CLBACT - Principles of Antimicrobial Action and Resistance

Antimicrobial agent

Substances that kill or inhibit the growth of microorganisms.

Antibiotic

Obtained and purified from other microbial organisms, for example penicillin from Penicillium mold.

Synthetic / semisynthetic

Chemically manufactured or modified.

Bacteriostatic agents

• Inhibit bacterial growth but do not kill the organism.
• Require a functional host immune system to clear the infection.

Bactericidal agents

• Directly kill the target organism.
• Preferred for severe infections or in immunocompromised patients.

Narrow-spectrum

Antibiotics that affect a certain range of different microbial types.

Broad-spectrum

Antibiotics that affect a broad range of gram-positive or gram-negative bacteria.

Pharmacodynamics

The effects of drugs in the body and the mechanism of their action.

Pharmacokinetics

Describes the four stages of absorption, distribution, metabolism and excretion of drugs.

Bioavailability

The extent to which a substance or drug becomes completely available to its intended biological destination(s); a measure of the rate and fraction of the initial dose of a drug that successfully reaches either the site of action or the bodily fluid domain from which the drug's intended targets have unimpeded access.

Drug administration route

Oral, IM, IV. Listed under pharmacodynamics in this lecture.

Anatomic approximation

Step 1 of the five basic steps required for antimicrobial activity.

Surface binding (adsorption)

Step 2 of the five basic steps required for antimicrobial activity.

Intracellular uptake

Step 3 of the five basic steps required for antimicrobial activity.

Target binding

Step 4 of the five basic steps required for antimicrobial activity.

Growth inhibition, or lysis and death

Step 5, the last of the five basic steps required for antimicrobial activity.

Five categories of antimicrobial action

• Inhibitors of Cell Wall Synthesis
• Inhibitors of Cell Membrane Functions
• Inhibitors of Protein Synthesis
• Inhibitors of Nucleic Acid Synthesis
• Inhibitors of Other Metabolic Processes

Beta-lactams

• A four-member, nitrogen-containing beta-lactam ring at the core structure.
• Bind penicillin-binding proteins, which halts cell wall synthesis.
• Death follows from osmotic instability.

Beta-lactam ring

Structurally similar to acyl-D-alanyl-D-alanine, the normal substrate required for the synthesis of the linear glycopeptide in the bacterial cell wall.

Penicillin-binding proteins

The enzymes essential for cell wall synthesis, anchored in the cell membrane. Binding halts cell wall synthesis.

Beta-lactam classes

Penicillins, cephalosporins, carbapenems, monobactams.

Glycopeptides

• Bind to the end of the peptidoglycan and interfere with transpeptidation, the incorporation of precursors.
• Cannot penetrate the outer membrane of most gram-negative bacteria.

Lipoglycopeptides

• Semisynthetic molecules; glycopeptides that contain hydrophobic chemical groups.
• Bind the bacterial cell membrane, increasing inhibition of cell wall synthesis and cell permeability, and causing depolarization of cell membrane potential.
• Inhibit the transglycosylation process.

Transpeptidation

The incorporation of precursors, blocked by glycopeptides.

Transglycosylation

The process inhibited by lipoglycopeptides.

Fosfomycin

• A synthetic, organic phosphonate derivative; fosfomycin tromethamine.
• Inactivates MurA.
• Single oral dose for uncomplicated UTIs, susceptible strains of Enterococcus faecalis and Escherichia coli.

MurA

UDP-N-acetylglucosamine enol-pyruvyl transferase, which catalyzes the first step in the synthesis of peptidoglycan. Inactivated by fosfomycin.

Lipopeptides

The cell membrane inhibitors of this lecture: daptomycin, and polymyxin B and E.

Daptomycin

Binds to the cytoplasmic membrane and inserts its hydrophobic tail into the membrane, increasing permeability; potent against gram-positive cocci.

Polymyxin

Polymyxin B and E; cyclic lipopeptide agents that act as detergents, interacting with phospholipids in the cell membranes to increase permeability.

Aminoglycosides

• Irreversibly bind protein receptors on the 30S ribosomal subunit.
• Interrupt the initial formation of the protein synthesis complex, accurate reading of the mRNA code, and formation of the ribosomal-mRNA complex.
• Bacterial uptake requires combination with cell-wall active antibiotics.

Macrolides

Reversibly bind the 23S rRNA on the bacterial 50S ribosomal subunit; disruption of the growing peptide chain by blocking translocation.

Lincosamides

Bind the 50S ribosomal subunit and prevent elongation, interfering with the peptidyl transfer.

Streptogramins

Naturally occurring cyclic peptides; bind irreversibly to the 50S subunit, producing a conformational change in the ribosome.

Ketolides

• Chemical derivatives of erythromycin A and other macrolides.
• Bind the 23S rRNA of the 50S ribosomal subunit.
• Effective against respiratory pathogens and intracellular bacteria.

Telithromycin

Maintains activity against most macrolide-resistant gram-positive organisms; does not induce a common macrolide resistance mechanism.

Oxazolidinones

• Synthetic antibacterial agents.
• Interact with the 23S rRNA of the 50S subunit and inhibit 70S initiation complex formation.
• Not expected to be affected by resistance mechanisms affecting other drug classes.
• Alternative treatment for patients who cannot tolerate vancomycin, for MRSA, multidrug-resistant streptococci and enterococci, or VRE.

Chloramphenicol

• Reversibly binds the 50S ribosomal subunit, inhibiting the addition of amino acids to the growing peptide chain, that is inhibiting transpeptidation.
• Limited clinical use because of severe toxicity: blood dyscrasia, aplastic anemia.

Tetracyclines

Bind reversibly to the 30S ribosomal subunit and interfere with the binding of the tRNA to the ribosome, preventing peptide chain elongation.

Glycylcyclines

Semisynthetic tetracycline derivatives; reversibly bind the 30S ribosomal subunit.

Tigecycline

The first glycylcycline approved for clinical use.

Mupirocin

• Pseudomonic acid A; a topical antibacterial agent from Pseudomonas fluorescens.
• Inhibition of isoleucyl-tRNA synthetase.
• Treatment of skin infections, staphylococci.
• Successful in decolonizing patients with nasal carriage of MRSA.

Isoleucyl-tRNA synthetase

The target of mupirocin, and the only synthetase target in this lecture.

30S inhibitors

Aminoglycosides, tetracyclines, glycylcycline-tigecycline.

50S inhibitors

Erythromycin, clindamycin, chloramphenicol, oxazolidinone-linezolid, streptogramine-DQ.

Irreversible binding

Aminoglycosides bind irreversibly to the 30S, and streptogramins bind irreversibly to the 50S. Every other agent in this lecture binds reversibly.

Initiation

The stage of translation blocked by aminoglycosides and oxazolidinones.

Elongation

The stage of translation blocked by macrolides, lincosamides, amphenicols, tetracyclines and glycylcyclines.

Fluoroquinolones

• Derivatives of nalidixic acid.
• Bind to and interfere with the DNA gyrase enzymes.
• Newer fluoroquinolones inhibit topoisomerase IV.
• Tendinitis and rupture of the Achilles tendon: greater risk at age over 60 years.

Moxifloxacin

The fluoroquinolone for anaerobic bacteria.

Ciprofloxacin, levofloxacin, delafloxacin

The only oral antibiotics with reliable activity against non-UTI P. aeruginosa.

Metronidazole

• A nitro group in the chemical structure.
• Reduction by nitroreductase generates cytotoxic compounds and free radicals.
• Requires low redox potential, that is an anaerobic environment.
• Most potent against anaerobic and microaerophilic organisms.

Rifamycins

• Rifampin / rifampicin; semisynthetic antibiotics.
• Bind the enzyme DNA-dependent RNA polymerase and inhibit the synthesis of RNA.
• Used in combination with isoniazid, pyrazinamide and ethambutol for M. tuberculosis.

Rifaximin

A derivative of rifampin; broad spectrum activity against enteric pathogens.

DNA gyrase

The enzyme fluoroquinolones bind and interfere with.

Sulfonamides

• Target and bind dihydropteroate synthase, one of the enzymes in the folic acid pathway.
• Antagonistic with warfarin, phenytoin and oral hypoglycemic agents.

Trimethoprim

• Targets and binds dihydrofolate reductase, the other folate enzyme.
• Frequently combined with a sulfonamide, that is sulfamethoxazole.

Dihydropteroate synthase

The folate-pathway enzyme targeted by sulfonamides.

Dihydrofolate reductase

The folate-pathway enzyme targeted by trimethoprim.

Nitrofurantoin

• A nitro group on a heterocyclic ring.
• Converted by nitroreductases to reactive intermediates that bind bacterial ribosomal proteins and rRNA, disrupting RNA, DNA and protein synthesis.
• Treatment of complicated and recurrent cystitis: E. coli, S. saprophyticus, S. aureus, enterococci, Klebsiella, Enterobacter spp.

Resistance

The ability of a microorganism to survive and grow in the presence of an antimicrobial agent.

Environmentally-mediated resistance

• Resistance directly resulting from physical or chemical characteristics of the environment.
• Acts by altering the antimicrobial agent, or altering the microorganism's normal physiologic response to the agent.
• Examples: pH, atmosphere, cation concentrations, thymidine content.

Microorganism-mediated resistance

Antimicrobial resistance from genetically encoded traits of the microorganism. Split into intrinsic and acquired.

Intrinsic resistance

Also inherent; a natural and consistently inherited characteristic.

Acquired resistance

Altered cellular physiology and structure caused by changes in a microorganism's genetic makeup.

Enzymatic destruction

Mechanism 1 of 4: production of enzymes that destroy the drug, for example beta-lactamases.

Altered target site

Mechanism 2 of 4: the drug's target is mutated so it cannot bind effectively, for example altered PBPs in MRSA.

Decreased uptake

Mechanism 3 of 4: the cell reduces drug entry, for example loss of porins in gram-negative bacteria.

Efflux pumps

Mechanism 4 of 4: active transport pumps the drug out of the cell before it can act, for example tetracycline resistance.

Common pathways of resistance

The six pathways the slide 34 figure gives, which are not the same as the four mechanisms in the slide 35 text:
• Enzymatic degradation or modification of the antimicrobial agent
• Decreased uptake or accumulation of the antimicrobial agent
• Altered antimicrobial target
• Circumvention of the consequences of antimicrobial action
• Uncoupling of antimicrobial agent-target interactions and subsequent effects on bacterial metabolism
• Any combination of mechanisms 1 through 5

Circumvention

Circumvention of the consequences of antimicrobial action; a common pathway of resistance that appears only in the slide 34 figure.

Uncoupling

Uncoupling of antimicrobial agent-target interactions and subsequent effects on bacterial metabolism; a common pathway of resistance that appears only in the slide 34 figure.

Beta-lactamases

The most common method of enzymatic destruction of beta-lactams; hydrolyze the beta-lactam ring.

ESBLs

Extended spectrum beta-lactamases; able to hydrolyze extended-spectrum cephalosporin.

Carbapenemase

Hydrolyzes carbapenems.

PBP2a / PBP2a' (mecA)

The altered penicillin-binding protein of MRSA.

PBP5

The altered penicillin-binding protein of E. faecium and E. faecalis.

MexAB-OprM

The efflux system named for P. aeruginosa.

OmpA, OmpC and OmpF

The outer membrane proteins named for E. coli.

Ambler class A

• Active site: serine.
• Enzyme: ESBL.
• Bush-Jacoby-Medeiros: Group II.

Ambler class B

• Active site: metal, Zn.
• Enzyme: metallo beta-lactamases.
• Bush-Jacoby-Medeiros: Group III.

Ambler class C

• Active site: serine.
• Enzyme: AmpC beta-lactamases.
• Bush-Jacoby-Medeiros: Group I.

Ambler class D

• Active site: serine.
• Enzyme: OXA beta-lactamases.
• Bush-Jacoby-Medeiros: Group II.

Ambler's classification

Splits beta-lactamases based on active site: classes A, C and D are the serine, non-metal site enzymes; class B is the metal site enzyme.

Bush-Jacoby-Medeiros Group I

Ambler class C.

Bush-Jacoby-Medeiros Group II

Ambler classes A and D.

Bush-Jacoby-Medeiros Group III

Ambler class B.

Serine beta-lactamases

Attack the ring with a serine hydroxyl through a tetrahedral intermediate.

Metallo beta-lactamases

Use a zinc-activated water molecule to attack the ring.

Beta-lactamase inhibitors

• Enhance the therapeutic range of beta-lactam antibiotics by preventing the degradation of beta-lactam antibiotics.
• Have little antimicrobial activity of their own; used in combination with specific antibiotics.
• Used in treating infections caused by gram-negative bacteria.

Tazobactam

A beta-lactamase inhibitor against the serine enzymes.

Avibactam

A beta-lactamase inhibitor against the serine enzymes.

Bisthiazolidines

Beta-lactamase inhibitors against the metallo enzymes.

Boronic acid derivatives

A class of beta-lactamase inhibitor named only in the slide 40 figure.

vanA and vanB

The genes of altered cell wall precursors in enterococci, unable to bind vancomycin with sufficient avidity.

D-Ala-D-Lac

A resistant cell terminates its precursor D-Ala-D-Lac. A susceptible cell terminates D-Ala-D-Ala, which vancomycin binds.

Overproduction of the peptidoglycan layer

A glycopeptide resistance route: binds excessive amounts of glycopeptide molecule.

APH

Aminoglycoside O-phosphotransferase; phosphorylation of hydroxyl groups. Confers very high levels of resistance, in Enterobacterales, P. aeruginosa and Acinetobacter spp.

ANT

Aminoglycoside O-adenyltransferase; adenylation of hydroxyl groups.

AAC

Aminoglycoside N-acetyltransferase; acetylation of amine groups.