Chapter 12 Antimicrobial Therapy Antibiotics
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3/31/2010 Chapter 12 Antimicrobial Therapy Topics: • Ehrlich (1900’s) – compound 606 to - Antimicrobial Therapy treat syphilis. Coined the term - Selective Toxicity “selective toxicity” - Survey of Antimicrobial Drug • Fleming (1928) – discovered - Microbial Drug Resistance penicillin - Drug and Host Interaction • Domagk (1930s) – “prontosil” was modified in the body to an active compound (first sulfa drug!) An ideal antimicrobic: Chemotherapy is the use of any chemical - soluble in body fluids, agent in the treatment of disease. - selectively toxic , - nonallergenic, An antibiotic agent is usually considered to - reasonable half life (maintained at a be a chemical substance made by a constant therapeutic concentration) microorganism that can inhibit the growth or - unlikely to elicit resistance, kill another microorganisms. - has a long shelf life, An antimicrobic or antimicrobial agent is - reasonably priced. a chemical substance similar to an There is no ideal antimicrobic!!! antibiotic, but may be synthetic. Selective Toxicity - Drugs that specifically target microbial processes, and not the human host’s. Antibiotics Spectrum of antibiotics and targets • Naturally occurring antimicrobials – Metabolic products of bacteria and fungi – Reduce competition for nutrients and space • Bacteria – Streptomyces, Bacillus, • Molds – Penicillium, Cephalosporium * * 1 3/31/2010 The mechanism of action for different 5 General Mechanisms of Action for antimicrobial drug targets in bacterial cells Antibiotics - Inhibition of Cell Wall Synthesis - Disruption of Cell Membrane Function - Inhibition of Protein Synthesis - Inhibition of Nucleic Acid Synthesis - Anti-metabolic activity Antibiotics weaken the cell wall, and cause the cell to lyse . Cell wall synthesis • Bactericidal • Vancomycin – hinders peptidoglycan elongation • Penicillin and cephalosporins – binds and blocks peptidases involved in cross-linking the glycan molecules Fig. 12.2 The consequences of exposing a growing cell to antibiotics that prevent cell wall synthesis. Affect cell wall synthesis The mechanism of cell wall inhibition by penicillins and cephalosporins Penicillin – Figure 13.11 - Penicillin G - drug of choice for streptococci, meningococci, pneumococci, spirochetes, clostridia, aerobic gram-positive rods treponemes - administered parenterally - other than by mouth - why? - Not absorved in the intestines!!! - Penicillin V, ampicillin or other analogues may be used for oral administration 2 3/31/2010 Penicillin Penicillin continued • Penicillium chrysogenum • Resistance – if bacteria contain st nd rd • A diverse group (1 , 2 , 3 generations) penicillinases - βββ-lactamase – Natural (penicillin G and V) • Inhibits cell wall synthesis – Semisynthetic (ampicillin, amoxicillin) • Effective against Gram+ bacteria • Structure –Beta-lactam ring – Variable side chain (R group) Effect of β-lactamase on penicillin Cephalosporin - beta lactam • Cephalosporium acremonium (mold) • Widely administered today – Diverse group (natural and semisynthetic- 4th generation!) • Structure – similar to penicillin except • Main ring is different • Two sites for R groups Cephalosporin continued… II. Nucleic acid synthesis • Resistant to most penicillinases • Chloroquine – binds and cross-links the double helix • Broad-spectrum – inhibits cell wall synthesis • Other quinolones – inhibits DNA unwinding enzymes (gyrase) and block rd • 3 generation drugs used to treat replication. Ciprofloxacin is an example enteric bacteria, respiratory, skin, urinary and nervous system infections • Viruses – Analogs of purines and pyrimidines - sometimes considered antimetabolites 3 3/31/2010 Rifampin - blocks transcription - can “Red Man Syndrome” cause red man syndrome - a result of accumulation of metabolic products of the antimicrobic in secretions Examples of different antibiotics and their Mostly sites of inhibition on the prokaryotic ribosome seen with anti-viral agents purine purine pyrimidine pyrimidine Protein Synthesis Inhibitors analog analog III. Protein synthesis Aminoglycosides • Aminoglycosides • Streptomyces and Micromonospora – Binds the 30S ribosome changes shape • Broad-spectrum – Misreads mRNA • Commonly used to treat bubonic • Tetracyclines plague and sexually transmitted – Binds the 30S ribosome diseases – Blocks attachment of tRNA to A site • Chloramphenicol • Inhibits protein synthesis - binds 30S – Binds to the 50S ribosome ribosomal subunit misreading of – Prevents peptide bond formation mRNA 4 3/31/2010 Tetracycline • Streptomyces • Broad spectrum and low cost • Commonly used to treat sexually transmitted diseases • Side effects – gastrointestinal disruption, deposition in hard tissues • Inhibits proteins synthesis - Binds the 30S ribosome and blocks attachment of tRNA Chloramphenicol Erythromycin • Streptomyces • Streptomyces • Broad-spectrum • Structure – macrolide ring • Only made synthetically today • Broad-spectrum • Treat typhoid fever, brain abscesses • Commonly used as prophylactic drug prior to surgery • Side effects – aplastic anemia • Side effects - low toxicity • Inhibits protein synthesis - binds 50S ribosome subunit preventing peptide • Inhibits protein synthesis - bind to bond formation 50S ribosome subunit- prevents translocation IV. Cell membrane Polyenes • Antifungal • Polymyxins • Commonly used for skin infections – Interact with membrane phospholipids • Targets the membrane - loss of selective – Distorts the cell surface permeability – Leakage of proteins and nitrogen bases • Polyenes – Amp B and Nystatin • Anti-fungal - Polyenes • Amphotericin B - binds to ergosterol found – Amphotericin B and Nystatin- bind to in fungi and protozoa, but not in human sterols on cells membranes. cells - increases membrane permeability – Potentially toxic to humans!! • Side effects are numerous due to toxicity of the drug 5 3/31/2010 V. ANTIMETABOLITES Sulfonamides (sulfa drugs) Act either through competitive inhibition or erroneous incorporation – molecular • Synthetic drug mimicry • Based on sulfanilamides • Used in combination with other Sulfonamides - block synthesis of folic synthetics such as trimethoprim acid - and as a result, nucleic acid • Commonly used to treat pneumonia synthesis in AIDS patients Isoniazid - antimetabolite for two vitamins • Inhibits folic acid synthesis The mode of action of sulfa drug Other types of antimicrobials • Antiprotozoan – metronidazole - most are fairly toxic - black hairy tongue – Treat Giardia and amebiasis • Antimalarial – Chlorquinine – malaria • Antihelminthic – mebendazole – Tapeworms, roundworms Antiviral • Limited drugs available Flagyl • Difficult to maintain selective toxicity • Effective drugs – target viral replication cycle (DNA Polymerase or RT in HIV) – Entry – Nucleic acid synthesis – Assembly/release (Amantidine – influenza) • Interferon – artificial antiviral drug 6 3/31/2010 Antimicrobial Resistance 5 Mechanisms of Resistance 1) Alteration of Targets – usually affects • Resistance factors – R plasmids ribosomes – Transformation, Conjugation, 2) Alteration of Membrane Permeability- Change Transduction in the receptor that binds the drug – Transposons 3) Development of Enzymes – βββ-lactamase • 5 main mechanisms of resistance 4) Efflux pumps – Membrane proteins many Gram negatives that pump out drug • New approaches 5) Alteration of Metabolic Pathway – Development of alternate pathway Examples of mechanisms of acquired drug resistance Human Misuse of βββ-lactamase Antibiotics!!! Membrane permeability BAD!!! Efflux pumps Alter targets GOOD!!! Alternate metabolism Limiting Resistance New approaches • Increase drug resistance requires new 1) Constant exposure to high levels of approaches for developing effective antibiotic antimicrobials – Prevent iron –scavenging capabilities 2) Use of multiple antibiotics – Inhibit genetic controls - (riboswitches) – Probiotics and prebiotics 3) Restricted use of antibiotics – Combination therapy (synergism) – Phage therapy 7 3/31/2010 Drug and Host Interaction • Toxicity to organs • Allergic reactions • Suppress/alter microflora • Effective drugs ↓ Pressure Main Types of Side Effects Development of disease following Associated with Antimicrobial broad spectrum antimicrobic therapy Treatment - Pseudomembranous colitis (antibiotic 1)Toxicity associated diarrhea) - often caused by Clostridium difficile 2)Allergy – actual drug or breakdown products - Yeast infection – after broad antibiotic use to treat UTI caused by E. coli 3)Disruption of Normal Microflora Can Lead to SUPERINFECTIONS!! Disrupting the microflora in the intestine can result in superinfections Effective drugs • Identify infectious agent • Sensitivity testing • Minimum Inhibitory Concentration (MIC) – visual call 8 3/31/2010 An example of the Kirby-Bauer Test The E-test is an alternative to the Kirby-Bauer procedure MIC MIC Antimicrobics have helped us deal with disease, but on the other hand, improper use of antimicrobics have created new difficulties. 9.