Mechanism of action

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Chapter 12 Antimicrobial Drugs

Mechanism of action of antibacterial agents

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Hooper; Mechanisms of Action of Negative interactions adderall lexapro Five bacterial targets have been exploited in the development of antimicrobial drugs: Because resistance to drugs that interact with these targets is widespread, new antimicrobials and an understanding of their mechanisms of action are vital.

Ternary complexes of drug, enzyme, and DNA block progress of the replication fork. Cytotoxicity of fluoroquinolones is likely a 2-step process involving 1 conversion of the topoisomerase-quinolone-DNA complex to an irreversible form and 2 generation of a double-strand break by denaturation of the topoisomerase.

The molecular factors necessary for the transition from step 1 to step 2 remain unclear, but downstream pathways for cell death may overlap with those used by other bactericidal antimicrobials. Studies of fluoroquinolone-resistant mutants and purified topoisomerases indicate that many quinolones have differing activities against the two targets. Drugs with similar activities against both targets may prove less likely to select de novo resistance. Development of antimicrobials for clinical use has been most successful in targeting essential components of 5 general areas of bacterial metabolism: It is mechanism of action of antibacterial agents the scope of this discussion to cover each of these areas in detail.

Instead, I focus on some recent developments in novel inhibitors that target dual steps in cell wall synthesis, in understanding the pathways by which antimicrobial agents of diverse types may effect cell killing, and in understanding the interactions of quinolones with their dual targets and the consequences of these interactions.

On the basis of the number of antimicrobials in clinical use, bacterial cell wall synthesis has been perhaps the target area most extensively exploited for antimicrobial development, although bacterial protein synthesis may be a close second.

The components of the cell wall synthesis machinery are appealing antimicrobial targets because of the absence of counterparts in human biology, thereby providing intrinsic target selectivity.

The sequential late steps in cell wall synthesis include the cytoplasmic synthesis of building blocks composed of N -acetyl muramic acid M linked to N -acetyl glucosamine G with an attached pentapeptide P side chain referred to as MGP subunits.

Linkage of an MGP subunit to a lipid II molecule allows subsequent translocation across the cytoplasmic membrane to the cell exterior or periplasmic space. Transglycosylase enzymes then assemble the MGP subunits into a linear backbone by catalyzing glycosidic linkages between the M and G components of the MGP subunits.

Linearly linked MGP subunits constitute an immature peptidoglycan structure. Transpeptidase enzymes then act to cross-link the peptide side chains with pentaglycine bridges, cleaving the terminal 2 D-alanines of the peptide side chain in the process, thereby producing the mature, lattice-like peptidoglycan that provides the bacterium with its shape and osmotic stability.

Because of the sequential nature of these steps in some bacteria such as Escherichia coli [ 1 ], it is possible to determine the site in the synthesis pathway at which an antimicrobial acts by measuring accumulation of precursors and blocks in development of immature and mature peptidoglycan.

Vancomycin is a glycopeptide that is known to bind tightly to the terminal 2 D-alanines of the peptide side chain of the immature peptidoglycan [ 4 ]. By use of measurements of the sites of inhibition of late cell wall synthesis, recent analyses of vancomycin analogs with modified carbohydrate moieties have identified unexpected additional target interactions that appear to account for the activity of these analogs against vancomycin-resistant bacteria [ 5 ].

These analogs, mechanism of action of antibacterial agents, containing either a chlorobiphenyl or an n -decyl substituent linked to the free rare abdominal cancers group of the vancosamine sugar of vancomycin, produce inhibition of both mature and immature peptidoglycan without inhibition of formation of the lipid intermediate, thus behaving similarly to the known transglycosylase inhibitor, bambermycin.

Furthermore, the derivatized disaccharide moiety alone, which does not mechanism of action of antibacterial agents to the original D-Ala-D-Ala target of vancomycin, produces a similar pattern of inhibition. These findings thus expose the possibility of developing modified glycopeptides that act on dual targets, D-Ala-D-Ala as well as transglycosylase, and thus retain activity against vancomycin-resistant enterococci, which have modifications of the D-Ala-D-Ala structure as the mechanism of resistance [ 6 ].

Furthermore, such derivatives with dual targets of action would be unlikely to provide selection pressures that promote vancomycin resistance.

Subsequent events are required, but the molecular nature of these events has been elusive. Early work in E. Mutations causing an ampicillin-tolerance phenotype were mapped to a genetic locus, hipA [ 7 ]. This locus contains 2 genes in an operon, hipA and hipB. The function of the products of these 2 genes is not yet clear, but hipB appears to be involved in regulating the expression of hipAoverexpression of which is toxic in E. Mutations in hipA that cause ampicillin tolerance also produce a cold-sensitive block in cell division and synthesis of peptidoglycan and other macromolecules [ 8 ].

Interestingly, the pathway involving hipAB also appears to be involved in the bactericidal activity of quinolones.

Mutants in hipA also exhibit reduced killing activity by quinolones without affecting their bacteriostatic activity [ 9 ]. Furthermore, mutants selected for tolerance to quinolones also exhibited tolerance to ampicillin. A locus necessary for quinolone tolerance, hipQwas mapped to region of the chromosome distinct from the site of hipA [ 9 ]. Autolytic enzymes, or autolysins, participate in the remodeling of peptidoglycan structure during cell growth. Although these enzymes appear to be constitutively expressed, their activity is tightly regulated to protect the cell from global destruction of peptidoglycan.

The LytA protein of S. The LytB protein, a glucosaminidase, also appears to have autolytic activity and to be involved in cell separation [ 12 ]. Recently, additional components of the autolytic pathway were identified by screening a library of pneumococcal mutants for loss of penicillin-induced autolysis [ 13 ].

One mutant was found to be tolerant to vancomycin also. Analysis of this mutant identified disruption of the gene for a putative histidine kinase of a 2-component regulatory system, termed vncSwhich is contiguous to a gene encoding a putative response regulator, vncR. Additional mutants in vncS and vncR were constructed, and they confirmed that disruption of vncS but not vncR produces tolerance to vancomycin.

These findings led to a proposed model of regulation based on that of the VanS B and VanR B proteins, which act as sensor and response components to regulate expression of vancomycin resistance in VanB enterococci [ 14 mechanism of action of antibacterial agents. Thus, a loss-of-function vncS mutant is unable to relieve repression of autolytic activity. VncS is thus hypothesized to function under normal conditions as a sensor of a stimulus as yet undefined that activates its phosphatase activity, resulting in dephosphorylation of VncR-P, thereby relieving repression of autolytic how much acetaminophen can liver take. The vancomycin tolerance phenotype of the vncS mutant was also demonstrated in a mouse meningitis model, and 3 vncS mutants were found in a screen of clinical pneumococcal isolates for vancomycin tolerance.

There has also been a recent report of one patient with recurrence of pneumococcal meningitis after treatment with vancomycin and third-generation cephalosporins whose initial pneumococcal isolate from cerebrospinal fluid exhibited tolerance to vancomycin and cephalosporins in vitro [ 15 ]. The isolate, however, was not characterized for vncS or other mutations.

Thus, VncS and VncR appear to be components of a pathway that mediates cell death in response to several diverse drug-target interactions, mechanism of action of antibacterial agents. This cross-tolerance phenotype of a vncS mutant implies that triggering of autolysis may be at least 1 final effector of cell death produced by these antimicrobials in pneumococci. Killing mechanisms independent of lysis by LytA, however, have been identified [ 16 ]. One signal, a secreted peptide, Pep 27 [ 17 ], encoded by a gene upstream of vncRSmay be involved in autolysis that occurs when pneumococci enter stationary phase, but its involvement in antimicrobial-induced killing is not yet defined.

Further work in this area will be important to dissect the common factors mediating cell death in response to diverse antimicrobials. Understanding of the cell death pathway or pathways would be a major advance for defining mechanisms of action of many antimicrobials and might identify new drug targets with broad applicability.

Quinolones are now known to interact with 2 related but distinct targets within the bacterial cell, DNA gyrase and faa plan of action cfi IV.

DNA gyrase was the first quinolone target identified on the basis of initial genetic studies with nalidixic acid-resistant mutants of E. The discovery of E. Proof that topoisomerase IV is another quinolone target came from studies in which first-step quinolone resistance mutations were found in parC also termed grlA in Staphylococcus aureus [ 2224 ] and second-step parC [ 25 ] and parE [ 26mechanism of action of antibacterial agents, 27 ] mutations were shown to cause increments in quinolone resistance in the presence of gyrA mutations.

For many other gram-negative bacteria, as for E. In contrast, for many gram-positive bacteria, as for S. The primary drug target enzyme as defined by first-step resistance mutations thus often differs between gram-positive and gram-negative bacteria, mechanism of action of antibacterial agents.

Furthermore, resistance mutations in the subunits of the secondary drug target enzyme topoisomerase IV for E. Exceptions to this rule, however, have been reported with low-level increments in MICs associated with mutations in a secondary target in S.

These patterns appear to result from the relative sensitivities of DNA gyrase and topoisomerase IV to particular quinolones in a given organism. Thus, the more sensitive enzyme generally determines the primary drug target for a given organism. Interaction of a quinolone with a primary drug target thus determines cell susceptibility independent of the sensitivity of the secondary drug target. However, mechanism of action of antibacterial agents, there have been a few notable exceptions to this general rule.

Furthermore, unexpectedly, purified S. The explanation for the apparently anomalous behavior of sparfloxacin, gatifloxacin, and clinafloxacin is as yet mechanism of action of antibacterial agents. The recognition of dual drug target enzymes also has important implications for the development of resistance. For resistance mutations in the primary target enzyme, the increment in resistance may be limited by the level of sensitivity of the unmutated secondary target enzyme, which becomes the more sensitive enzyme when the primary target is resistant.

This scheme implies that for different quinolones, the level of resistance conferred by a mutation in the primary target enzyme would decrease as the level of drug sodium bicarbonate plus ascorbic acid of the secondary target approaches that of the primary target.

Furthermore, it implies that concurrent dual mutations in both target enzymes would be required for resistance due to target alteration for any quinolone that had equal potency against DNA gyrase and topoisomerase IV. This limit appears to be approximated in S.

Later-step mutants with mutations in both gyrA and parCmechanism of action of antibacterial agents, however, exhibit substantial resistance. For quinolones with differing activities against the 2 target enzymes, mechanism of action of antibacterial agents, potency and pharmacokinetic properties may also have effects on the likelihood of resistance.

In particular, if drug concentrations in vivo exceed the MIC of that quinolone for first-step resistance mutants, a value that has been termed the mutant prevention concentration, then it is predicted that resistance will also be unlikely to occur [ 37 ]. Similar activity of a quinolone against the 2 target enzymes operates in this mechanism of action of antibacterial agents by producing a lower increment in MIC for a first-step mutant.

One such example is the activity of moxifloxacin against specific mutants of S. In contrast, a single mutation in gyrA causes only a minimal increase in MIC, to 0. For both kinds of single mutants, the MIC of moxifloxacin is substantially below the peak concentrations that are achieved in serum with usual dosing, 3.

It is not yet clear if the similarity of activity against dual targets by a quinolone in one species predicts similar behavior in other species. In at least 3 species, Mycobacterium tuberculosis, Helicobacter pyloriand Treponema pallidumdual targeting is not possible because these organisms lack genes for topoisomerase IV, as determined by complete genome sequencing [ 4042 ].

The molecular nature of the interaction of quinolones with their target enzymes is only incompletely understood, mechanism of action of antibacterial agents.

The x-ray crystallographic structures of the fragments of the gyrase A and B mechanism of action of antibacterial agents and the related yeast enzyme topoisomerase II, the domains of which have homology to GyrB N terminus and GyrA C terminus [ 4345 ], have provided further information about the localization of amino acids that affect quinolone interaction with their targets.

These amino acids are clustered in alpha helices near the active-site tyrosine, which is involved in DNA breakage. In the GyrA fragment, these amino acids are located along a positively charged surface to which DNA likely binds, a finding based on structural modeling. Thus this site represents a hypothetical site at which mechanism of action of antibacterial agents bind to mechanism of action of antibacterial agents enzyme-DNA complex.

One amino acid change in this domain has been shown to reduce drug binding to the enzyme-DNA complex [ 46 ]. This potential quinolone-binding site in GyrA may in some enzyme conformations be in proximity to the domains of GyrB that appear also to be involved in quinolone action [ 47 ], suggesting that regions of both GyrA and GyrB may make up the binding site. Consequences of quinolone interaction with target enzyme-DNA complexes.

These effects are mediated by the mechanism of action of antibacterial agents of quinolones to stabilize complexes of DNA and type II topoisomerases.

This process has been studied in vitro with purified DNA replication systems. This collision also blocked progression of the replication fork. A mutant topoisomerase IV that lacks DNA cleavage capability bound DNA but was unable in the presence of a quinolone to block progression of the replication complex. Interaction of DnaB with the quinolone-DNA-topoisomerase IV complex, however, does not itself convert this complex to an irreversible form, mechanism of action of antibacterial agents.

In vivo, DNA synthesis inhibition by mechanism of action of antibacterial agents interaction with DNA gyrase occurs rapidly [ 52 ], but inhibition due to interaction with topoisomerase IV occurs with some delay [ 4849 ].

This difference is thought to relate to differences in the localization of DNA gyrase and topoisomerase IV on the bacterial chromosome.

 

Mechanism of action of antibacterial agents

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