Chapter 20 Antimicrobial Drugs Lectures prepared by Christine L. Case Copyright Lectures prepared by Christine L. Case
The History of Chemotherapy Learning Objectives 20-1 Identify the contributions of Paul Ehrlich and Alexander Fleming to chemotherapy. 20-2 Name the microbes that produce most antibiotics.
Antimicrobial Drugs Chemotherapy: the use of drugs to treat a disease Antimicrobial drugs: interfere with the growth of microbes within a host Antibiotic: a substance produced by a microbe that, in small amounts, inhibits another microbe Selective toxicity: killing harmful microbes without damaging the host
Figure 19.5 Drug-induced thrombocytopenic purpura. Platelet Drug (hapten) Drug binds to platelet, forming hapten platelet complex. Complex induces formation of antibodies against hapten. Hapten platelet complex Anti-hapten antibody Action of antibodies and complement causes platelet destruction. Complement
Antimicrobial Drugs 1928: Fleming discovered penicillin, produced by Penicillium 1940: Howard Florey and Ernst Chain performed first clinical trials of penicillin
Figure 20.1 Laboratory observation of antibiosis.
Table 20.1 Representative Sources of Antibiotics Insert Table 20.1
Check Your Understanding Who coined the term magic bullet? 20-1 More than half our antibiotics are produced by a certain genus of bacteria. What is it? 20-2
The Spectrum of Antimicrobial Activity Learning Objectives 20-3 Describe the problems of chemotherapy for viral, fungal, protozoan, and helminthic infections. 20-4 Define the following terms: spectrum of activity, broad-spectrum antibiotic, superinfection.
The Spectrum of Antimicrobial Activity Broad spectrum Narrow spectrum Superinfection ANIMATION Chemotherapeutic Agents: Modes of Action
Table 20.2 The Spectrum of Activity of Antibiotics and Other Antimicrobial Drugs
Check Your Understanding Identify at least one reason why it is so difficult to target a pathogenic virus without damaging the host s cells. 20-3 Why are antibiotics with a very broad spectrum of activity not as useful as one might first think? 20-4
The Action of Antimicrobial Drugs Learning Objective 20-5 Identify five modes of action of antimicrobial drugs.
The Action of Antimicrobial Drugs Bactericidal Kill microbes directly Bacteriostatic Prevent microbes from growing
Figure 4.13a Bacterial cell walls. N-acetylglucosamine (NAG) N-acetylmuramic acid (NAM) Side-chain amino acid Cross-bridge amino acid Tetrapeptide side chain Peptide cross-bridge NAM Peptide bond Carbohydrate backbone Structure of peptidoglycan in gram-positive bacteria.
Figure 20.3 The inhibition of bacterial cell synthesis by penicillin. Rod-shaped bacterium before penicillin. The bacterial cell lysing as penicillin weakens the cell wall.
Figure 20.4 The inhibition of protein synthesis by antibiotics. Protein synthesis site 5 30S Growing polypeptide Tunnel 50S 3 mrna Three-dimensional detail of the protein synthesis site showing the 30S and 50S subunit portions of the 70S prokaryotic ribosome Growing polypeptide trna 50S portion Chloramphenicol Binds to 50S portion and inhibits formation of peptide bond Protein synthesis site Messenger RNA 30S portion Direction of ribosome movement Streptomycin Changes shape of 30S portion, causing code on mrna to be read incorrectly 70S prokaryotic ribosome Translation Tetracyclines Interfere with attachment of trna to mrna ribosome complex Diagram indicating the different points at which chloramphenicol, the tetracyclines, and streptomycin exert their activities
Figure 20.5 Injury to the plasma membrane of a yeast cell caused by an antifungal drug.
Table 20.3 Antibacterial Drugs (Part 1 of 3)
Table 20.3 Antibacterial Drugs (Part 2 of 3)
Table 20.3 Antibacterial Drugs (Part 3 of 3)
Inhibiting the Synthesis of Essential Metabolites
Figure 20.2 Major Action Modes of Antimicrobial Drugs. 1. Inhibition of cell wall synthesis: penicillins, cephalosporins, bacitracin, vancomycin 2. Inhibition of protein synthesis: chloramphenicol, erythryomycin, tetracyclines, streptomycin DNA mrna Transcription Translation Protein Replication Enzyme 4. Injury to plasma membrane: polymyxin B 5. Inhibition of essential metabolite synthesis: sulfanimide, trimethoprim 3. Inhibition of nucleic acid replication and transcription: quinolones, rifampin
Check Your Understanding What cellular function is inhibited by tetracyclines? 20-5
Commonly Used Antimicrobial Drugs Learning Objectives 20-6 Explain why the drugs described in this section are specific for bacteria. 20-7 List the advantages of each of the following over penicillin: semisynthetic penicillins, cephalosporins, and vancomycin. 20-8 Explain why isoniazid (INH) and ethambutol are antimycobacterial agents.
Commonly Used Antimicrobial Drugs Learning Objectives 20-9 Describe how each of the following inhibits protein synthesis: aminoglycosides, tetracyclines, chloramphenicol, macrolides. 20-10 Compare the mode of action of polymyxin B, bacitracin, and neomycin. 20-11 Describe how rifamycins and quinolones kill bacteria. 20-12 Describe how sulfa drugs inhibit microbial growth.
Inhibitors of Cell Wall Synthesis Penicillin Natural penicillins Semisynthetic penicillins Extended-spectrum penicillins
Figure 20.6a The structure of penicillins, antibacterial antibiotics. Natural penicillins Common nucleus Penicillin G (requires injection) β lactam ring Penicillin V (can be taken orally)
Figure 20.6b The structure of penicillins, antibacterial antibiotics. Semisynthetic penicillins Common nucleus Oxacillin: Narrow spectrum, only gram-positives, but resistant to penicillinase β lactam ring Ampicillin: Extended spectrum, many gram-negatives.
Figure 20.7 Retention of penicillin G. Penicillin G (injected intramuscularly) Concentration in blood Penicillin G (oral) Procaine penicillin Benzathine penicillin 0 2 4 6 12 18 24 30 Time (hr)
Figure 20.8 The effect of penicillinase on penicillins. β lactam ring Penicillinase Penicillin Penicilloic acid
β Lactam Antibiotics Penicillin Penicillinase-resistant penicillins Penicillins + β-lactamase inhibitors Carbapenems Substitute a C for an S, add a double bond Monobactam Single ring
Inhibitors of Cell Wall Synthesis Cephalosporins First-generation: narrow spectrum; act against grampositive bacteria Second-generation: extended spectrum includes gram-negative bacteria Third-generation: includes pseudomonads; injected Fourth-generation: oral
Figure 20.9 The nuclear structures of a cephalosporin and penicillin compared. β lactam ring Insert Fig 20.9 Cephalosporin nucleus Penicillin nucleus
Inhibitors of Cell Wall Synthesis Polypeptide antibiotics Bacitracin Topical application Against gram-positives Vancomycin Glycopeptide Important last line against antibiotic-resistant S. aureus
Inhibitors of Cell Wall Synthesis Antimycobacterial antibiotics Isoniazid (INH) Inhibits mycolic acid synthesis Ethambutol Inhibits incorporation of mycolic acid
Check Your Understanding One of the most successful groups of antibiotics targets the synthesis of bacterial cell walls; why does the antibiotic not affect the mammalian cell? 20-6 What phenomenon prompted the development of the first semisynthetic antibiotics, such as methicillin? 20-7 In what genus of bacteria do we find mycolic acids in the cell wall? 20-8
Inhibitors of Protein Synthesis Chloramphenicol Broad spectrum Binds 50S subunit; inhibits peptide bond formation
Figure 20.10 The structure of the antibacterial antibiotic chloramphenicol. Chloramphenicol
Inhibitors of Protein Synthesis Aminoglycosides Streptomycin, neomycin, gentamicin Broad spectrum Change shape of 30S subunit
Inhibitors of Protein Synthesis Tetracyclines Broad spectrum Interfere with trna attachment
Figure 20.11 The structure of the antibacterial antibiotic tetracycline. Tetracycline
Inhibitors of Protein Synthesis Glycylcyclines MRSA and Acinetobacter baumanii Bind 30S subunit; inhibit translation
Inhibitors of Protein Synthesis Macrolides Gram-positives Bind 50S; prevent translocation
Figure 20.12 The structure of the antibacterial antibiotic erythromycin, a representative macrolide. Macrocyclic lactone ring Erythromycin
Inhibitors of Protein Synthesis Streptogramins Gram-positives Bind 50S subunit; inhibit translation
Inhibitors of Protein Synthesis Oxazolidinones Linezolid MRSA Bind 50S subunit; prevent formation of 70S ribosome
Inhibitors of Protein Synthesis Pleuromutilins From the mushroom Pleurotis mutilus MRSA Bind 50S; prevent translocation
Injury to the Plasma Membrane Lipopeptides Structural changes in the membrane, followed by arrest of the synthesis of DNA, RNA, and protein MRSA Polymyxin B Topical Combined with bacitracin and neomycin in over-the-counter preparation
Check Your Understanding Why does erythromycin, a macrolide antibiotic, have a spectrum of activity limited largely to gram-positive bacteria even though its mode of action is similar to that of the broad-spectrum tetracyclines? 20-9 Of the three drugs often found in over-thecounter antiseptic creams polymyxin B, bacitracin, and neomycin which has a mode of action most similar to that of penicillin? 20-10
Inhibitors of Nucleic Acid Synthesis Rifamycin Inhibits RNA synthesis Antituberculosis Quinolones and fluoroquinolones Nalidixic acid: urinary infections Ciprofloxacin Inhibit DNA gyrase Urinary tract infections
Competitive Inhibitors Sulfonamides (sulfa drugs) Inhibit folic acid synthesis Broad spectrum
Figure 5.7bc Enzyme inhibitors. Action of Enzyme Inhibitors Competitive inhibitor Altered active site Noncompetitive inhibitor Allosteric site
Figure 20.13 Actions of the antibacterial synthetics trimethoprim and sulfamethoxazole. PABA PABA Sulfamethoxazole, a sulfonamide that is a structural analog of PABA, competitively inhibits the synthesis of dihydrofolic acid from PABA. Sulfamethoxazole Dihydrofolic acid Sulfamethoxazole Dihydrofolic acid Trimethoprim, a structural analog of a portion of dihydrofolic acid, competitively inhibits the synthesis of tetrahydrofolic acid. Trimethoprim Tetrahydrofolic acid Trimethoprim Precursors of proteins, DNA, RNA DNA RNA
Check Your Understanding What group of antibiotics interferes with the DNA-replicating enzyme DNA gyrase? 20-11 Both humans and bacteria need the essential nutrient para-aminobenzoic acid; why, then, are only bacteria affected by sulfa drugs? 20-12
Commonly Used Antimicrobial Drugs Learning Objectives 20-13 Explain the modes of action of currently used antifungal drugs. 20-14 Explain the modes of action of currently used antiviral drugs. 20-15 Explain the modes of action of currently used antiprotozoan and antihelminthic drugs.
Antifungal Drugs: Inhibition of Ergosterol Synthesis Polyenes Amphotericin B
Figure 20.14 The structure of the antifungal drug amphotericin B, representative of the polyenes. Amphotericin B
Antifungal Drugs: Inhibition of Ergosterol Synthesis Azoles Miconazole Triazole Allylamines For azole-resistant infections
Figure 20.15 The structure of the antifungal drug miconazole, representative of the imidazoles. Miconazole
Antifungal Drugs: Inhibiting Cell Wall Synthesis Echinocandins Inhibit synthesis of β-glucan Cancidas is used against Candida and Pneumocystis
Inhibition of Nucleic Acids Flucytosine Cytosine analog interferes with RNA synthesis Pentamidine isethionate Anti-Pneumocystis; may bind DNA
Other Antifungal Drugs Griseofulvin Inhibits microtubule formation Superficial dermatophytes Tolnaftate Action unknown
Check Your Understanding What sterol in the cell membrane of fungi is the most common target for antifungal action? 20-13
Figure 20.16a The structure and function of the antiviral drug acyclovir. Guanine Deoxyguanosine Acyclovir Acyclovir structurally resembles the nucleoside deoxyguanosine.
Figure 20.16bc The structure and function of the antiviral drug acyclovir. Phosphate Normal thymidine kinase Nucleoside Guanine nucleotide DNA polymerase Incorporated into DNA The enzyme thymidine kinase combines phosphates with nucleosides to form nucleotides, which are then incorporated into DNA. Phosphate Acyclovir (resembles nucleoside) Thymidine kinase in virus-infected cell DNA polymerase blocked by false nucleotide. Assembly of DNA stops. Acyclovir has no effect on a cell not infected by a virus, that is, with normal thymidine kinase. In a virally infected cell, the thymidine kinase is altered and converts the acyclovir (which resembles the nucleoside deoxyguanosine) to a false nucleotide, which blocks DNA synthesis by DNA polymerase. False nucleotide (acyclovir triphosphate)
Antiviral Drugs: Enzyme Inhibitors Protease inhibitors Indinavir: HIV Integrase inhibitors HIV
Antiviral Drugs: Entry Inhibitors Entry inhibitors Amantadine: influenza Fusion inhibitors Zanamivir: influenza Block CCR5: HIV
Antiviral Drugs: Interferons Prevent spread of viruses to new cells Alpha interferon: Viral hepatitis Imiquimod Promotes interferon production
Antiprotozoan Drugs Chloroquine Inhibits DNA synthesis Malaria Artemisinin Kills Plasmodium sporozoites Metronidazole Interferes with anaerobic metabolism Trichomonas and Giardia
Antihelminthic Drugs Niclosamide Prevents ATP generation Tapeworms Praziquantel Alters membrane permeability Flatworms Mebendazole and albendazole Interfere with nutrient absorption Intestinal roundworms Ivermectin Paralysis of helminths Intestinal roundworms
Check Your Understanding One of the most widely used antivirals, acyclovir, inhibits the synthesis of DNA. Humans also synthesize DNA, so why is the drug still useful in treating viral infections? 20-14 What was the first drug available for use against parasitic infections? 20-15
Tests to Guide Chemotherapy Learning Objective 20-16 Describe two tests for microbial susceptibility to chemotherapeutic agents.
Tests to Guide Chemotherapy MIC: minimal inhibitory concentration MBC: minimal bactericidal concentration Antibiogram
Figure 20.17 The disk-diffusion method for determining the activity of antimicrobials.
Figure 20.18 The E test (for epsilometer), a gradient diffusion method that determines antibiotic sensitivity and estimates minimal inhibitory concentration (MIC). MIC MIC
Figure 20.19 A microdilution, or microtiter, plate used for testing for minimal inhibitory concentration (MIC) of antibiotics. Doxycycline (Growth in all wells, resistant) Sulfamethoxazole (Trailing end point; usually read where there is an estimated 80% reduction in growth) Streptomycin (No growth in any well; sensitive at all concentrations) Ethambutol Kanamycin (Growth in fourth wells; equally sensitive to ethambutol and kanamycin) Decreasing concentration of drug
Check Your Understanding In the disk-diffusion (Kirby-Bauer) test, the zone of inhibition indicating sensitivity around the disk varies with the antibiotic. Why? 20-16
Resistance to Antimicrobial Drugs Learning Objective 20-17 Describe the mechanisms of drug resistance.
Figure 20.21 The development of an antibiotic-resistant mutant during antibiotic therapy. Initiation of antibiotic therapy Antibiotic resistance of bacterial population measured by amount of antibiotic needed to control growth 10 8 50 Bacteria (number/ml) 10 7 10 6 10 5 10 4 Bacteria count 40 30 20 10 Antibiotic resistance (mg/ml) 10 3 0 1 2 3 4 5 6 7 8 9 10 11 Days
Antibiotic Resistance A variety of mutations can lead to antibiotic resistance Resistance genes are often on plasmids or transposons that can be transferred between bacteria
Antibiotic Resistance Misuse of antibiotics selects for resistance mutants Misuse includes: Using outdated or weakened antibiotics Using antibiotics for the common cold and other inappropriate conditions Using antibiotics in animal feed Failing to complete the prescribed regimen Using someone else s leftover prescription ANIMATION Antibiotic Resistance: Origins of Resistance ANIMATION Antibiotic Resistance: Forms of Resistance
Figure 20.20 Bacterial Resistance to Antibiotics. 1. Blocking entry Antibiotic Antibiotic Antibiotic 2. Inactivation by enzymes Altered target molecule Enzymatic action 3. Alteration of target molecule Inactivated antibiotic 4. Efflux of antibiotic
Clinical Focus Antibiotics in Animal Feed Linked to Human Disease, Figure A. Cephalosporin-resistance in E. coli transferred by conjugation to Salmonella enterica in the intestinal tracts of turkeys. E. coli S. enterica S. enterica after conjugation Resistance plasmid
Clinical Focus Antibiotics in Animal Feed Linked to Human Disease, Figure B. Percent FQ-resistant Campylobacter 30 25 20 15 10 5 Flouroquinolone-resistant Campylobacter jejuni in the United States, 1986 2008. FQ for humans FQ for poultry FQ for poultry discontinued 0 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Year
Check Your Understanding What is the most common mechanism that a bacterium uses to resist the effects of penicillin? 20-17
Effects of Combinations of Drugs Learning Objective 20-18 Compare and contrast synergism and antagonism.
Effects of Combinations of Drugs Synergism occurs when the effect of two drugs together is greater than the effect of either alone Antagonism occurs when the effect of two drugs together is less than the effect of either alone
Figure 20.23 An example of synergism between two different antibiotics. Area of synergistic inhibition, clear Area of growth, cloudy Disk with antibiotic amoxicillin-clavulanic acid Disk with antibiotic aztreonam
Antibiotic Safety Therapeutic index: risk versus benefit
Check Your Understanding Tetracycline sometimes interferes with the activity of penicillin. How? 20-18
Future of Chemotherapeutic Agents Learning Objective 20-19 Identify three areas of research on new chemotherapeutic agents.
Future of Chemotherapeutic Agents Antimicrobial peptides Broad-spectrum antibiotics Nisin (lactic acid bacteria) Defensins (human) Magainin (frogs) Squalamine (sharks) Phage therapy
Figure 9.14 Gene silencing could provide treatments for a wide range of diseases. Nucleus DNA RNA transcript An abnormal gene, cancer gene, or virus gene is transcribed in a host cell. mrna sirna binds mrna. sirna RISC breaks down the RNA complex. Cytoplasm No protein expression occurs.
Check Your Understanding What are defensins? 20-19