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ANTIMICROBIAL THERAPHY

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Antimicrobial theraphy in daily life

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Content

• Overview of the mechanism of action of antimicrobial drugs

• Antimicrobial sensitivity test• Mechanism of antibiotic resistance

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Introduction

Discovered by Paul Ehrlich (chemical killing pathogenic m/org wtout injuring the host)

Antimicrobial theraphy ChemotheraphyAntibiotic (antibiosis) – chemical (against life)2 types synthetic

semisynthetic

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General properties

• Selective toxicity• The spectrum of activity • Modes of action• Side effects• Resistance of m/org

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PART 1 – GENERAL PROPERTIES

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Selective toxicity

• To way of actions

internal (eat) external (topical)• Internal – should be selective toxicity (harm to

microbes not the host cell)• Most of the antibiotic act depending on 2 levels - toxic dosage level (cause host damage) - therapeutic dosage level ( eliminates the pathogenic organism)

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Relationship of antibiotic and host

• Measured by chemotherapeutic index –

max dose ÷ min dose = range 1 till 8 kg bw kg bw

• 8 = > effective, < toxic• 1 = > toxic, < effective• Eg. arsenic, mercury, antimony = toxic and effective towards

pathogen• Eg. Treatment for worm infection = damage to parasites and host

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The spectrum of activity

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Modes of action

1. Inhibition of cell wall synthesis2. Disruption of cell membrane function3. Inhibition of protein synthesis4. Inhibition of nucleic acid synthesis5. Action as antimetabolites

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1. Inhibition of cell wall synthesis• Normal mechanism, the peptidoglycan can helps to

maintain the osmotic pressure of the cell• Allow the membrane of the effected microbe to

rupture and release the cell content• Usually affecting Gram positive bacteria• Examples – penicillin, bacitracin, vancomycin and

cephalosporin• All the respective antibiotic contain β-lactam ring that

attach to enzyme and cross-link with peptidoglycan• However, those lack with peptidoglycan (Fungi and

Archaea) were unaffected

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2. Disruption of cell membrane function

• Dissolve the membrane of interfere with the movement of substances into or out of the cells

Polypeptide antibiotic (Polymyxins)• act on bacteria as detergent and distort

bacterial cell membranes by binding with the phospholipid bilayer

• Effects on Gram negative bacteria (rich with phospholipids)

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Cont.

Polyene antibiotic (amphotericin B)• Bind to particular sterols, present in the

membranes of fungal and animal cell

• Polymyxins – do not act on fungi• Polyenes - do not act on bacteria

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3. Inhibition of protein synthesis

• Basically protein synthesis requires DNA, RNA and ribosomes (bacterial – 50S + 30S / animal – 60S + 40S)

• Prevent the growth of microbes by disrupting ribosomes or otherwise, interfering with the process of translation

• Thru selective toxicity (respective attack)Examples: • Streptomycin – from amino acids and glycosidic bonds that

act on 30S portion interfering the translation process• Tetracycline • Erythromycin, chloramphenicol – act on 50S portion and

interfering the growing of polypeptide

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4. Inhibition of nucleic acid synthesis

• Interfere with the synthesis RNA (transcription) or DNA (replication) and disrupt the formation these molecules contain

• The enzyme used by bacterial and animal cells to synthesize nucleic acids provide a means selective action of antimicrobial agents

• Example: rifamycin family (transcription), quinolones (DNA replication) and metronidazole

• Bind to a bacterial RNA polymerase and inhibit RNA synthesis

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5. Action as antimetabolites

• Affect normal metabolites by competitively inhibiting microbial enzymes or by being erroneously incorporated into important molecules such as nucleic acids

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Side effects

• Most of the host will show adverse effects such as toxicity, allergy and disruption of normal microflora

• Most of the antibiotic acts on pathogen, and also the normal flora

• Leads to superinfections will happens where the normal flora will demolished and no host defends in particular area

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Resistance of m/org

• Antibiotic resistance can be a result of horizontal gene transfer, and also of unlinked point mutations in the pathogen genome and a rate of about 1 in 108 per chromosomal replication.

• The antibiotic action against the pathogen can be seen as an environmental pressure; those bacteria which have a mutation allowing them to survive will live on to reproduce.

• They will then pass this trait to their offspring, which will result in a fully resistant colony.

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The four main mechanisms:1. Drug inactivation or modification: e.g. enzymatic deactivation of

Penicillin G in some penicillin-resistant bacteria through the production of β-lactamases.

2. Alteration of target site: e.g. alteration of PBP—the binding target site of penicillins—in MRSAand other penicillin-resistant bacteria.

3. Alteration of metabolic pathway: e.g. some sulfonamide-resistant bacteria do not require para-aminobenzoic acid (PABA), an important precursor for the synthesis of folic acid and nucleic acids in bacteria inhibited by sulfonamides. Instead, like mammalian cells, they turn to utilizing preformed folic acid.

4. Reduced drug accumulation: by decreasing drug permeability and/or increasing active efflux(pumping out) of the drugs across the cell surface.

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Diagram of resistance

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History of resistance• The first antibiotic,

penicillin, was discovered in 1929 by Sir Alexander Fleming, who observed inhibition of Staphylococci on an agar plate contaminated by aPenicillium mold

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In simple words..

• Hereditary drug resistance (R) factors are carried by plasmids and transposons

• Resistance may be due to enzymatic destruction of a drug, prevention of penetration of the drug to its target site, cellular or metabolic changes at target sites, or rapid efflux of the antibiotic

• Resistance can be minimized by the discrimination use of drugs in appropriate concentrations and dosages

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Examples

• Staphylococcus aureus• Pseudomonas aeruginosa• Streptococcus and Enterococcus• Clostridium difficile• Salmonella and E. Coli• Acinetobacter baumannii

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PART 2 – DETERMINING MICROBIAL SENSITIVITY

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1. The Disk Diffusion Method

• Also known as Kirby-Bauer method• The sensitivity compared by size of inhibition

zone around the disk referred to a table of standard measurements

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The sensitivity level

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2. The Dilution Method

• Constant inoculum is placed into broth cultures or well wt differing known quantities of chemotherapeutic agents

2 level of determination:• MIC (minimum inhibitory concentration) - lowest

concentration of growth / no growth at all• MBC (minimum bactericidal concentration) –

lowest cont. in which subculturing of broth yields no growth

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3. Serum Killing Power

• To determine the capability of antibiotic in killing the pathogen

• By adding bacterial suspension to the patient’s serum that already taken the antibiotic for certain period of time

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4. Automated Methods

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Antibacterial agents

• Refer to : slide on modes of action

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Antifungal

• Imidazoles and Triazoles• Polyenes• Griseofulvin• Other agents

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Antiviral agents

• Purine and Pyrimidine Analogs• Amantadine• Interferons and Immunoenhancers

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Antiprotozoan agents

• Quinine• Chloroquine and Primaquine• Metronidazole• Other agents

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Antihelminthic agents

• Niclosamide• Mebendazole• Other agents

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Homeworks

• Please study regarding each of the antimicrobial agents listed in this lecture.

• Focus on the mechanism of actions, resistances and laboratory diagnostic.

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THE END