antimicrobial agents sofronio agustin professor sofronio agustin professor lectures in microbiology...

Antimicrobial AgentsAntimicrobial Agents

Sofronio Agustin

Professor

Sofronio Agustin

Professor

LECTURES IN MICROBIOLOGYLECTURES IN

MICROBIOLOGY

LESSON 8LESSON 8

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Lesson 8 TopicsLesson 8 Topics

Antimicrobial Therapy

Selective Toxicity

Survey of Antimicrobial Agents

Microbial Drug Resistance

Drug-Host Interaction

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The Ideal Antimicrobial DrugThe Ideal Antimicrobial Drug

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Selective ToxicitySelective Toxicity

An ideal in chemotherapy that an

antimicrobial drug kills only pathogenic

microbes without harming the host.

Historically, reminiscent of the “magic bullet” of Paul Ehrlich.

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Terms in ChemotherapyTerms in Chemotherapy

Chemotherapy - use of drugs to treat diseases.

Antimicrobials - any drug used in treating infectious diseases.

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

Synthetic drugs - antimicrobial compounds synthesized in the laboratory.

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Historical Note in ChemotherapyHistorical Note in Chemotherapy

1928 – Alexander Fleming discovered penicillin from Penicillium notatum.

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

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AntibioticsAntibiotics

Naturally occurring Metabolic products of bacteria and fungiReduce competition for nutrients and space

Examples:

Bacteria- Streptomyces, Bacillus

Molds -Penicillium, Cephalosporium

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Antimicrobial ActivityAntimicrobial Activity

Narrow-spectrum

Broad-spectrum

Bactericidal

Bacteriostatic

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Antimicrobial ActivityAntimicrobial Activity

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Modes of ActionModes of Action

Primary target sites of antimicrobial drugs in bacterial cells.

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Cell Wall Active AgentsCell 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

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Cell Wall Active AgentsCell Wall Active Agents

Antibiotics weaken the cell wall and cause the cell to lyse.

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Cell Wall Active AgentsCell Wall Active Agents

Penicillins and cephalosporins destroy the peptidoglycan layer by disrupting the peptide cross bridges.

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Cell Wall Active AgentsCell Wall Active Agents

Penicillin Natural penicillins Semi-synthetic penicillins

Molecular Structure Thiazolidine ring Beta-lactam ring Variable side chain (R group)

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PenicillinsPenicillins

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PenicillinasePenicillinase

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PenicillinsPenicillins

Penicillinase-resistant penicillins

Extended-spectrum penicillins

Penicillins + -lactamase inhibitors

Carbapenems

Monobactam

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PenicillinsPenicillins

BactericidalNarrow spectrum.

Used to treat:

Streptococcal

Staphylococcal

Meningococcal, and

Spirochaete infections.

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CephalosporinsCephalosporins

Derived from Cephalosporium acremonium

Beta lactam antibiotic

like penicillin

Main ring different

from penicillin 2 sites for R groups

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CephalosporinsCephalosporins

Inhibit cell wall synthesis

Broad-spectrum or extended spectrum

antibiotic

2nd, 3rd, 4th generations more effective

against Gram-negatives

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CephalosporinsCephalosporins

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

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Polypeptide AntibioticsPolypeptide Antibiotics

Bacitracin

Topical application

Effective against Gram-positives

Vancomycin

Glycopeptide

Important "last line" against antibiotic resistant S. aureus

Hinders peptidoglycan elongation

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Mycolic Acid InhibitorsMycolic Acid Inhibitors

Antimycobacterial antibiotics

Isoniazid (INH) - inhibits mycolic acid synthesis

Ethambutol - inhibits incorporation of

mycolic acid into cell wall

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Inhibition of Protein SynthesisInhibition of Protein Synthesis

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

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Inhibitors of Protein SynthesisInhibitors of Protein Synthesis

Aminoglycosides

Broad-spectrum antibioticsChanges shape of 30S subunit

Treatment of bubonic plague,STD, and Gram-negative infections

Examples: Streptomycin, neomycin,

gentamycin

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Aminoglycoside StructureAminoglycoside Structure

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

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Streptomyces Streptomyces

Streptomyces sp.

synthesizes many

antibiotics such

as:

aminoglycosides,

tetracycline,

chloramphenicol,

and erythromycin.

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Tetracycline Tetracycline

Broad spectrum

Interferes with tRNA attachment

Treat intracellular infections

Risk to pregnant women

Chemical Structure of Tetracycline

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Chloramphenicol Chloramphenicol

Broad-spectrumBinds 50S subunit,

inhibits peptide bond formation

Cheap syntheticTreat typhoid fever Side effects: Aplastic

anemia

Nitrobenzene ring of chloramphenicol

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Erythromycin Erythromycin

A macrolide

Bactericidal

Binds 50s, prevents

translocation

Gram positives

Side effects:

GI

disturbance

Lactone ring of erythromycin

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Streptogramins Streptogramins

A combination drug of quinopristin and dalfopristin

Bactericidal

Binds 50s, inhibits translation

Affect Gram-positives

Example: Synercid

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Oxazolidinones Oxazolidinones

Bactericidal

Binds 50S, prevents formation of 70S

ribosome

Affect Gram-positives

Example: Linezolid

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Injury to Cell MembraneInjury to Cell Membrane

PolymyxinsInteract with membrane phospholipidsTopicalCombined with Bacitracin and Neomycin as over-the counter antibiotic

Amphotericin BAnit-fungal agentForms complexes with sterols in the membraneCauses cytoplasmic leakageCan affect human cell membranes (toxicity)

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Nucleic Acid Synthesis InhibitorsNucleic Acid Synthesis Inhibitors

RifamycinInhibits RNA synthesis

Anti-tuberculosis drug

Quinolones and fluoroquinolones

inhibits DNA unwinding enzymes (gyrases)

Urinary tract infectionsCiprofloxacin

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Nucleic Acid Synthesis InhibitorsNucleic Acid Synthesis Inhibitors

Chloroquinebinds and cross-links the double helixanti-malarial

Quinolones - e.g. Cirpofloxacininhibits DNA unwinding enzymes (gyrases)

Azidothymidine (AZT)AntiviralAnalogs of purines and pyrimidines

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Sulfa Drugs Sulfa Drugs

Analogs of important metabolites (folic acid)

Competitive enzyme inhibition

Prevents the metabolism of DNA, RNA, and amino acid

Examples: Sulfonamides, and

trimethoprim

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Sulfa Drugs Sulfa Drugs

Sulfonamides compete

with PABA for the active

site on the enzyme.

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SulfonamidesSulfonamides

Attachment of

different R groups

to the main

structural nucleus

affords versatility

of sulfonamides.

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SulfonamidesSulfonamides

Synthetic drug derived from dyes

(Prontosil of Domagk)

Synergistic combination as

Trimethoprim/Sulfamethoxazole

Treatment of pneumonia in AIDS

patients

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Antifungal Drugs Antifungal Drugs

(a) Polyenes (b) Azoles (c) Fluorocytosine

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Antifungal Drugs Antifungal Drugs

Amphotericin B

Polyene derivativeAffects sterols in fungal membraneCauses cytoplasmic leakage

Can affect human cell membranes

(nephrotoxicity)

For systemic fungal infections

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Antifungal Drugs Antifungal Drugs

Azoles- Miconazole, Triazoles

Inhibit ergosterol synthesis

For cutaneous fungal infections

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Antifungal Drugs Antifungal Drugs

Echinocandins

Inhibit synthesis of -glucan, cell wall component in yeasts

Used against Candida and Pneumocystis infections

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Antifungal Drugs Antifungal Drugs

Fluorocytosine (5-FC)

Cytosine analog, interferes with RNA synthesis

Used in serious systemic fungal infections

For Amphotericin B resistant fungi

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Antifungal Drugs Antifungal Drugs

Pentamidine isothionateMay bind DNA

For Pneumocystis infections

GriseofulvinInhibition of microtubules (mitosis)

For superficial mycoses

Tolnaftate Action unknown

For Athlete’s foot

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Antiprotozoal Drugs Antiprotozoal Drugs

Chloroquine

Inhibits DNA synthesis

For Malaria

Metronidazole

Damages DNA

For Entamoeba, Trichomonas infections

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Antihelminthic Drugs Antihelminthic Drugs

Niclosamide

Prevents ATP generation

For Tapeworms

Praziquantel

Alters membrane permeability

For Flatworms

Pyrantel pamoate

Neuromuscular block

Intestinal roundworms

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Antihelminthic Drugs Antihelminthic Drugs

Mebendazole

Inhibits nutrient absorption

For intestinal roundworms

Ivermectin

Paralyzes worm

For intestinal roundworms

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Antiviral DrugsAntiviral Drugs

Few antiviral drugs available Selective toxicity difficult - viruses are

intracellular in host cellsTargets in viral replication cycle:

-Entry-Nucleic acid synthesis-Assembly and release

Interferons – natural or artificial

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Antiviral DrugsAntiviral Drugs

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Antiviral DrugsAntiviral Drugs

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Antiviral DrugsAntiviral Drugs

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Antiviral DrugsAntiviral Drugs

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Antimicrobial AgentsAntimicrobial Agents

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Antimicrobial AgentsAntimicrobial Agents

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Antimicrobial AgentsAntimicrobial Agents

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Antimicrobial TherapyAntimicrobial Therapy

Identify infectious agentSusceptibility testingMinimum Inhibitory Concentration (MIC)Minimum Bactericidal Concentration

(MBC)

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Kirby-Bauer TestKirby-Bauer Test

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Kirby-Bauer TestKirby-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-TestE-Test

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

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Dilution MethodsDilution 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 MIC

Comparative MIC values for sample bacterial isolates

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Combination Therapy 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 Synergism

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Drug-Host InteractionDrug-Host Interaction

Toxicity to organsAllergic reactions

Suppression or alteration of

microbiota

Effective drugs

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Drug-Induced Side EffectsDrug-Induced Side Effects

Tetracycline treatments can cause teeth discoloration

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Disruption of MicrobiotaDisruption of Microbiota

Disrupting the microbiota in the intestine can result in superinfections

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Drug ToxicityDrug Toxicity

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Antimicrobial ResistanceAntimicrobial 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 ResistanceAntimicrobial Resistance

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Antimicrobial ResistanceAntimicrobial Resistance

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

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Natural SelectionNatural Selection

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Antimicrobial ResistanceAntimicrobial 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

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New ApproachesNew Approaches

To counter emergence of drug resistance

requires new approaches to drug

development.Prevent iron –scavenging

capabilities of microbesInhibit genetic controls (riboswitches)Probiotics and prebiotics

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Future ApproachesFuture 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