New class of competitive inhibitor of bacterial histidine kinases

Bacterial histidine kinases have been proposed as targets for the discovery of new antibiotics, yet few specific inhibitors of bacterial histidine kinases have been reported. We report here a novel thienopyridine (TEP) compound that inhibits bacterial histidine kinases competitively with respect to ATP but does not comparably inhibit mammalian serine/threonine kinases. Although it partitions into membranes and does not inhibit the growth of bacterial or mammalian cells, TEP could serve as a starting compound for a new class of histidine kinase inhibitors with antibacterial activity.

Kinetic and mechanistic analyses of new classes of inhibitors of two-component signal transduction systems using a coupled assay containing HpkA-DrrA from Thermotoga maritima

Two-component signal transduction systems (TCSs) play fundamental roles in bacterial survival and pathogenesis and have been proposed as targets for the development of novel classes of antibiotics. A new coupled assay was developed and applied to analyse the kinetic mechanisms of three new kinds of inhibitors of TCS function. The assay exploits the biochemical properties of the cognate HpkA-DrrA histidine kinase-response regulator pair from Thermotoga maritima and allows multiple turnovers of HpkA, linear formation of phosphorylated DrrA, and Michaelis-Menten analysis of inhibitors. The assay was validated in several ways, including confirmation of competitive inhibition by adenosine 5'-beta,gamma-imidotriphosphate (AMP-PNP). The coupled assay, autophosphorylation and chemical cross-linking were used to determine the mechanisms by which several compounds inhibit TCS function. A cyanoacetoacetamide showed non-competitive inhibition with respect to ATP concentration in the coupled assay. The cyanoacetoacetamide also inhibited autophosphorylation of histidine kinases from other bacteria, indicating that the coupled assay could detect general inhibitors of histidine kinase function. Inhibition of HpkA autophosphorylation by this compound was probably caused by aggregation of HpkA, consistent with a previous model for other hydrophobic compounds. In contrast, ethodin was a potent inhibitor of the combined assay, did not inhibit HpkA autophosphorylation, but still led to aggregation of HpkA. These data suggest that ethodin bound to the HpkA kinase and inhibited transfer of the phosphoryl group to DrrA. A peptide corresponding to the phosphorylation site of DrrA appeared to inhibit TCS function by a mechanism similar to that of ethodin, except that autophosphorylation was inhibited at high peptide concentrations. The latter mechanism of inhibition of TCS function is unusual and its analysis demonstrates the utility of these approaches to the kinetic analyses of additional new classes of inhibitors of TCS function.

Novel lipoglycopeptides as inhibitors of bacterial signal peptidase I

Signal peptidase (SPase) I is responsible for the cleavage of signal peptides of many secreted proteins in bacteria. Because of its unique physiological and biochemical properties, it serves as a potential target for development of novel antibacterial agents. In this study, we report the production, isolation, and structure determination of a family of structurally related novel lipoglycopeptides from a Streptomyces sp. as inhibitors of SPase I. Detailed spectroscopic analyses, including MS and NMR, revealed that these lipoglycopeptides share a common 14-membered cyclic peptide core, an acyclic tripeptide chain, and a deoxy-alpha-mannose sugar, but differ in the degree of oxidation of the N-methylphenylglycine residue and the length and branching of the fatty acyl chain. Biochemical analysis demonstrated that these peptides are potent and competitive inhibitors of SPase I with K(i) 50 to 158 nm. In addition, they showed modest antibacterial activity against a panel of pathogenic Gram-positive and Gram-negative bacteria with minimal inhibitory concentration of 8-64 microm against Streptococcus pneumonniae and 4-8 microm against Escherichia coli. Notably, they mechanistically blocked the protein secretion in whole cells as demonstrated by inhibiting beta-lactamase release from Staphylococcus aureus. Taken together, the present discovery of a family of novel lipoglycopeptides as potent inhibitors of bacterial SPase I may lead to the development of a novel class of broad-spectrum antibiotics.

Glycopeptide carboxamides active against vancomycin-resistant enterococci

Glycopeptide antibiotics were synthesized via the PyBOP mediated condensation of aliphatic, heterocyclic and aromatic amines with the C-terminus of vancomycin, LY264826 (A82846B) and semi-synthetic derivatives of these natural products. Amides of LY264826 and vancomycin demonstrated excellent activity against staphylococci and streptococci as compared to the parent natural product. However, the amides of N-alkylated LY264826 and N-alkylated vancomycin were active against vancomycin-resistant enterococci as well as other gram-positive pathogens such as Staphylococcus aureus, S. haemolyticus, S. epidermidis and Streptococcus pneumoniae.

Mechanism of action of oritavancin and related glycopeptide antibiotics

Oritavancin (LY333328) is a semisynthetic glycopeptide antibiotic having excellent bactericidal activity against glycopeptide-susceptible and -resistant Gram-positive bacteria. Oritavancin is the N-alkyl-p-chlorophenylbenzyl derivative of chloroeremomycin (LY264826) and is currently in phase III clinical trials for use in Gram-positive infections. Studies show that oritavancin and related alkyl glycopeptides inhibit bacterial cell wall formation by blocking the transglycosylation step in peptidoglycan biosynthesis in a substrate-dependent manner. As with other glycopeptide antibiotics, including vancomycin, the effects of oritavancin on cell wall synthesis are attributable to interactions with dipeptidyl residues of peptidoglycan precursors. Unlike vancomycin, however, oritavancin is strongly dimerized and can anchor to the cytoplasmic membrane, the latter facilitated by its alkyl side chain. Cooperative interactions derived from dimerization and membrane anchoring in situ can be of sufficient strength to enable binding to either dipeptidyl or didepsipeptidyl peptidoglycan residues of vancomycin-susceptible and -resistant enterococci, respectively. This review describes the antibacterial activity of oritavancin, and examines the evidence supporting the proposed mechanism of action for this agent and related analogs.

Vancomycin tolerance induced by erythromycin but not by loss of vncRS, vex3, or pep27 function in Streptococcus pneumoniae

Vancomycin-tolerant Streptococcus pneumoniae is a growing problem among drug-resistant human pathogens. Some vancomycin-tolerant pneumococci have been reported to carry mutations in loci encoding a two-component regulatory system designated VncRS or in a proximal ABC transporter, Vex. A model was advanced proposing that the tolerance phenotype resulted from the inability of a vncS mutant to respond to the Vex-transported Pep27 "death peptide" signal and dephosphorylate VncR, thereby preventing relief of repression of autolytic and other cell death functions in response to antibiotics. To explore this hypothesis, we constructed mutations in vncS, vncR, vex3, and pep27 in S. pneumoniae strain R6 and two additional genetic backgrounds. The lytic responses of the isogenic DeltavncS, Deltavex3, DeltavncR, and Deltapep27 mutants, but not a DeltalytA strain, to vancomycin were indistinguishable from that of the parent strain. DeltavncS strains also failed to exhibit tolerance to vancomycin at various doses in multiple media and showed wild-type sensitivity to other classes of autolysis-inducing antibiotics. In contrast, addition of subinhibitory levels of the antibiotic erythromycin led to tolerance to vancomycin during late, but not early, exponential-phase growth in a DeltavncS strain, in the parent strain R6, and in two other strains bearing erythromycin resistance markers, namely, a DeltavncR strain and an unrelated DeltacomD strain that is defective in competence-quorum sensing. Thus, this tolerance effect resulted from changes in cell growth or other erythromycin-dependent phenomena and not inactivation of vncS per se. Consistent with these results, and in contrast to a previous report, we found that a synthetic form of Pep27 did not elicit lytic or nonlytic killing of pneumococci. Finally, microarray transcriptional analysis and beta-galactosidase reporter assays revealed VncS-dependent regulation of the vex123 gene cluster but did not support a role for VncRS in the regulation of autolytic or other putative cell death loci. Based on these findings, we propose that vancomycin tolerance in S. pneumoniae does not result from loss of vncS function alone.

Genome of the bacterium Streptococcus pneumoniae strain R6

Streptococcus pneumoniae is among the most significant causes of bacterial disease in humans. Here we report the 2,038,615-bp genomic sequence of the gram-positive bacterium S. pneumoniae R6. Because the R6 strain is avirulent and, more importantly, because it is readily transformed with DNA from homologous species and many heterologous species, it is the principal platform for investigation of the biology of this important pathogen. It is also used as a primary vehicle for genomics-based development of antibiotics for gram-positive bacteria. In our analysis of the genome, we identified a large number of new uncharacterized genes predicted to encode proteins that either reside on the surface of the cell or are secreted. Among those proteins there may be new targets for vaccine and antibiotic development.

Induction of VanA vancomycin resistance genes in Enterococcus faecalis: use of a promoter fusion to evaluate glycopeptide and nonglycopeptide induction signals

To characterize induction of VanA resistance a plasmid was constructed in which the gene for firefly luciferase lucA was placed under the control of the promoter for the VanA resistance genes, the vanH promoter. This system afforded convenient quantitative measurement of induction of the VanA genes. Glycopeptide antibiotics and antibiotics representing 19 different mechanisms of action were evaluated for their ability to induce. Antibiotics that acted as inducers were all inhibitors of late steps of peptidoglycan synthesis. These included moenomycin, bacitracin, tunicamycin, ramoplanin and glycopeptides, but not penicillin or other beta-lactam antibiotics. Glycopeptide antibiotics were the most potent inducers. Both glycopeptides with little or no antimicrobial activity and semisynthetic glycopeptides active against VanA resistant enterococci were inducers. Overall, results suggest that an induction response may involve both an internal signal, such as precursor accumulation, and the glycopeptide molecule itself as a signal. The system may be useful as a screen for new antimicrobial agents.

Gene disruption studies of penicillin-binding proteins 1a, 1b, and 2a in Streptococcus pneumoniae

The effects of inactivation of the genes encoding penicillin-binding protein 1a (PBP1a), PBP1b, and PBP2a in Streptococcus pneumoniae were examined. Insertional mutants did not exhibit detectable changes in growth rate or morphology, although a pbp1a pbp1b double-disruption mutant grew more slowly than its parent did. Attempts to generate a pbp1a pbp2a double-disruption mutant failed. The pbp2a mutants, but not the other mutants, were more sensitive to moenomycin, a transglycosylase inhibitor. These observations suggest that individually the pbp1a, pbp1b, and pbp2a genes are dispensable but that either pbp1a or pbp2a is required for growth in vitro. These results also suggest that PBP2a is a functional transglycosylase in S. pneumoniae.

Enzymatic deacylation of teicoplanin followed by reductive alkylation: synthesis and antibacterial activity of new glycopeptides

Novel glycopeptides derived from teicoplanin were prepared and evaluated for activity against antibiotic-resistant gram-positive pathogens. Removal of the fatty acid sidechains of teicoplanin was accomplished by enzymatic deacylation. The resulting deacylated teicoplanin was subjected to reductive alkylation resulting in mono- and di-alkylated compounds at the 2 possible primary amines. Deacylated teicoplanin was less active than teicoplanin against enterococci and staphylococci (MIC > or =32 microg/ml). All mono- and di-alkylated products regained some activity, and some had potent activity against both staphylococci and glycopeptide-resistant enterococci. MICs of the most potent di-alkylated compounds ranged from 0.25 approximately 2 microg/ml against glycopeptide-resistant enterococci.

Novel glycopeptide antibiotics: N-alkylated derivatives active against vancomycin-resistant enterococci

LY264826 (A82846B) is a naturally-occurring glycopeptide antibiotic, differing from vancomycin in the stereochemistry of the amino-sugar of the disaccharide function, and the presence of a third sugar attached at the benzylic position of amino acid residue 6. Despite these seemingly subtle differences, LY264826 is approximately 10 times more active than vancomycin against the enterococci. In the pursuit of new antibiotics active against multiresistant Gram-positive organisms, an extensive side chain SAR was developed focusing on the reductive alkylation of LY264826 at the amino function of the disaccharide moiety. A new series of derivatives having varying degrees of structural diversity in the side chain (e.g. varying lengths and degrees of rigidity) was found to have potent activity against vancomycin-resistant enterococci (MIC's < 1.0 microgram/ml) as well as activity against staphylococci and streptococci as good or better than vancomycin.

Beyond vancomycin: new therapies to meet the challenge of glycopeptide resistance

The incidence of infections caused by resistant Gram-positive pathogens is increasing, while emergence of vancomycin resistance is reducing the number of therapeutic options. New agents are being rapidly evaluated as candidates to replace vancomycin; some of the most promising include semisynthetic glycopeptides, quinupristin-dalfopristin, oxazolidinones and everninomycins. Alternative strategies, including immunization and therapeutic vaccines, may also have a role.

Inhibition of peptidoglycan biosynthesis in vancomycin-susceptible and -resistant bacteria by a semisynthetic glycopeptide antibiotic

LY191145 is a p-chlorobenzyl derivative of LY264826 (A82846B) with activity against both vancomycin-susceptible and -resistant enterococci. Incorporation of L-[14C]lysine into peptidoglycan of intact vancomycin-susceptible and -resistant Enterococcus faecium was inhibited by LY191145 (50% inhibitory concentrations of 1 and 5 microgram/ml, respectively). Inhibition was accompanied by accumulation of UDP-muramyl-peptide precursors in the cytoplasm. This agent inhibited late-stage steps in peptidoglycan biosynthesis in permeabilized E. faecium when either the UDP-muramyl-pentapeptide precursor from vancomycin-susceptible E. faecium or the UDP-muramyl-pentadepsipeptide precursor from vancomycin-resistant E. faecium was used as a substrate. Inhibition of late-stage steps led to accumulation of an N-acetyl-[14C]glucosamine-labeled lipid intermediate indicative of inhibition of the transglycosylation step. Inhibition of peptidoglycan polymerization without affecting cross-linking in a particulate membrane-plus-wall-fragment assay from Aerococcus viridans was consistent with this explanation. The fact that inhibition of peptidoglycan biosynthesis by LY191145 was not readily antagonized by an excess of free acyl-D-alanyl-D-alanine or acyl-D-alanyl-D-lactate ligands indicates that the manner in which this compound inhibits transglycosylation may not be identical to that of vancomycin.

Semisynthetic glycopeptide antibiotics derived from LY264826 active against vancomycin-resistant enterococci

Certain derivatives of the glycopeptide antibiotic LY264826 with N-alkyl-linked substitutions on the epivancosamine sugar are active against glycopeptide-resistant enterococci. Six compounds representing our most active series were evaluated for activity against antibiotic-resistant, gram-positive pathogens. For Enterococcus faecium and E. faecalis resistant to both vancomycin and teicoplanin, the MICs of the six semisynthetic compounds for 90% of the strains tested were 1 to 4 micrograms/ml, compared with 2,048 micrograms/ml for vancomycin and 256 micrograms/ml for LY264826. For E. faecium and E. faecalis resistant to vancomycin but not teicoplanin, the MICs were 0.016 to 1 micrograms/ml, compared with 64 to 1,024 micrograms/ml for vancomycin. The compounds were highly active against vancomycin-susceptible enterococci and against E. gallinarum and E. casseliflavus and showed some activity against isolates of highly vancomycin-resistant leuconostocs and pediococci. The MICs for 90% of the strains of methicillin-resistant Staphylococcus aureus tested were typically 0.25 to 1 micrograms/ml, compared with 1 microgram/ml for vancomycin. Against methicillin-resistant S. epidermidis MICs ranged from 0.25 to 2 micrograms/ml, compared with 1 to 4 micrograms/ml for vancomycin and 4 to 16 micrograms/ml for teicoplanin. The spectrum of these new compounds included activity against teicoplanin-resistant, coagulase-negative staphylococci. The compounds exhibited exceptional potency against pathogenic streptococci, with MICs of < or = 0.008 microgram/ml against Streptococcus pneumoniae, including penicillin-resistant isolates. In in vivo studies with a mouse infection model, the median effective doses against a challenge by S. aureus, S. pneumoniae, or S. pyogenes were typically 4 to 20 times lower than those of vancomycin. Overall, these new glycopeptides, such as LY307599 and LY333328, show promise for use as agents against resistant enterococci, methicillin-resistant S. aureus, and penicillin-resistant pneumococci.

Solid-Phase Total Synthesis of Bacitracin A

An efficient solid-phase method for the total synthesis of bacitracin A is reported. This work was undertaken in order to provide a general means of probing the intriguing mode of action of the bacitracins and exploring their potential for use against emerging drug-resistant pathogens. The synthetic approach to bacitracin A involves three key features: (1) linkage to the solid support through the side chain of the L-asparaginyl residue at position 12 (L-Asn(12)), (2) cyclization through amide bond formation between the alpha-carboxyl of L-Asn(12) and the side chain amino group of L-Lys(8), and (3) postcyclization addition of the N-terminal thiazoline dipeptide as a single unit. To initiate the synthesis, Fmoc L-Asp(OH)-OAllyl was attached to a PAL resin. The chain of bacitracin A was elaborated in the C-to-N direction by sequential piperidine deprotection/HBTU-mediated coupling cycles with Fmoc D-Asp(OtBu)-OH, Fmoc L-His(Trt)-OH, Fmoc D-Phe-OH, Fmoc L-Ile-OH, Fmoc D-Orn(Boc)-OH, Fmoc L-Lys(Aloc)-OH, Fmoc L-Ile-OH, Fmoc D-Glu(OtBu)-OH, and Fmoc L-Leu-OH. The allyl ester and allyl carbamate protecting groups of L-Asn(12) and L-Lys(8), respectively, were simultaneously and selectively removed by treating the peptide-resin with palladium tetrakis(triphenylphosphine), acetic acid, and triethylamine. Cyclization was effected by PyBOP/HOBT under the pseudo high-dilution conditions afforded by attachment to the solid support. After removal of the N-terminal Fmoc group, the cyclized peptide was coupled with 2-[1'(S)-(tert-butyloxycarbonylamino)-2'(R)-methylbutyl]-4(R)-carboxy-Delta(2)-thiazoline (1). The synthetic peptide was deprotected and cleaved from the solid support under acidic conditions and then purified by reverse-phase HPLC. The synthetic material exhibited an ion in the FAB-MS at m/z 1422.7, consistent with the molecular weight calculated for the parent ion of bacitracin A (MH(+) = C(73)H(84)N(10)O(23)Cl(2), 1422.7 g/mol). It was also indistinguishable from authentic bacitracin A by high-field (1)H NMR and displayed antibacterial activity equal to that of the natural product, thus confirming its identity as bacitracin A. The overall yield for the solid-phase synthesis was 24%.

Reductive alkylation of glycopeptide antibiotics: synthesis and antibacterial activity

Reductive alkylation of the A82846 family of glycopeptide antibiotics has the potential of producing seven products. N-Alkylation of the disaccharide amino function can be accomplished selectively, and offers the greatest increase in antibacterial activity. Products resulting from N-alkylation of LY264826 (A82846B) provide the most potent derivatives as compared to other members of this class of antibiotics. Two of these derivatives, LY307599 and LY333328 are approximately 500 times more active than vancomycin against vancomycin-resistant enterococci.

Activities of the semisynthetic glycopeptide LY191145 against vancomycin-resistant enterococci and other gram-positive bacteria

LY191145 is the prototype of a series of compounds with activities against vancomycin-resistant enterococci derived by modification of the glycopeptide antibiotic LY264826. LY191145 had MICs for vancomycin- and teicoplanin-resistant enterococci of < or = 4 micrograms/ml for 50% of isolates and < or = 16 micrograms/ml for 90% of isolates. Its MICs for vancomycin-resistant, teicoplanin-susceptible enterococci were 1 to 8 micrograms/ml. LY191145 retains the potent activities of its parent compound against staphylococci and streptococci. In vivo studies in a mouse infection model confirmed these activities. This compound indicates the potential of semisynthetic glycopeptides as agents against antibiotic-resistant gram-positive bacteria.

beta-Lactamase inhibitors derived from N-tosyloxy-beta-lactams

Electrophilic N-tosyloxy-beta-lactams, N-tosyloxy-4-phenyl-2-azetidinone (2b) and N-tosyloxy-3-(S)-phthalimido-4-(S)-2-azetidinone (2c), are described. These agents are novel potent beta-lactamase inhibitors.

In-vitro synergy and mechanism of interaction between vancomycin and ciprofloxacin against enterococcal isolates

In-vitro synergy between vancomycin and ciprofloxacin against 44 enterococcal isolates was studied. Synergy occurred in chequerboard MIC determinations with six Enterococcus faecium strains resistant to both vancomycin and ciprofloxacin. The combination was additive for strains susceptible to one or both antibiotics. Time-kill studies involving selected strains with different susceptibility patterns confirmed the chequerboard results. The effect of ciprofloxacin on the induction of vancomycin resistance was compared in two vancomycin-resistant strains of E. faecium. Sub-inhibitory concentrations of ciprofloxacin prevented induction of vancomycin resistance in a ciprofloxacin-resistant strain, but not in a ciprofloxacin-susceptible strain. Membranes isolated from vancomycin-resistant ciprofloxacin-resistant cultures grown with vancomycin and ciprofloxacin at < or = 8 mg/L (0.125 x MIC) expressed a 39.5-kDa membrane protein involved in the expression of vancomycin resistance, but the protein was not detected in membranes from cultures grown in ciprofloxacin 16 mg/L. These findings indicated that a vancomycin-ciprofloxacin combination can be synergic against enterococci resistant to both vancomycin and ciprofloxacin, but would be unlikely to offer any advantage in the treatment of enterococcal infections because of the high concentrations required.

Inhibitors of two-component signal transduction systems: inhibition of alginate gene activation in Pseudomonas aeruginosa

Pseudomonas aeruginosa strains infecting cystic fibrosis patients often produce copious amounts of the exopolysaccharide alginate. Expression of alginate genes in P. aeruginosa is regulated by several proteins including members of the two-component bacterial signal transduction systems. Two of these regulatory proteins are AlgR1, the DNA-binding response regulator that transcriptionally activates alginate gene expression, and AlgR2, the kinase that modifies AlgR1 via phosphorylation to enhance its activity. In this paper, we report the identification of compounds that inhibit alginate gene expression by inhibiting (i) the phosphorylation/dephosphorylation of AlgR2 and (ii) the DNA-binding activity of AlgR1. Compounds with these activities may have potential as components of therapy for eliminating P. aeruginosa infection from the cystic fibrosis lung. In addition, we describe the effect of these compounds on the autophosphorylation activity of other known two-component kinases and show the ability of one compound to significantly inhibit the kinase activities of CheA, NRII, and KinA.

Modes of action and inhibitory activities of new siderophore-beta-lactam conjugates that use specific iron uptake pathways for entry into bacteria

We describe here the mechanism of inhibition of two new siderophore-beta-lactam conjugates against Escherichia coli X580. One conjugate is a spermidine-based catechol siderophore-carbacephalosporin (JAM-2-263), and the other is an N5-acetyl-N5-hydroxy-L-ornithine tripeptide hydroxamate siderophore-carbacephalosporin (EKD-3-88). In an agar diffusion test, both conjugates produced large inhibitory zones against strain X580. Resistant strains (i.e., JAMR and EKDR) could be isolated after exposure of X580 to the conjugates JAM-2-263 and EKD-3-88, respectively. No cross-resistance was observed in these individual isolates. JAMR and EKDR were studied further to elucidate the mechanism of inhibition of each conjugated drug. The affinities of JAM-2-263 and EKD-3-88 for penicillin-binding proteins (PBPs) of isolated inner membranes were determined by a competition assay with 125I-penicillin V. JAM-2-263 targeted primarily PBPs 1A/B and 5/6, while EKD-3-88 targeted PBPs 1A/B and 3. Strains X580, JAMR, and EKDR showed similar PBP affinities for the conjugates. However, marked changes were observed in the iron-regulated outer membrane proteins of resistant isolates grown on agar plates depleted of iron. EKDR lost the expression of FhuA (78 kDa) and its sensitivity to phages T1 and T5, whereas JAMR lost the expression of Cir (74 kDa) and its sensitivity to colicin Ia. These results revealed the requirement of FhuA and Cir for the inhibitory activities of EKD-3-88 and JAM-2-263, respectively. In an antibiotic diffusion assay, ferrichrome (1 microM) strongly antagonized the activities of both conjugates against X580 and JAMR, including the residual activity of JAM-2-263 against JAMR. However, the susceptibility of strain EKDR lacking the ferrichrome receptor (FhuA-) to the two conjugates remained the same in the presence of ferrichrome. The antagonistic effect of ferrichrome on the activity of JAM-2-263 may also indicate a role for FhuA in the activity of this beta-lactam conjugate. A FhuA- Cir- double mutant confirmed this hypothesis, since it showed a higher level of resistance to JAM-2-263. To reproduce iron-restricted in vivo growth conditions, we grew X580 and EKDR cells in diffusion chambers implanted in the peritoneal cavities of rats. Strain EKDR showed impaired growth in such a cultivation system. This is the first report of beta-lactam drug transport into E. coli cells that involves the FhuA outer membrane protein.

Activity of glycopeptides against vancomycin-resistant gram-positive bacteria

Gram-positive bacteria resistant to vancomycin are rare; but they include members of the genera Leuconostoc, Lactobacillus, and Pediococcus, as well as recently emerging vancomycin-resistant strains of Enterococcus faecium and Enterococcus faecalis. Vancomycin, teicoplanin, and several vancomycin derivatives were tested for their activities against vancomycin-resistant gram-positive bacteria. Vancomycin-resistant E. faecium and E. faecalis were generally cross-resistant to other glycopeptides, but some N-substituted vancomycin derivatives were active against the resistant strains, with MICs of 2 to 32 micrograms/ml. These vancomycin derivatives also had significant levels of activity against intrinsically vancomycin-resistant organisms such as Leuconostoc sp. While vancomycin resistance in E. faecium and E. faecalis was inducible, resistance in members of the genera Leuconostoc, Lactobacillus, and Pediococcus appeared to be expressed constitutively. Antibody to a vancomycin-induced membrane protein found in membranes of resistant enterococci did not detect a cross-reacting protein in other vancomycin-resistant species.

Characterization of vancomycin resistance in Enterococcus faecium and Enterococcus faecalis

Vancomycin resistance in Enterococcus faecium 180, a clinical isolate from England, was studied. Resistance to vancomycin was transferable by conjugation to other enterococci. Expression of resistance was inducible and coincided with the appearance of a new membrane protein.

Cloning and characterization of elastase genes from Pseudomonas aeruginosa

A gene bank was constructed from Pseudomonas aeruginosa PAO1 and used to complement three P. aeruginosa elastase-deficient strains. One clone, pRF1, contained a gene which restored elastase production in two P. aeruginosa isolates deficient in elastase production (PA-E15 and PAO-E105). This gene also encoded production of elastase antigen and activity in Escherichia coli and is the structural gene for Pseudomonas elastase. A second clone, pHN13, contained a 20-kilobase (kb) EcoRI insert which was not related to the 8-kb EcoRI insert of pRF1 as determined by restriction analysis and DNA hybridization. A 2.2-kb SalI-HindIII fragment from pHN3 was subcloned into pUC18, forming pRB1822-1. Plasmid pRB1822-1 restored normal elastolytic activity to PAO-E64, a mutant for elastase activity. Clones derived from pHN13 failed to elicit elastase antigen or enzymatic activity in E. coli.

Expression in Escherichia coli and function of Pseudomonas aeruginosa outer membrane porin protein F

The gene encoding porin protein F of Pseudomonas aeruginosa was cloned onto a cosmid vector into Escherichia coli. Protein F was expressed as the predominant outer membrane protein in a porin-deficient E. coli background and was clearly visible on one-dimensional sodium dodecyl sulfate-polyacrylamide gels in a porin-sufficient background. The identity of the protein F from the E. coli clone and native P. aeruginosa protein F was demonstrated by their identical mobilities on sodium dodecyl sulfate-polyacrylamide gel electrophoretograms, 2-mercaptoethanol modifiabilities, and reactivities with monoclonal antibodies specific of two separate epitopes of protein F. In the course of gene subcloning, a 2-kilobase DNA fragment was isolated, with an apparent truncation of the part of the gene encoding the carboxy terminus of protein F. This subclone produced a 24,000-molecular-weight, outer membrane-associated, truncated protein F derivative which was not 2-mercaptoethanol modifiable and which reacted with only one of the two classes of protein F-specific monoclonal antibodies. The 2-kilobase fragment was used in Southern blot hybridizations to construct a restriction map of the cloned and subcloned fragments and to demonstrate with restriction digests of whole P. aeruginosa DNA that only one copy of the protein F gene was present in the P. aeruginosa chromosome. The protein F produced by the original cosmid clone in a porin-deficient E. coli background was purified. To demonstrate retention of porin function after cloning, the protein F from the E. coli clone was incorporated into black lipid bilayer membranes. Two major classes of channels were revealed. The predominant class of channels had an average conductance of 0.36 nS in 1 M KCl, whereas larger channels (4 to 7 nS) were seen at a lower frequency. Similar channel sizes were observed for porin protein F purified by the same method from P. aeruginosa outer membranes.

Production of elastase and other exoproducts by environmental isolates of Pseudomonas aeruginosa

Pseudomonas aeruginosa isolates from environmental sources and bacteremic patients were compared for their levels of elastolytic activity. No significant differences were found. The incidence of production of toxin A, phospholipase C, alkaline protease, and elastase among the environmental strains was also as high as that previously reported for clinical isolates.

The role of exoenzyme S in infections with Pseudomonas aeruginosa

To define the contribution of exoenzyme S to the pathogenesis of infections with Pseudomonas aeruginosa, we have compared the ability of an exoenzyme S-deficient mutant, 388 exs1::Tn1, and that of its exoenzyme S-producing parent to colonize and disseminate in burned mice infected with this organism. Both the exoenzyme S-deficient mutant and the parent strain proliferated in burned skin, but only the parent strain was able to effectively disseminate to blood and other tissues. The reduced ability of the mutant to disseminate was not due to alterations in serum sensitivity, lipopolysaccharide composition, or motility. The exoenzyme S-deficient mutant was able to disseminate in the presence of the exoenzyme S-producing parent. Antibody to purified exoenzyme S was able to greatly reduce dissemination of the exoenzyme S-producing parent strain but did not prevent colonization in the burned skin. These data suggest that exoenzyme S does not contribute to the initial colonization but does contribute to the establishment of disseminated infection.

The contribution of exoproducts to virulence of Pseudomonas aeruginosa

Pseudomonas aeruginosa produces a large number of extracellular products which may contribute to its virulence. We have employed a genetic approach to determine the contribution of toxin A, exoenzyme S, elastase and alkaline protease to the pathogenesis of P. aeruginosa. Mutations have been introduced with chemicals or transposons. Mutants have been identified using immunological, chemical, or toxicity assays. Mutants were extensively characterized in vitro to ascertain that they were identical to their parent strain except for the production of the desired product. Appropriate mutants were compared with their parent strains in several animal models: the burned mouse model, the mouse corneal infection model, and a rat model of chronic lung infection. The data indicate that virulence of P. aeruginosa is multifactorial. Further, the relative contribution of a given P. aeruginosa product may vary with the type of infection.

Role of exoenzyme S in chronic Pseudomonas aeruginosa lung infections

Exoenzyme S is an extracellular ADP-ribosyltransferase enzyme produced by Pseudomonas aeruginosa. Mutants of Pseudomonas aeruginosa deficient in this enzyme have been shown to have reduced virulence in infections of burned mice. The contribution of exoenzyme S to the pathogenesis of chronic lung infections with this organism was evaluated by examining the incidence of exoenzyme S production by Pseudomonas aeruginosa strains isolated from cystic fibrosis patients and comparing an exoenzyme S deficient mutant and its exoenzyme S producing parent in a rat chronic lung infection model. Of 51 isolates examined, 43% produced detectable levels of exoenzyme S. While both the exoenzyme S deficient mutant and its parent strain were equally capable of colonizing and persisting in rat lungs, the exoenzyme S producing parent elicited a greater degree of lung damage. These data suggest that exoenzyme S contributes to the pathogenesis of chronic lung infections.

Contribution of exoenzyme S to the virulence of Pseudomonas aeruginosa

Exoenzyme S is an extracellular ADPR transferase produced by P. aeruginosa. Forms of this enzyme that have thus far been purified are not toxic, however, exoenzyme S clearly contributes to the virulence of strain 388. Thus, a Tn1 mutant deficient in exoenzyme S was found to be markedly less virulent than its exoenzyme S-producing parental strain in both a burned mouse infection model and a rat chronic lung infection model. Exoenzyme S does not appear to contribute to initial colonization of the rat lung or the burned mouse skin. Exoenzyme S does, however, appear to contribute to local tissue damage in the rat lung, and to dissemination of P. aeruginosa from the skin into the blood and distant organs of the burned mouse. Perhaps our most important observation is that specific antibody against exoenzyme S confers a high level of protection against subsequent infection of burned mice. While these results must be extended to include additional strains they are encouraging, and they underscore the relative importance of exoenzyme S in the pathogenesis of P. aeruginosa infections.

Genetic approaches to study Pseudomonas aeruginosa protein antigens

Pseudomonas aeruginosa produces a large number of extracellular products which may play a role in pathogenesis. We have used genetic techniques to elucidate the relative contribution of these proteins to virulence, and as a method of producing safe toxoids. A mutant has been isolated which produces an immunologically reactive nontoxic form of toxin A, the most toxic extracellular protein produced by P. aeruginosa. Although there are difficulties in production of sufficient quantities of this CRM toxoid, these are likely to be solved by further genetic manipulation. Protection studies with toxin A antibody and studies of mutants deficient in toxin A have confirmed that toxin A plays a role in pathogenesis while clearly showing that toxin A alone cannot totally account for the virulence of P. aeruginosa. Studies of mutants specifically altered in three other products, exoenzyme S, and the two major proteases of P. aeruginosa, elastase and alkaline protease, have clarified the contribution of these products to virulence. Demonstration by genetic studies that exoenzyme S was a major factor in the virulence for one P. aeruginosa strain allowed us to correctly predict that antibody to this product would be protective against infection with that strain.

Isolation and characterization of transposon-induced mutants of Pseudomonas aeruginosa deficient in production of exoenzyme S

Exoenzyme S is an extracellular product of Pseudomonas aeruginosa. This enzyme catalyzes the transfer of ADP-ribose from NAD to a number of as yet unidentified eucaryotic proteins, but it is distinct from toxin A. To evaluate the role of exoenzyme S in the pathogenicity of P. aeruginosa, we isolated transposon-induced mutants of strain 388, a clinical isolate that produces exoenzyme S but no toxin A. The transposon Tn1 was introduced by using a temperature-sensitive derivative of plasmid RP1. A Tn1-induced mutant was found which had no detectable exoenzyme S activity or antigen in culture supernatants or in cell lysates. Except for its lack of exoenzyme S and resistance to carbenicillin, this mutant was indistinguishable from the parent strain. When tested in an experimental mouse burn infection model, this Tn1-induced mutant was reduced in virulence by at least 2,000-fold, suggesting a role for exoenzyme S in the virulence of this strain.

Alteration of susceptibility to EDTA, polymyxin B and gentamicin in Pseudomonas aeruginosa by divalent cation regulation of outer membrane protein H1

Induction of outer membrane protein H1 in Pseudomonas aeruginosa results in decreased susceptibility to aminoglycosides, polymyxin B, and EDTA. We have previously shown that protein H1 can become the major cellular protein in cells grown in low (0.02 mM) Mg2+. The induction of protein H1 was prevented by supplementation of low Mg2+ medium with Mg2+, Ca2+, Mn2+, or Sr2+ (each at 0.5 mM), but not with Zn2+, Ba2+, Sn2+, Al3+ or Na+ (each at 0.5 mM). Only cells grown in the presence of those cations which failed to prevent H1 induction were resistant to the cationic antibiotics, polymyxin B and gentamicin, and to chelators of divalent cations. Cells grown in Ca2+, but not in Mg2+, were susceptible to outer membrane permeabilization by the Ca2+ specific chelator EGTA, whereas both were susceptible to EDTA. In agreement with this, cells grown in Mg2+, Ca2+, Mn2+, or Zn2+ showed enhanced levels of these cations respectively as their major cell envelope-associated cation. When cells were shifted from low to high Mg2+ medium, the time course of the decrease in the levels of protein H1 correlated well with the increase in sensitivity to EDTA and polymyxin B. These results support the hypothesis that protein H1 acts to replace divalent cations at a critical outer membrane site attacked by cationic antibiotics and chelators of divalent cations, and suggest that only a small proportion of the total divalent cation-binding sites in the outer membrane are susceptible to attack by these agents.

Pseudomonas aeruginosa outer membrane permeability: isolation of a porin protein F-deficient mutant

A mutant of Pseudomonas aeruginosa severely deficient in outer membrane protein F levels was isolated by screening heavily mutagenized strains for membrane protein alterations on sodium dodecyl sulphate-polyacrylamide gel electrophoresis. To provide a basis for phenotypic comparison, three independent spontaneous revertants with normal protein F levels were isolated. Neither the protein F-deficient mutant nor its revertants had gross surface alterations as judged by their sensitivities to 31 phages with diverse receptors and their low degrees of leakage of periplasmic beta-lactamase into the supernatant. Outer membrane permeability was measured in whole cells by examining the rates of hydrolysis of a chromogenic beta-lactam, nitrocefin, by periplasmic RP1-encoded beta-lactamase. It was found that the outer membrane permeabilities of wild-type and protein F revertant strains were similar, but low when compared with those of Escherichia coli and an antibiotic-supersusceptible mutant Z61 of P. aeruginosa. The loss of protein F caused a further significant decrease in outer membrane permeability. The results suggest that protein F is a pore-forming protein in vivo and that only a small proportion, as few as 1 in 400, of the protein F molecules form active functional channels in vivo.

Outer membrane proteins of Pseudomonas aeruginosa serotype strains

The basis of differentiation of Pseudomonas aeruginosa into the 17 serotypes of the International Antigenic Typing Scheme is differences in an outer membrane glycolipid, lipopolysaccharide (LPS). This observation, together with the high toxicity and pyrogenicity of LPS, has led to the search for alternative "common" antigens for use as vaccines. The relation between the major outer membrane proteins of serotype strains was studied in three ways. By demonstrating conservation of outer membrane protein receptors for bacteriophages, a high similarity of outer membrane protein patterns on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gels, and antigenic cross-reactivity of major outer membrane proteins, it was shown that the major outer membrane proteins were closely related. Radioiodinated antibodies to outer membrane proteins interacted with outer membrane proteins after SDS-PAGE separation and electrophoretic blotting of the separated outer membrane proteins into nitrocellulose paper. This demonstrated that major outer membrane proteins F, H2, and I were antigenically related in all serotype strains.

Involvement of the outer membrane in gentamicin and streptomycin uptake and killing in Pseudomonas aeruginosa

Induction of a major outer membrane protein, H1, in Pseudomonas aeruginosa resulted in decreased susceptibility to gentamicin and streptomycin. Mutants which overproduce protein H1 and cells in which H1 is induced in response to growth conditions had altered kinetics of uptake and killing. It was further demonstrated that gentamicin and streptomycin interact with the outer membrane to permeabilize it to lysozyme and to increase the permeation of a chromogenic beta-lactam, nitrocefin. Experiments with inhibitors of aminoglycoside uptake showed that uptake was not required to increase permeability. Mg2+ at 1 mM totally inhibited aminoglycoside-mediated outer membrane permeabilization. We propose that the uptake and killing by these aminoglycosides requires interaction with an Mg2+ binding site at the outer membrane, permitting aminoglycoside uptake into the periplasm.

Outer membrane protein H1 of Pseudomonas aeruginosa: involvement in adaptive and mutational resistance to ethylenediaminetetraacetate, polymyxin B, and gentamicin

It is well established that Pseudomonas aeruginosa cells grown in Mg2+-deficient medium acquire nonmutational resistance to the chelator ethylenediaminetetraacetate and to the cationic antibiotic polymyxin B; this type of resistance can be reversed by transferring the cells to Mg2+-sufficient medium for a few generations. Stable mutants resistant to polymyxin B were isolated and shown to have also gained ethylenediaminetetraacetate resistance. Both the mutants and strains grown on Mg2+-deficient medium had greatly enhanced levels of outer membrane protein H1 when compared with the wild-type strain or with revertants grown in Mg2+-sufficient medium. It was determined that in all strains and at all medium Mg2+ concentrations, the cell envelope Mg2+ concentration varied inversely with the amount of protein H1. In addition, the increase in protein H1 in the mutants was associated with an increase in resistance to another group of cationic antibiotics, the aminoglycosides, e.g., gentamicin. We propose that protein H1 acts by replacing Mg2+ at a site on the lipopolysaccharide which can otherwise be attacked by the cationic antibiotics or ethylenediaminetetraacetate.

Relationship between gentamicin susceptiblity criteria and therapeutic serum levels for Pseudomonas aeruginosa in mouse infection model

In this study estimations of in vivo and in vitro gentamicin susceptibility for a series of strains of Pseudomonas aeruginosa were compared. The series included an extremely susceptible strain, typically susceptible strains by current susceptibility criteria, and strains with enzymatic and permeability-mediated resistance. In vivo testing was done by using a mice protection test involving six 1-h doses of gentamicin and an inoculum of 50 50% lethal doses of P. aeruginosa. Both normal mice and cyclophosphamide-treated mice were used. It was found that peak serum levels and serum levels of gentamicin obtained just prior to the sixth dose (fifth dose trough levels) required for protection were much higher than minimal inhibitory concentrations (MICs) or minimal bactericidal concentrations (MBCs) obtained in high-cation medium. However, first dose trough levels were similar to MICs or MBCs. Only an extremely susceptible strain, 280, could be treated at antibiotic dosages and serum levels which are considered likely to be safe in humans. A distinct inoculum effect was found in the mice tests, with a 10-fold increase in inoculum producing a 4-fold increase in the amount of gentamicin required, but no inoculum effect was found for MICs. These results suggest that current susceptibility criteria in use for gentamicin and P. aeruginosa overestimate gentamicin susceptibility, particularly when low-cation growth medium is used for susceptibility testing and when treating disseminated infection.