LECTURES IN MICROBIOLOGY Antimicrobial Agents LESSON 8 Sofronio Agustin Professor

Lesson 8 Topics

     Antimicrobial Therapy Selective Toxicity Survey of Antimicrobial Agents Microbial Drug Resistance Drug-Host Interaction 2

The Ideal Antimicrobial Drug 3

Selective Toxicity

 An ideal in chemotherapy that an antimicrobial drug kills only pathogenic microbes without harming the host.

 Historically, reminiscent of the bullet” of Paul Ehrlich.

“magic 4

Terms in Chemotherapy  Chemotherapy diseases.

- use of drugs to treat  Antimicrobials - any drug used in treating infectious diseases.

 Antibiotics - substances produced by some microbes that inhibit or kill other microbes.

 Synthetic drugs - antimicrobial compounds synthesized in the laboratory.

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Historical Note in Chemotherapy  1928 – Alexander Fleming discovered penicillin from

Penicillium notatum

.

 1940 Chain – Howard Florey and Ernst performed first clinical trials of penicillin.

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Antibiotics  Naturally occurring  Metabolic products of bacteria and fungi  Reduce competition for nutrients and space  Examples: Bacteria-

Streptomyces, Bacillus

Molds -

Penicillium, Cephalosporium

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Antimicrobial Activity  Narrow-spectrum  Broad-spectrum  Bactericidal  Bacteriostatic 8

Antimicrobial Activity 9

Modes of Action Primary target sites of antimicrobial drugs in bacterial cells.

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Cell Wall Active Agents   Bactericidal Penicillin and Cephalosporins – binds and blocks peptidases involved in cross-linking the glycan molecules.

  Vancomycin – prevents peptidoglycan elongation Cycloserine – inhibits the formation of the basic peptidoglycan subunits 11

Cell Wall Active Agents Antibiotics weaken the cell wall and cause the cell to lyse.

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Cell Wall Active Agents Penicillins and cephalosporins destroy the peptidoglycan layer by disrupting the peptide cross bridges.

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Cell Wall Active Agents Penicillin  Natural penicillins  Semi-synthetic penicillins  Molecular Structure  Thiazolidine ring  Beta-lactam ring  Variable side chain (R group) 14

Penicillins 15

Penicillinase 16

Penicillins  Penicillinase-resistant penicillins  Extended-spectrum penicillins  Penicillins +  -lactamase inhibitors  Carbapenems  Monobactam 17

Penicillins  Bactericidal  Narrow spectrum.

 Used to treat:  Streptococcal  Staphylococcal  Meningococcal, and  Spirochaete infections.

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Cephalosporins  Derived from

Cephalosporium acremonium

 Beta lactam antibiotic like penicillin  Main ring different from penicillin  2 sites for R groups 19

Cephalosporins  Inhibit cell wall synthesis  Broad-spectrum or extended spectrum antibiotic  2nd, 3rd, 4th generations more effective against Gram-negatives 20

Cephalosporins Different R groups allow for versatility and improved effectiveness of cephalosporins.

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Polypeptide Antibiotics Bacitracin  Topical application  Effective against Gram-positives Vancomycin  Glycopeptide  Important "last line" against antibiotic resistant

S. aureus

 Hinders peptidoglycan elongation 22

Mycolic Acid Inhibitors  Antimycobacterial antibiotics  Isoniazid (INH) - inhibits mycolic acid synthesis  Ethambutol - inhibits incorporation of mycolic acid into cell wall 23

Inhibition of Protein Synthesis Various antibiotics and their sites of protein synthesis inhibition on the prokaryotic ribosome.

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Inhibitors of Protein Synthesis Aminoglycosides  Broad-spectrum antibiotics  Changes shape of 30S subunit  Treatment of bubonic plague,STD, and Gram-negative infections  Examples: Streptomycin, neomycin, gentamycin 25

Aminoglycoside Structure

Amino sugars and a six-carbon ring (aminocyclitol) in Streptomycin.

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Streptomyces

Streptomyces sp.

synthesizes many antibiotics such as: aminoglycosides, tetracycline, chloramphenicol, and erythromycin.

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Tetracycline Chemical Structure of Tetracycline  Broad spectrum  Interferes with tRNA attachment  Treat intracellular infections  Risk to pregnant women 28

Chloramphenicol Nitrobenzene ring of chloramphenicol  Broad-spectrum  Binds 50S subunit, inhibits peptide bond formation  Cheap synthetic  Treat typhoid fever  Side effects: Aplastic anemia 29

Erythromycin Lactone ring of erythromycin  A macrolide  Bactericidal  Binds 50s, prevents translocation  Gram positives  Side effects: GI disturbance 30

Streptogramins  A combination drug of quinopristin and dalfopristin  Bactericidal  Binds 50s, inhibits translation  Affect Gram-positives Example: Synercid 31

Oxazolidinones  Bactericidal  Binds 50S, prevents formation of 70S ribosome  Affect Gram-positives Example: Linezolid 32

Injury to Cell Membrane  Polymyxins  Interact with membrane phospholipids  Topical  Combined with Bacitracin and Neomycin as over-the counter antibiotic  Amphotericin B  Anit-fungal agent  Forms complexes with sterols in the membrane  Causes cytoplasmic leakage  Can affect human cell membranes (toxicity) 33

Nucleic Acid Synthesis Inhibitors Rifamycin  Inhibits RNA synthesis  Anti-tuberculosis drug Quinolones and fluoroquinolones  inhibits DNA unwinding enzymes (gyrases)  Urinary tract infections  Ciprofloxacin 34

Nucleic Acid Synthesis Inhibitors Chloroquine  binds and cross-links the double helix  anti-malarial Quinolones - e.g. Cirpofloxacin  inhibits DNA unwinding enzymes (gyrases) Azidothymidine (AZT)  Antiviral  Analogs of purines and pyrimidines 35

Sulfa Drugs     Analogs of important metabolites (folic acid) Competitive enzyme inhibition Prevents the metabolism of DNA, RNA, and amino acid Examples: Sulfonamides, and trimethoprim 36

Sulfa Drugs Sulfonamides compete with PABA for the active site on the enzyme.

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Sulfonamides Attachment of different R groups to the main structural nucleus affords versatility of sulfonamides.

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Sulfonamides  Synthetic drug derived from dyes (Prontosil of Domagk)  Synergistic combination as Trimethoprim/Sulfamethoxazole  Treatment of pneumonia in AIDS patients 39

Antifungal Drugs (a) Polyenes (b) Azoles (c) Fluorocytosine 40

Antifungal Drugs Amphotericin B  Polyene derivative  Affects sterols in fungal membrane  Causes cytoplasmic leakage  Can affect human cell membranes (nephrotoxicity)  For systemic fungal infections 41

Antifungal Drugs  Azoles- Miconazole, Triazoles  Inhibit ergosterol synthesis  For cutaneous fungal infections 42

Antifungal Drugs Echinocandins  Inhibit synthesis of  -glucan, cell wall component in yeasts  Used against

Candida

and

Pneumocystis

infections 43

Antifungal Drugs Fluorocytosine (5-FC)  Cytosine analog, interferes with RNA synthesis  Used in serious systemic fungal infections  For Amphotericin B resistant fungi 44

Antifungal Drugs Pentamidine isothionate  May bind DNA  For Pneumocystis infections Griseofulvin  Inhibition of microtubules (mitosis)  For superficial mycoses Tolnaftate  Action unknown  For Athlete’s foot 45

Antiprotozoal Drugs Chloroquine  Inhibits DNA synthesis  For Malaria Metronidazole  Damages DNA  For Entamoeba, Trichomonas infections 46

Antihelminthic Drugs Niclosamide  Prevents ATP generation  For Tapeworms Praziquantel  Alters membrane permeability  For Flatworms Pyrantel pamoate  Neuromuscular block  Intestinal roundworms 47

Antihelminthic Drugs Mebendazole  Inhibits nutrient absorption  For intestinal roundworms Ivermectin  Paralyzes worm  For intestinal roundworms 48

Antiviral Drugs

 Few antiviral drugs available  Selective toxicity difficult - viruses are intracellular in host cells  Targets in viral replication cycle: -Entry -Nucleic acid synthesis -Assembly and release  Interferons – natural or artificial 49

Antiviral Drugs

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Antiviral Drugs

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Antiviral Drugs

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Antiviral Drugs

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Antimicrobial Agents 54

Antimicrobial Agents 55

Antimicrobial Agents 56

Antimicrobial Therapy  Identify infectious agent  Susceptibility testing  Minimum Inhibitory Concentration (MIC)  Minimum Bactericidal Concentration (MBC) 57

Kirby-Bauer Test 58

Kirby-Bauer Test The Kirby-Bauer Test is used to determine the effectiveness of a drug by measuring the zone of inhibition.

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E-Test The E-test as an alternative method to the Kirby-Bauer test 60

Dilution Methods  The dilution test determines actual MIC values.

 Correlated with in vivo reactions  More accurate and standardized  Modern micro-dilution techniques are used in automated methods.

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MIC Comparative MIC values for sample bacterial isolates 62

Combination Therapy  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.

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Synergism 64

Drug-Host Interaction  Toxicity to organs  Allergic reactions  Suppression or alteration of microbiota  Effective drugs 65

Drug-Induced Side Effects Tetracycline treatments can cause teeth discoloration 66

Disruption of Microbiota Disrupting the microbiota in the intestine can result in superinfections 67

Drug Toxicity 68

Antimicrobial Resistance    A variety of mutations can lead to antibiotic resistance.

Mechanisms of antibiotic resistance 1. Enzymatic destruction of drug 2. Prevention of penetration of drug 3. Alteration of drug's target site 4. Rapid ejection of the drug Resistance genes are often on plasmids or transposons that can be transferred between bacteria.

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Antimicrobial Resistance 70

Antimicrobial Resistance Intermicrobial transfer of plasmids bearing resistance genes R factors) by conjugation, transformation, and transduction.

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Natural Selection 72

Antimicrobial Resistance Misuse of antibiotics selects for resistance mutants. Misuse includes:  Using outdated, weakened antibiotics  Using antibiotics for the common cold and other inappropriate conditions   Use of antibiotics in animal feed Failure to complete the prescribed regimen  Using someone else's leftover prescription 73

New Approaches To counter emergence of drug resistance requires new approaches to drug development.

 Prevent iron –scavenging capabilities of microbes  Inhibit genetic controls (riboswitches)  Probiotics and prebiotics 74

Future Approaches Antimicrobial peptides broad spectrum antibiotics from plants and animals  Squalamine (sharks)  Protegrin (pigs)  Magainin (frogs) Antisense agents -complementary DNA or peptide nucleic acids that binds to a pathogen's virulence gene(s) and prevents transcription 75