Antimicrobial Therapy

Chapter 10

History of Antimicrobials

• 1600s

• Quinine for malaria

• Emetine for amebiasis (Entamoeba histolytica)

• 1900-1910

• Arsphenamines for syphilis

• 1935

• Sulfonamides - broadly active

• 1940

• Penicillin - substantially more active than sulfa drugs

• Originally discovered in 1929 by Alexander Fleming (Scottish)

• Nobel Prize, 1945

• Knighted, 1944

• Produced by fungus Penicillium chrysogenum

Mechanisms of Action of Anitmicrobial Drugs

• Selective toxicity

• Antimicrobials must be toxic to the microbe, but not to the host

• Unfortunately, no such antibiotic exists

• Mechanisms of action

• Cell wall synthesis inhibitors

• Cell membrane inhibitors

• Protein synthesis inhibitors

• Nucleic acid synthesis inhibitors

• Metabolic Pathways

Cell Wall Inhibitors

• Cell wall

• Outer layer of bacterial cell

• Barrier to outside

• Maintains osmotic pressure

• Peptidoglycan (polymer)

• Polysaccharide and cross-linked peptides (transpeptidation)

• N-acetylglucosamine (NAG)

• N-acetylmuramic acid* (NAM)

• *Only found in bacteria

• Synthesis of peptidoglycan layer is performed by several enzymes

• Gram+ have substantially thicker peptidoglycan layer

• Penicillin and Cephalosporin

• Highly insoluble in natural form

• Usually converted to a salt to increase solubility

• Contains a β-lactam ring that interferes with cell wall synthesis

• Penicillin is first bound by cellular penicillin binding receptors (PBP)

• This binding interferes with transpeptidation reaction

• This prevents peptidoglycan synthesis

Cell Wall Inhibitors

Cell Wall Inhibitors

Semisynthetic penicillins

Cell Membrane Function Inhibitors

• The cell membrane is a biochemically-rich compartment

• Polymyxins

• Contain detergent-like (amphipathic) cyclic peptides

• These damage membranes containing phosphatidylethanolamine

• Novobiocin - inhibits teichoic acid synthesis

• Ionophores - disrupt ion transport

• Discharge membrane potential

• Disrupts oxidative phosphorylation

Protein Synthesis Inhibitors

Protein Synthesis Inhibitors

•Most interfere with ribosomes

•By preventing ribosome function, polypeptide synthesis is inhibited

•Compounds• Aminoglycosides (e.g., streptomycin)

• Bind to 30S subunit

• Interferes with initiation complex

• mRNA localization to P site

• fMet tRNA

• Incorrect amino acid is incorporated into polypeptide

• Tetracyclines

• Bind to 30S subunit

• Prevents IF3 binding

• No tRNA binding

•Others• Macrolides - initiation complex, translocation

• Azalides - initiation complex, translocation

• Ketolides - initiation complex, translocation

• Lincomycins - initiation complex, translocation

• Glycylcyclines - Tet analogs; bind with higher affinity

• Chloramphenicol - Inhibits peptidyl transferase

• Streptogramins - Irreversible binding to 50S subunit; unknown mechanism

• Oxazolidinones - Inhibit fMet tRNA binding to P site

Protein Synthesis Inhibitors

Nucleic Acid Synthesis Inhibitors

• Types

• DNA/RNA polymerase inhibitors

• Base analogs

• Rifampin

• Binds with high affinity to β subunit of DNA-dependent RNA polymerase

• Prevents RNA synthesis

• Quinolones - inhibit bacterial DNA gyrase

• Sulfonamides

• Structural homologs of p-aminobenzoic acid (PABA)

• PABA is required for folic acid synthesis by dihydropteroate synthetase (DHPS)

• Folic acid is a nucleotide precursor

• Sulfa compounds compete with PABA for the active site of DHPS

Nucleic Acid Synthesis Inhibitors

DHPS

Resistance to Antimicrobial Drugs

• Mechanisms of resistance

• Enzymes that cleave or otherwise inactivate antibiotics

• β-lactamases

• Changes in bacterial permeabilities

• Prevents entry of antibiotic into cell

• Mutation in target molecule

• Alter binding characteristics of the antibiotics

• Alteration of metabolic pathways

• Some resistant bacteria can acquire PABA from the environment

• Molecular pumps (efflux systems)

• Secretion systems that export antibiotics faster than the rate of import

Nongenetic Origins of Drug Resistance

• Low replication rates

• Antibiotic is metabolized or neutralized before it act

• Mycobacteria spp.

• Alteration of cellular physiology

• Bacterial L forms are cell wall-free

• Streptococcus spp., Treponema spp., Bacillius spp., others

• Colonization of sites where antibiotics cannot reach

• Gentamicin cannot enter cells

• Salmonella are thus resistant to gentamicin

• Chromosomal Resistance

• Genes that regulate susceptibility

• Often found in enzymes, rRNA and secretion system genes

• Mutations in RNApol render it resistant to the effects of rifampin

• Efflux pumps with specificity for antibiotics

• Found in all bacteria

• All possess large hydrophobic cavity for binding antibiotics

Genetic Origins of Drug Resistance

Five efflux pumps (“antiporters”) that regulate antibiotic resistance (Paulsen, 2003)

• Extrachromosomal Resistance

• Often account for interspecies acquisition of resistance

• Contribute to multi-drug resistance (MDR)

• Genetic elements are:

• Plasmids

• Transposons

• Conjugation

• Transduction

• Transformation

Genetic Origins of Drug Resistance

Drug Resistance

Antimicrobial Activity In Vivo

• Drug-Pathogen Relationships

• Environment

• State of metabolic activity: slow-growing or dormant bacteria less susceptible

• Distribution of drug: CNS is often exclusionary

• Location of organisms: Some drugs do not enter host cells

• Interfering substances: pH, damaged tissues, etc.

• Concentration

• Absorption: some cannot be taken orally

• Distribution: some accumulate in certain tissues

• Variability of concentration: peaks and troughs

• Postantibiotic effect: delayed regrowth of bacteria

•Host-Pathogen Relationships

•Alteration of tissue response

•Suppression of microbe can reduce inflammatory responses

•Alteration of immune response

•Prevention of autoimmune antibodies (e.g., rheumatic fever)

•Alteration of microbial flora

•Expansion of harmful flora (e.g., C. difficile)

Antimicrobial Activity In Vivo

Clinical Use of Antibiotics

• Selection of appropriate antibiotic

• Accurate diagnosis is critical

• Susceptibility testing should be performed if:

• Isolated microbe is often antibiotic resistant

• Infection would likely be fatal if incorrect drug is selected

• Need rapidly bactericidal activity (e.g., endocarditis)

• Susceptibility testing is often performed with antibiotic discs

• A large zone of clearance suggest sensitivity

Minimal Inhibitory Concentration

•The MIC determines the dose of antibiotic necessary to kill or retard bacteria

•It is usually done as a tube test (i.e., liquid phase)

•Serial dilutions of an antibiotic is made, then a defined number of bacteria are added to the tubes

•Tubes are read the following day (or days) for the endpoint

Minimal Inhibitory Concentration

Dangers of Indiscriminate Use

•In some countries antibiotics are available OTC

•This has led to the emergence of antibiotic resistance

•Often the wrong antibiotic is used

•The full regimen is not completed

•Hypersensitivities (e.g., penicillin anaphylaxis)

•Hepatotoxicity

•Changes in normal flora

Antimicrobial Chemoprophylaxis

•Exposure to specific pathogens (e.g., N. meningitidis)

•Health-related susceptibilities

•Heart disease/valve replacement

•Respiratory disease (e.g., influenza, measles)

•Recurrent urinary tract infections

•Opportunistic infections

•Post surgery

•Disinfectants

•Medical devices (e.g., catheters)

Antifungal Drugs

Antiprotozoal and Antihelminth Drugs

Antiprotozoal and Antihelminth Drugs

Toxic Side Effects

• Penicillins: Hypersensitivity

• Cephalosporins: Hypersensitivity, nephritis, hemolytic anemia

• Tetracyclines: Discoloring of teeth

• Chloramphenicol: Disruption of RBC production

• Erythromycins: Hepatitis

• Vancomycin: Deafness, leukopenia, renal damage

• Sulfonamides: Hemolytic anemia, bone marrow depression