfering with other post-translational modifications. While N-(ε)-lysine ac(et)ylation was shown to be reversible, dynamically regulated by lysine acetyltransferases and lysine deacetylases, for N-(α)-ac(et)ylation only N-terminal acetyltransferases were identified and so far no deacetylases were discovered neither in bacteria nor in mammals. To this end, N-terminal ac(et)ylation is regarded as being irreversible. Besides enzymatic ac(et)ylation, recent data showed that ac(et)ylation of lysine side chains and of the proteins N-termini can also occur non-enzymatically by the high-energy molecules acetyl-coenzyme A and acetyl-phosphate. Acetyl-phosphate is supposed to be the key molecule that drives non-enzymatic ac(et)ylation in bacteria. Non-enzymatic ac(et)ylation can occur site-specifically with both, the protein primary sequence and the three dimensional structure affecting its efficiency. Ac(et)ylation is tightly controlled by the cellular metabolic state as acetyltransferases use ac(et)yl-CoA as donor molecule for the ac(et)ylation and sirtuin deacetylases use NAD⁺ as co-substrate for the deac(et)ylation. Moreover, the accumulation of ac(et)yl-CoA and acetyl-phosphate is dependent on the cellular metabolic state. This constitutes a feedback control mechanism as activities of many metabolic enzymes were shown to be regulated by lysine ac(et)ylation. Our knowledge on lysine ac(et)ylation significantly increased in the last decade predominantly due to the huge methodological advances that were made in fields such as mass-spectrometry, structural biology and synthetic biology. This also includes the identification of additional acylations occurring on lysine side chains with supposedly different regulatory potential. This review highlights recent advances in the research field. Our knowledge on enzymatic regulation of lysine ac(et)ylation will be summarized with a special focus on structural and mechanistic characterization of the enzymes, the mechanisms underlying non-enzymatic/chemical ac(et)ylation are explained, recent technological progress in the field are presented and selected examples highlighting the important physiological roles of lysine ac(et)ylation are summarized.

Protein acetyltransferases mediate bacterial adaptation to a diverse environment

Protein lysine acetylation is a conserved post-translational modification that modulates several cellular processes. Protein acetylation and its physiological implications are well understood in eukaryotes; however, its role is emerging in bacteria. Lysine acetylation in bacteria is fine-tuned by the concerted action of lysine acetyltransferases (KATs), protein deacetylases (KDACs), metabolic intermediates- acetyl-coenzyme A (Ac-CoA) and acetyl phosphate (AcP). AcP mediated nonenzymatic acetylation is predominant in bacteria due to its high acetyl transfer potential whereas, enzymatic acetylation by bacterial KATs (bKAT) are considered less abundant. SePat, the first bKAT discovered in Salmonella enterica, regulates the activity of the central metabolic enzyme- acetyl-CoA synthetase, through its acetylation. Recent studies have highlighted the role of bKATs in stress responses like pH tolerance, nutrient stress, persister cell formation, antibiotic resistance and pathogenesis. Bacterial genomes encode many putative bKATs of unknown biological function and significance. Detailed characterization of putative and partially characterized bKATs is important to decipher the acetylation mediated regulation in bacteria. Proper synthesis of information about the diverse roles of bKATs is missing to date, which can lead to the discovery of new antimicrobial targets in future. In this review, we provide an overview of the diverse physiological roles of known bKATs, and their mode of regulation in different bacteria. We also highlight existing gaps in the literature and present questions that may help understand the regulatory mechanisms mediated by bKATs in adaptation to a diverse habitat.

Small-Molecule Acetylation by GCN5-Related N-Acetyltransferases in Bacteria

Acetylation is a conserved modification used to regulate a variety of cellular pathways, such as gene expression, protein synthesis, detoxification, and virulence. Acetyltransferase enzymes transfer an acetyl moiety, usually from acetyl coenzyme A (AcCoA), onto a target substrate, thereby modulating activity or stability. Members of the GCN5- N -acetyltransferase (GNAT) protein superfamily are found in all domains of life and are characterized by a core structural domain architecture. These enzymes can modify primary amines of small molecules or of lysyl residues of proteins. From the initial discovery of antibiotic acetylation, GNATs have been shown to modify a myriad of small-molecule substrates, including tRNAs, polyamines, cell wall components, and other toxins. This review focuses on the literature on small-molecule substrates of GNATs in bacteria, including structural examples, to understand ligand binding and catalysis. Understanding the plethora and versatility of substrates helps frame the role of acetylation within the larger context of bacterial cellular physiology.

Structure of the full-length Serratia marcescens acetyltransferase AAC(3)-Ia in complex with coenzyme A

Acyl-coenzyme A-dependent N-acetyltransferases (AACs) catalyze the modification of aminoglycosides rendering the bacteria carrying such enzymes resistant to this class of antibiotics. Here we present the crystal structure of AAC(3)-Ia enzyme from Serratia marcescens in complex with coenzyme A determined to 1.8 Å resolution. This enzyme served as an architype for the AAC enzymes targeting the amino group at Position 3 of aminoglycoside main aminocyclitol ring. The structure of this enzyme has been previously determined only in truncated form and was interpreted as distinct from subsequently characterized AACs. The reason for the unusual arrangement of secondary structure elements of AAC(3)-Ia was not further investigated. By determining the full-length structure of AAC(3)-Ia we establish that this enzyme adopts the canonical AAC fold conserved across this family and it does not undergo through significant rearrangement of secondary structure elements upon ligand binding as was proposed previously. In addition, our results suggest that the C-terminal tail in AAC(3)-Ia monomer forms intramolecular hydrogen bonds that contributes to formation of stable dimer, representing the predominant oligomeric state for this enzyme.

Insights into the Function of the N-Acetyltransferase SatA That Detoxifies Streptothricin in Bacillus subtilis and Bacillus anthracis

Acylation of epsilon amino groups of lysyl side chains is a widespread modification of proteins and small molecules in cells of all three domains of life. Recently, we showed that Bacillus subtilis and Bacillus anthracis encode the GCN5-related N-acetyltransferase (GNAT) SatA that can acetylate and inactivate streptothricin, which is a broad-spectrum antibiotic produced by actinomycetes in the soil. To determine functionally relevant residues of B. subtilis SatA (BsSatA), a mutational screen was performed, highlighting the importance of a conserved area near the C terminus. Upon inspection of the crystal structure of the B. anthracis Ames SatA (BaSatA; PDB entry 3PP9), this area appears to form a pocket with multiple conserved aromatic residues; we hypothesized this region contains the streptothricin-binding site. Chemical and site-directed mutagenesis was used to introduce missense mutations into satA, and the functionality of the variants was assessed using a heterologous host (Salmonella enterica). Results of isothermal titration calorimetry experiments showed that residue Y164 of BaSatA was important for binding streptothricin. Results of size exclusion chromatography analyses showed that residue D160 was important for dimerization. Together, these data advance our understanding of how SatA interacts with streptothricin.IMPORTANCE This work provides insights into how an abundant antibiotic found in soil is bound to the enzyme that inactivates it. This work identifies residues for the binding of the antibiotic and probes the contributions of substituting side chains for those in the native protein, providing information regarding hydrophobicity, size, and flexibility of the antibiotic binding site.

Encoding of Promiscuity in an Aminoglycoside Acetyltransferase

Aminoglycoside antibiotics are a large family of antibiotics that can be divided into two distinct classes on the basis of the substitution pattern of the central deoxystreptamine ring. Although aminoglycosides are chemically, structurally, and topologically diverse, some aminoglycoside-modifying enzymes (AGMEs) are able to inactivate as many as 15 aminoglycosides from the two main classes, the kanamycin- and neomycin-based antibiotics. Here, we present the crystal structure of a promiscuous AGME, aminoglycoside- N3-acetyltransferase-IIIb (AAC-IIIb), in the apo form, in binary drug (sisomicin, neomycin, and paromomycin) and coenzyme A (CoASH) complexes, and in the ternary neomycin-CoASH complex. These data provide a structural framework for interpretation of the thermodynamics of enzyme-ligand interactions and the role of solvent in the recognition of ligands. In combination with the recent structure of an AGME that does not have broad substrate specificity, these structures allow for the direct determination of how antibiotic promiscuity is encoded in some AGMEs.

Aminoglycosides: An Overview

Aminoglycosides are natural or semisynthetic antibiotics derived from actinomycetes. They were among the first antibiotics to be introduced for routine clinical use and several examples have been approved for use in humans. They found widespread use as first-line agents in the early days of antimicrobial chemotherapy, but were eventually replaced in the 1980s with cephalosporins, carbapenems, and fluoroquinolones. Aminoglycosides synergize with a variety of other antibacterial classes, which, in combination with the continued increase in the rise of multidrug-resistant bacteria and the potential to improve the safety and efficacy of the class through optimized dosing regimens, has led to a renewed interest in these broad-spectrum and rapidly bactericidal antibacterials.

Bacterial GCN5-Related N-Acetyltransferases: From Resistance to Regulation

The GCN5-related N-acetyltransferases family (GNAT) is an important family of proteins that includes more than 100000 members among eukaryotes and prokaryotes. Acetylation appears as a major regulatory post-translational modification and is as widespread as phosphorylation. N-Acetyltransferases transfer an acetyl group from acetyl-CoA to a large array of substrates, from small molecules such as aminoglycoside antibiotics to macromolecules. Acetylation of proteins can occur at two different positions, either at the amino-terminal end (αN-acetylation) or at the ε-amino group (εN-acetylation) of an internal lysine residue. GNAT members have been classified into different groups on the basis of their substrate specificity, and in spite of a very low primary sequence identity, GNAT proteins display a common and conserved fold. This Current Topic reviews the different classes of bacterial GNAT proteins, their functions, their structural characteristics, and their mechanism of action.

Enzymatic Adenylylation of Streptomycin and Spectinomycin by R-Factor-Resistant Escherichia coli

A resistance (R) factor- containing strain of Escherichia coli which is known to inactivate streptomycin by adenylylation has been shown to be spectinomycin resistant. An osmotic shockate of this strain catalyzes the formation of the biologically inactive spectinomycin adenylate, in which the adenylyl residue is probably attached to a d-threo methylamino alcohol moiety of spectinomycin. Both the streptomycin and spectinomycin adenylylating activities show the same temperature inactivation profile, and both are present in a protein fraction purified for the streptomycin inactivating enzyme. Mutants obtained from this strain which were sensitive to either spectinomycin or streptomycin were shown to lack both enzymatic activities when tested in vitro. Revertants of these mutants selected for recovery of either streptomycin resistance or spectinomycin resistance regain both activities. Therefore, we conclude that the inactivation of the two drugs is catalyzed by the same enzyme. Examination of a number of R factor-carrying strains has shown that those strains which are resistant to streptomycin and spectinomycin contain the adenylylating enzyme, whereas strains resistant to streptomycin but sensitive to spectinomycin inactivate streptomycin by phosphorylation.

Kinetic and structural analysis of bisubstrate inhibition of the Salmonella enterica aminoglycoside 6'-N-acetyltransferase

Aminoglycosides are antibacterial compounds that act by binding to the A site of the small 30S bacterial ribosomal subunit and inhibiting protein translation. Clinical resistance to aminoglycosides is generally the result of the expression of enzymes that covalently modify the antibiotic, including phosphorylation, adenylylation, and acetylation. Bisubstrate analogs for the aminoglycoside N-acetyltransferases are nanomolar inhibitors of Enterococcus faecium AAC(6')-Ii. However, in the case of the Salmonella enterica aac(6')-Iy-encoded aminoglycoside N-acetyltransferase, we demonstrate that a series of bisubstrate analogs are only micromolar inhibitors. In contrast to studies with AAC(6')-Ii, the inhibition constants toward AAC(6')-Iy are essentially independent of both the identity of the aminoglycoside component of the bisubstrate and the number of carbon atoms that are used to link the CoA and aminoglycoside components. The patterns of inhibition suggest that the CoA portion of the bisubstrate analog can bind to the enzyme-aminoglycoside substrate complex and that the aminoglycoside portion can bind to the enzyme-CoA product complex. However, at the high concentrations of bisubstrate analog used in crystallization experiments, we could crystallize and solve the three-dimensional structure of the enzyme-bisubstrate complex. The structure reveals that both the CoA and aminoglycoside portions bind in essentially the same positions as those previously observed for the enzyme-CoA-ribostamycin complex, with only a modest adjustment to accommodate the "linker". These results are compared to previous studies of the interaction of similar bisubstrate analogs with other aminoglycoside N-acetyltransferases.

A new electro-optical approach to rapid assay of cell viability

A new electro-optical (EO) approach was developed and applied to rapidly assay cell viability by using phage M13K07. Since phage M13K07 can replicate only in living bacteria and cannot replicate in the presence of inhibitors, the difference between the EO signals obtained in the presence and absence of the phage can be used as an important factor for evaluating cell viability. Variation in the electrophysical parameters of Escherichia coli XL-1 during its interaction with phage M13K07 was studied under exposure of the cells to various inhibitors of cellular metabolism. Significant changes in the EO signal were found during incubation of living E. coli cells with phage M13K07. At the same time, no changes were recorded during cell incubation with the phage after pretreatment of E. coli XL-1 cells with sodium azide, carbonyl cyanide 3-chlorophenyl hydrazone, chloramphenicol, and kanamycin. This finding can be explained by the decrease in the number of living cells in the culture after preliminary incubation with the chemical agents, and it was confirmed by colony counts by conventional plating onto solid LB medium before and after treatment of the cells with the inhibitors. The EO approach can be used as a rapid method for evaluation of the inhibitory effects of various chemical agents and drugs, and it has the potential for the study of the molecular mechanisms underlying cell death.

Pleiotropic antibiotic resistance mutations associated with ribosomes and ribosomal subunits in Mycobacterium smegmatis

Viomycin-resistant strains isolated from Mycobacterium smegmatis demonstrated pleiotropic resistance to tuberactinomycin-N, capreomycin, streptomycin, and kanamycin as a result of mutational alteration of ribosomes, even though they were selected for resistance to a single antibiotic. The pleiotropic drug resistance of three mutants isolated by stepwise selection for resistance to viomycin was due to alteration of the 30S ribosomal subunit. One mutant, strain A, isolated independently by multiple-step selection to viomycin resistance, was resistant to viomycin, tuberactinomycin-N, and capreomycin through an alteration of the 50S ribosomal subunit, whereas it was sensitive to kanamycin but resistant to streptomycin through an alteration of the 30S ribosomal subunit. Three streptomycin-resistant strains, which were isolated by one-step selection at a high concentration of streptomycin, did not show significant co-resistance to any other antibiotics tested in culture and cell-free systems; streptomycin resistance in these mutants was localized on the 30S ribosomal subunit.

Aminoglycoside-modifying enzymes: mechanisms of catalytic processes and inhibition

The most prevalent mechanism for resistance to aminoglycoside antibiotics is mediated through their enzymatic modification in resistant organisms. Dozens of aminoglycoside-modifying enzymes are known at the gene sequence level, but few have been characterized in the details of their mechanism. This review summarizes the state of knowledge of the best studied of these enzymes, focusing on their catalytic mechanisms and inhibition.

Semisynthetic aminoglycoside antibiotics: Development and enzymatic modifications

The critical resistance mechanisms of aminoglycoside antibiotics in bacteria of clinical importance are the enzymatic N-acetylation, O-phosphorylation, and O-nucleotidylation that generally result in the inactivation of aminoglycosides. To overcome such resistance mechanisms, dibekacin (3',4'-dideoxykanamycin B) was developed as the first rationally designed semisynthetic aminoglycoside, based on the enzymatic 3'-O-phosphorylation of kanamycin. Subsequently, amikacin, netilmicin, and isepamicin were developed by introducing (S)-4-amino-2-hydroxybutyryl (AHB), ethyl, and (S)-3-amino-2-hydroxypropionyl side chains into the 1-amino group of kanamycin, sisomicin, and gentamicin B, respectively. These side chains are believed to block the access of a variety of aminoglycoside-modifying enzymes to their target sites. The latest semisynthetic aminoglycoside of clinical use in Japan is arbekacin (1-N-AHB-dibekacin), which has been extensively used since its approval as an anti-methicillin-resistant Staphylococcus aureus (MRSA) agent in 1990. Although it has several possible modification sites for aminoglycoside acetyltransferases (AACs), arbekacin-resistant MRSA strains that have emerged in the past 8 years have been those with a low or moderate level of resistance, due to a bifunctional enzyme, AAC(6')/APH(2"), at low incidence. To overcome AAC(6')/APH(2")-dependent arbekacin-resistant MRSA strains, 2"-amino-2"-deoxyarbekacin and its 5-epiamino derivative have been already synthesized. However, simulative modification studies using AACs from aminoglycoside-producing Streptomyces strains have revealed that AAC(3) and AAC(2') converted arbekacin to 3"-N-acetyl and 2'-N-acetyl derivatives, respectively, which retain high antibiotic activity. By contrast, the same acetylations of amikacin (3"-N-) and dibekacin (3-N-) resulted in their inactivation. Thus, these new findings confirmed the steric hindrance effect of the 1-N-acyl side chain and illuminated the novel aspect of arbekacin distinct from the other semisynthetic aminoglycosides, indicating that MRSA strains cannot be arbekacin-resistant even if they have acquired the aac(3) or aac(2') gene.

Bacterial resistance to aminoglycoside antibiotics

The aminoglycoside antibiotics are broad-spectrum antibacterial compounds that are used extensively for the treatment of many bacterial infections. In view of the current concerns over the global rise in antibiotic-resistant microorganisms, there has been renewed interest in the mechanisms of resistance to the aminoglycosides, including the superfamily of aminoglycoside-modifying enzymes.

Biosafety of kanamycin-resistant transgenic plants

Kanamycin resistance is one of the most frequently used selection markers for obtaining transgenic plants. The introduction of these transgenic plants into agricultural practice will cause the kanamycin resistance gene and the gene product to be present on a large scale. The desirability of this situation is analysed. The nature, properties and applications of the antibiotic kanamycin are briefly reviewed, as are the mechanisms of kanamycin resistance. It is argued that the gene used for resistance is an excellent choice because of the high substrate specificity of the enzyme encoded. Human or veterinary antibiotic therapies will not be compromised. Also, the physico-chemical characteristics of the antibiotic exclude the existence of selective conditions in the environment. Therefore, a transgenic plant or any other organism that might have acquired the gene will not get any selective advantage because of this gene. Evidence further suggests there is no toxicity or predictable harm of both gene or gene product for human or animal consumption. Full legislative clearance of this transgenic trait is therefore acceptable.

Nosocomial spread of an amikacin resistance gene on both a mobilized, nonconjugative plasmid and a conjugative plasmid

Resistance to amikacin among members of the family Enterobacteriaceae at a hospital in Venezuela rose from 2% in 1979 to 5% in 1984 and 10% in 1985 as amikacin usage rose 20-fold to exceed gentamicin usage. Resistance to gentamicin remained at 25 to 27%. We examined the plasmids from 21 isolates obtained in 1984 and 1985. Nine of eleven in 1984 and three of ten in 1985 carried aacA and sul on a 3.8-kb BamHI fragment of pBWH300, a 10.4-kb nonconjugative plasmid that had been mobilized into strains of six species by at least two different coresident conjugative plasmids. Six 1985 isolates of two species carried these genes on a similar BamHI fragment of the 104-kb conjugative plasmid pBWH303. One isolate in 1984 and one in 1985 carried the 69-kb conjugative plasmid pBWH301, which had aacA as the promoter-proximal gene of an operon that also encompassed the cat and aadB resistance genes. Another conjugative plasmid, pBWH302, was found in a single isolate. It carried a different aacA allele on the functional transposon Tn654, which appeared to be closely related to Tn1331, a transposon previously isolated in Argentina and Chile. Increased selection may thus have led to dissemination of an endemic aacA allele on two endemic plasmids, one spread by mobilization, with occasional intrusion of additional aacA alleles from outside.

Transient expression of chimeric CAT genes injected into early embryos of the domesticated silkworm Bombyx mori

In order to establish a transient expression system for genes introduced into early embryos of the silkworm, Bombyx mori, we tested various promoters ligated with CAT reporter genes. The embryos into which we injected supercoiled plasmid DNA of pFb(-860/+10)CAT containing the Bombyx fibroin promoter region ligated to the CAT gene showed a reasonably high CAT activity beginning around 30 h after oviposition. This high activity was observed only when the plasmid was injected before termination of the early nuclear cleavage stage, which was about 8 h after oviposition, but not after this stage. This means that the expression of injected DNA is closely related to the presence of cleavage nuclei in early embryos. Promoters originating from insect genes, like the Bombyx sericin-1 gene, Drosophila hsp70 and Drosophila copia LTR, functioned as strong promoters in the embryos. On the contrary, promoters from mammalian virus genes, such as the SV40 early and Moloney murine leukemia virus LTR genes, functioned as weak promoters. Moreover, linearized DNAs showed no or weak activity of expression in embryos. From these results, we conclude that the silkworm embryo transient expression system is a useful tool for studying the mechanism of regulation of insect genes.

Clinical isolate of a porinless Salmonella typhi resistant to high levels of chloramphenicol

We studied a clinical isolate of Salmonella typhi (strain 1895) characterized by resistance to 200 micrograms of chloramphenicol per ml despite the absence of chloramphenicol-inactivating activity. The outer membrane protein profile analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated a deficiency of one of the major protein species which may serve as a porin for entry of chloramphenicol. When the strain was transformed with a plasmid encoding chloramphenicol acetyltransferase, chloramphenicol added to the culture was not inactivated, suggesting a drastic reduction of permeability towards the drug. Moreover, transformants bearing a plasmid coding for the Escherichia coli OmpF porin became considerably more susceptible to chloramphenicol (40 micrograms/ml). On the other hand, transformants carrying a plasmid encoding the Salmonella typhi ompC gene remained as resistant to the drug as the parental strain, even though they overexpressed OmpC. These findings indicate that the lack of OmpF plays a major role in the resistance to chloramphenicol in strain 1895.

Metabolism of drugs and other xenobiotics in the gut lumen and wall

Metabolism in the gut lumen and wall can decrease the bioavailability and the pharmacological effects of a wide variety of drugs. Bacterial flora in the gut, the environmental pH and oxidative or conjugative enzymes present in the intestinal epithelial cells can all contribute to the process. Bacterial biotransformation is greatest in the colon, while gut wall metabolism is generally highest in the jejunum and decreases distally. Gut wall metabolism may be induced or inhibited by dietary or environmental xenobiotics or by co-administered drugs. Recent evidence suggests that some drugs, food-derived mutagens and other xenobiotics can be metabolized by gut flora and/or gut wall enzymes to reactive species which may cause tumors.

Mechanism of resistance to benzalkonium chloride by Pseudomonas aeruginosa

The mechanisms of resistance of Pseudomonas aeruginosa to benzalkonium chloride (BC) were studied. The effluence of cell components was observed in susceptible P. aeruginosa by electron microscopy, but resistant P. aeruginosa seemed to be undamaged. No marked changes in cell surface potential between Escherichia coli NIHJC-2 and a spheroplast strain were found. The contents of phospholipids (PL) and fatty and neutral lipids (FNL) in the cell walls of resistant P. aeruginosa were higher than those in the cell walls of susceptible P. aeruginosa. The amounts of BC adsorbed to PL and FNL of cell walls of BC-resistant P. aeruginosa were lower than those for BC-susceptible P. aeruginosa. Fifteen species of cellular fatty acids were identified by capillary gas chromatography and gas chromatography-mass spectrometry. The ability of BC to permeate the cell wall was reduced because of the increase in cellular fatty acids. These results suggested that the resistance of P. aeruginosa to BC is mainly a result of increased in the contents of PL and FNL. In resistant P. aeruginosa, the decrease in the amount of BC adsorbed is likely to be the result of increases in the contents of PL and FNL.

Heterogeneity of 6'-N-acetyltransferases of type 4 conferring resistance to amikacin and related aminoglycosides in members of the family Enterobacteriaceae

DNA-DNA hybridization and immunoblotting were used to assess the relatedness between the 6'-N-acetyltransferases of type 4 encoded by plasmid pAZ007 from a clinical isolate of Serratia marcescens and those encoded by NR79 and R5. The absence of detectable DNA-DNA homology and of immunological cross-reactivity suggests the existence of at least two distinct 6'-N-acetyltransferase type 4 genes that mediate amikacin resistance in gram-negative bacilli.

Tobramycin in ophthalmology

Selman Waksman's laboratory at Rutgers University discovered the first aminoglycoside antibiotic, streptomycin, in 1943. Other aminoglycoside antibiotics, such as gentamicin and tobramycin, soon followed. Tobramycin is compatible with most intravenous fluids and tear substitutes, but it is incompatible with heparin and some beta-lactam antibiotics such as penicillin and cephalosporins. Due to tobramycin's broad spectrum of activity, it has proven useful in controlling both superficial and deep infections of the eye and ocular adnexa (i.e., blepharitis, conjunctivitis, keratitis, and endophthalmitis). However, since tobramycin has been associated with neuromuscular blockade, as well as possessing ototoxic and nephrotoxic effects, care must be taken to minimize toxicity by monitoring patients undergoing systemic tobramycin therapy.

Survey of modifying enzymes and plasmids in amikacin-resistant Serratia marcescens

Forty amikacin-resistant strains of Serratia marcescens isolated from four different hospitals (A, B, C, and D) were examined for modifying enzymes and plasmids. Twenty-one of the isolates produced acetyltransferase that modified amikacin. Eighteen of the 21 acetyltransferase-bearing isolates were from different inpatients in hospital A and the other three were from hospital C. Amikacin resistance was mediated by conjugative plasmid of 24 megadaltons in 15 of the 18 acetyltransferase-bearing isolates of hospital A and by nonconjugative plasmids, derivatives of the 24-megadalton plasmids, in the remaining three isolates of the same hospital. The 24-megadalton plasmid determined aminoglycoside acetyltransferase (6') IV. This plasmid-borne enzyme conferred amikacin resistance on S. marcescens but not on Escherichia coli K12. The frequency of transfer of the 24-megadalton plasmid from the S. marcescens isolate to E. coli K12 by conjugation was approximately 10(-7) (transconjugants/donors) and was 0.1% of that between E. coli strains. In acetyltransferase-bearing isolates from hospital C, the enzyme was mediated by a nonconjugative plasmid in one case and could not be associated with a plasmid in the remaining two cases. Neither enzymes nor plasmids could be associated with amikacin resistance of the isolates of the other two hospitals.

Development of plasmid-mediated resistance in Vibrio cholerae during treatment with trimethoprim-sulfamethoxazole

The persistence of Vibrio cholerae, biotype el tor, in a patient treated with trimethoprim-sulfamethoxazole was due to the acquisition of a conjugative resistance plasmid. The plasmid, with a molecular size of 72 megadaltons, belonged to incompatibility group 6-C and conferred resistance to ampicillin, chloramphenicol, sulfonamide, and trimethoprim.

Purification of two forms of kanamycin acetyltransferase from Escherichia coli

Kanamycin acetyltransferase acylates aminoglycoside antibiotics using acetyl-CoA, and thereby conveys bacterial resistance to several clinically important antibiotics, notably amikacin. The enzyme was quantitatively and reproducibly released from Escherichia coli W677 harboring plasmid pMH67 by a modified osmotic shock procedure (bacterial cells are incubated overnight in sucrose and again without sucrose before onset of osmotic shock). The enzyme was purified by dye-ligand chromatography on Affi-Gel Blue in addition to antibiotic affinity chromatography on neomycin-Sepharose-4B. The activity did not increase with subsequent chromatography on ion-exchange, hydrophobic, or molecular-exclusion gels. However, both dye-ligand and molecular-exclusion chromatography, as well as disc-gel electrophoresis, separated the purified enzyme equally into two active protein fractions. Based on the more active of the two forms, the purification was 112-fold with a specific activity of 1.9 IU/mg. The less-active form has an unusual absorbance spectrum, with a maximum near 255 nm, which cannot be explained by the amino acid composition. Chromatography of this form alone regenerated both forms, suggesting that the enzyme is noncovalently conjugated to an uncharged chromophore, such as a lipid. The purified enzyme has a very sharp pH optimum at 5.5 with a plateau on the alkaline side, but is most stable between pH 8.5 and 9.5. Data from electrophoresis in the presence of sodium dodecyl sulfate and gel-filtration on Ultrogel AcA 44 are consistent with a tetrameric protein of 60-70,000 Da.

Evolutionary relationships among genes for antibiotic resistance

The genes that determine resistance to antibiotics are commonly found encoded by extrachromosomal elements in bacteria. These were described first in Enterobacteriaceae and subsequently in a variety of other genera; their spread is associated with the increased use of antibiotics in human and animal medicine. Antibiotic-resistance genes that determine the production of enzymes which modify (detoxify) the antibiotics have been detected in antibiotic-producing organisms. It has been suggested that the producing strains provided the source of antibiotic-resistance genes that were then 'picked-up' by recombination. Recent studies of the nucleotide sequence of certain antibiotic-resistance genes indicate regions of strong homology in the encoded proteins. The implications of these similarities are discussed.

Chloramphenicol resistance mutation in Escherichia coli which maps in the major ribosomal protein gene cluster

Localized mutagenesis and selection for streptomycin resistance were utilized to isolate a chloramphenicol resistance mutation in Escherichia coli K-12 linked to the strA (rpsL) locus. Bacteriophage P1 transduction verified the map position of the new resistance mutation at 72 min, placing it within a dense cluster of ribosomal protein genes. The map position differs from that of known cmlA and cmlB mutations, which map at 18 and 21 min, respectively. Ribosomes prepared from chloramphenicol-resistant and -sensitive isogenic transductants were analyzed in vitro for activity in formation of N-formylmethionyl-puromycin, polyphenylalanine, and polylysine in the presence of inhibitory concentrations of chloramphenicol. Comparisons were also made of 14C-chloramphenicol binding to 70S ribosomes and of the two-dimensional polyacrylamide gel electrophoresis pattern of ribosomal proteins from each strain. There was no detectable difference between ribosomes from sensitive and resistant strains as measured by these assays. Enzymatic modification by chloramphenicol acetyltransferase is not responsible for the observed phenotype.

Resistance mechanism of chloramphenicol in Streptococcus haemolyticus, Streptococcus pneumoniae and Streptococcus faecalis

The chloramphenicol resistance of Streptococcus haemolyticus, Streptococcus pneumoniae and Streptococcus faecalis isolated from clinical materials was proved to be due to an inactivating enzyme produced by these bacteria. The inactivated products of chloramphenicol were identified as 1-acetoxy, 3-acetoxy and 1,3-diacetoxy derivatives by thin-layer chromatography and infrared spectroscopy. The responsible enzyme was thus confirmed to be chloramphenicol acetyltransferase. The enzyme was inducible. It was partially purified by ammonium sulfate precipitation, DEAE-cellulose chromatography and gel filtration on Sephadex G-150. The enzymes obtained from S. haemolyticus, S. pneumoniae and S. faecalis have been compared with the conclusion that they are identical with respect to molecular weight (approximately 75,000-80,000), optimum pH and heat stability.

Evidence that the multi-drug-resistance of Streptococcus faecalis BIO-4R is not carried by a plasmid

The multi-drug-resistant strain Streptococcus faecalis BIO-4R was studied to see if it carried a plasmid responsible for antibiotic resistance. From results indicating that the antibiotic resistance of S. faecalis BIO-4R was not transferred to recipient bacteria, that the organism did not produce enzymes which inactivated antibiotics, and that the presence of covalently closed circular DNA was not demonstrated by dye-cesium chloride buoyant density gradient centrifugation, it was concluded that the organism did not carry such a plasmid. Studies on polyphenylalanine synthesis by cell-free extracts of the oganism showed that its ribosomes were resistant to chloramphenicol, tetracycline, streptomycin and kanamycin. These results, although rather indirectly, support the above notion that the multi-drug-resistance of S. faecalis BIO-4R is not mediated by a plasmid.

Colcemid sensitivity of fission yeast: permeability and detoxification properties of resistant mutants

Mutants of Schizosaccharomyces pombe, which were resistant to concentrations of colcemid inhibitory to the wild-type parent, were not diminished in their permeability to colcemid or colchicine. They also did not modify colcemid to a detectable derivative or to a product that was incapable of binding to brain tubulin. The resistance of these mutants was therefore via mechanisms different from those of permeability and detoxification known for animal cells.

Thiogalactoside transacetylase of the lactose operon as an enzyme for detoxification

Thigalactoside transacetylase, the lacA gene product, confers selective advantage to cells of Escherichia coli K-12 growing on beta-galactosides in the presence of non-metabolizable analogues.