Week 13:

Antimicrobial Drugs

Tuesday, April 18, 2023                                                                                  

Biochemistry, Microbiology and Immunology

Antimicrobial Drugs

• Chemicals used to treat microbial infections

• Before antimicrobials, large number of people died from common illnesses

• Now many illnesses easily treated with antimicrobials

• However, many antimicrobial drugs are becoming less useful

Antimicrobial Drugs

• Chemotherapeutic agent: Drugs that act against disease• Antimicrobial drug: Drugs that treat infections

• Different types of antimicrobial drugs:– Antibacterial drugs– Antifungal drugs– Antiprotozoan drugs: The protozoans, unlike bacteria and fungi,

do not have a cell wall. They have a nucleus and a cytoplasm that is surrounded by a selectively permeable cell membrane. The cytoplasm contains organelles similar to those found in other animal and plant cells (e.g., mitochondria, Golgi apparatus, and endoplasmic reticulum). Thus, most of the antibiotics effective in inhibiting bacteria are not active against protozoans.

– Antihelminthic drugs (parasitic worms)

Paul Ehrlich

• He observed that certain dyes stain bacterial cells and not animal cells

• Theorized that there could be a dye or chemical that would harm bacterial cells but not human cells

• Systematic search for chemical to cure syphilis

• 606th compound tested proved to be highly effective in treating laboratory animals

Gerhard Domagk• Prontosil resulted from research, directed by German chemist and

pathologist Gerhard Domagk, on the antibacterial action of azo dyes. A red azo dye of low toxicity, Prontosil was shown by Domagk to prevent mortality in mice infected with Streptococcus bacteria. The dye was also effective in controlling Staphylococcus infections in rabbits. Within a relatively short period, it was demonstrated that Prontosil was effective not only in combating experimental infections in animals but also against streptococcal diseases in humans, including meningitis and puerperal sepsis.

• Later it was found that Enzymes in animals’ blood split Prontosil molecule into sulfonamide- this acted against streptococcal.

• Prontosil has been replaced in clinical use by newer sulfonamide drugs, including sulfanilamide, sulfathiazole, sulfamethoxazole, and other Slpha drugs.

Alexander Fleming• Penicillin was discovered by chance, after Fleming accidentally left

a dish of staphylococcus bacteria uncovered for a few days. He returned to find the dish dotted with bacterial growth, apart from one area where a patch of mould (Penicillin notatum) was growing. The mould produced a substance, named penicillin by Fleming, which inhibited bacterial growth and was later found to be effective against a wide range of harmful bacteria.

• However, it was not until World War II that penicillin, the first antibiotic, was finally isolated by Howard Florey and Ernst Chain.

• Fleming, Florey and Chain received a Nobel prize in 1945, for their discovery which revolutionised medicine and led to the development of lifesaving antibiotics.

Figure 20.1

Features of Antimicrobial Drugs

• Most modern antibiotics come from species of microorganisms that live in the soil

• To commercially produce antibiotic:1. Select strain and grow in broth2. When maximum antibiotic concentration

reached, extract from medium3. Purify4. Chemically alter to make it more stable

Features of Antimicrobial Drugs: Selective Toxicity

• Cause greater harm to microorganisms than to host

• Chemotherapeutic index= lowest dose toxic to patient divided by dose typically used for therapy

Features of Antimicrobial Drugs: Antimicrobial Action

• Bacteriostatic: inhibit growth of microorganisms

• Bactericidal: Kill microorganisms

Features of Antimicrobial Drugs:

Spectrum of Activity

• Antimicrobial medications vary with respect to the range of microorganisms they kill or inhibit

• Some kill only limited range : Narrow-spectrum antimicrobial

• While others kill wide range of microorganisms: Broad-spectrum antimicrobial

Features of Antimicrobial Drugs:

Effects of Combining Drugs

• Combinations are sometimes used to fight infections

• Synergistic: action of one drug enhances the activity of another

• Antagonistic: activity of one drug interferes with the action of another

Routes of Administration

• – Topical application of drug for external infections• – Oral route requires no needles and is self-• administered• – Intramuscular administration delivers drug via• needle into muscle• – Intravenous administration delivers drug directly to• bloodstream• – Know how antimicrobial agent will be distributed to• infected tissues

The effect of route of administration on blood levels of

a chemotherapeutic

agent

Administration method

Oral

Intramuscular(IM)

Continuousintravenous

(IV)

Time (hours)

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Clinical Considerations in Prescribing Antimicrobial Drugs

• Safety and Side Effects– Toxicity

• Cause of many adverse reactions poorly understood

• Drugs may be toxic to kidneys, liver, or nerves• Consideration needed when prescribing drugs to

pregnant women

– Allergies• Allergic reactions are rare but may be life

threatening such as Anaphylactic shock

Some side effects resulting from toxicity of antimicrobial agents

Features of Antimicrobial Drugs:

Adverse Effects

1. Allergic Reactions: some people develop hypersensitivities to antimicrobials

2. Toxic Effects: some antimicrobials toxic at high concentrations or cause adverse effects

3. Suppression of normal flora: when normal flora killed, other pathogens may be able to grow to high numbers

Features of Antimicrobial Drugs:

Resistance to Antimicrobials

• Some microorganisms inherently resistant to effects of a particular drug

• Other previously sensitive microorganisms can develop resistance through spontaneous mutations or acquisition of new genes

Mechanisms of action of Antibacterial Drugs

1. Inhibit cell wall synthesis

2. Inhibit protein synthesis

3. Inhibit nucleic acid synthesis

4. Injury to plasma membrane

5. Inhibit synthesis of essential metabolites

Figure 20.2

Lactam Drugs- inhibit cell wall synthesis

• Irreversibly inhibit enzymes involved in the final steps of cell wall synthesis

• These enzymes mediate formation of peptide bridges between adjacent stands of peptidoglycan

• lactam ring similar in structure to normal substrate of enzyme

• Drug binds to enzyme, competitively inhibit enzymatic activity

Lactam Drugs

• Some bacteria produce -lactamase- enzyme that breaks the critical -lactam ring

• -lactam drugs include: penicillins and cephalosporins

Antibacterial medications that inhibit protein synthesis

• Target ribosomes of bacteria

• Aminoglycosides: bind to 30S subunit of ribosome causing it to distort and malfunction; blocks initiation of translation

• Tetracyclines: bind to 30S subunit blocking attachment of tRNA

• Macrolides: bind 50S subunit and prevents continuation of protein synthesis

Figure 20.4b

Antibacterial medications that inhibit nucleic acid synthesis

• Target enzymes required for nucleic acid synthesis

• Fluoroquinolones: inhibit enzymes that maintain the supercoiling of closed circular DNA

• Rifamycins: block prokaryotic RNA polymerase from initiating transcription

Antibacterial medications injure plasma membrane

• Polymyxin B: binds to membrane of G- bacteria (gram +, gram-) and alters permeability

• This leads to leakage of cellular contents and cell death

• These drugs also bind to eukaryotic cells to some extent, which limits their use to topical applications

Antibacterial drugs that inhibit synthesis of essential metabolites

• Competitive inhibition by substance that resembles normal substrate of enzyme

• Sulfa drugs

Antiviral Drugs

• Very few antiviral drugs approved for use in US

• Effective against a very limited group of diseases

• Targets for antiviral drugs are various points of viral reproduction

Nucleoside and Nucleotide analogs

• Acyclovir- used to treat genital herpes

• Cidofovir- used for treatment of cytomegaloviral infections of the eye

• Lamivudine- used to treat Hepatitus B

Antiretrovirals

• Currently implies, a drug used to treat HIV

• Tenofovir- nucleotide reverse transcriptase inhibitor

• Zidovudine- nucleoside analog

Other enzyme inhibitors

• Zanamivir (Relenza) and Oseltamivir phosphate (Tamiflu)- inhibitors of the enzyme neurominidase– Used to treat influenza

• Indinavir- protease inhibitors

Interferons

• Interferons inhibit further spread of the infection

• Alpha-interferon- drug for treatment of viral hepatitis infections

Antifungal drugs

• Since fungi are eukaryotic, it is more difficult to find point of selective toxicity in eukaryotes than in prokaryotes

• Targets of antifungal drugs:– Agents affecting fungal sterols– Agents affecting fungal cell walls– Agents inhibiting nucleic acids

Agents affecting fungal sterols

• Many antifungals target the sterols in the plasma membrane

• Polyenes- used in systemic fungal infections, very toxic to kidneys

• Azoles- used for athlete’s foot and vaginal yeast infections (miconizole)

Agents affecting fungal cell walls

• Primary target of selective toxicity is β-glucan

• Inhibition of synthesis of this glucan results in an incomplete cell wall, and results in lysis of the cell

• Caspofungin- first new class of antifungals in 40 years

Agents inhibiting nucleic acids

• Flucytin- nucleotide analog of cytosine, interferes with the biosynthesis of RNA, and therefore protein synthesis

Antiprotozoan drugs

• Quinine is the major one that still is used to control malaria

• Chloroquinone- synthetic derivative of Quinine that has largely replaced it

• Mefloquinone- used in areas where resistance to chloroquinone has developed

• Quinacrine- drug of choice for treating protozoan disease, giardiasis (is an intestinal infection caused by a microscopic, single-celled parasite known as Giardia lamblia).

http://www.umm.edu/graphics/images/en/18139.jpg

Antihelminthic drugs

• Praziquantel- used in treatment of tapeworms; kills worms by altering permeability of plasma membranes

Clinical Considerations in Prescribing Antimicrobial Drugs

• Efficacy– Ascertained by

• Diffusion susceptibility test• Minimum inhibitory concentration test• Minimum bactericidal concentration test

Kirby-Bauer method for determining drug susceptibility

1. Bacteria spread on surface of agar plate

2. 10-12 disks, each with different antimicrobial drug, placed on agar plate

3. Incubated- drugs diffuse outward and kill susceptible bacteria

4. Zone of inhibition around each disk

5. Compare size of zone to chart

Zones of inhibition in a diffusion susceptibility (Kirby-Bauer) test

Bacterial lawn Zone of inhibition

Minimum inhibitory concentration (MIC) test in test tubes

Turbid tubes Clear tubes

Increasing concentration of drug

An Etest combines aspects of Kirby-Bauer and MIC tests

A minimum bactericidal concentration (MBC) test

ClearMIC tube

Concentration of antibacterial drug (µg/ml)

8 µg/ml 16 µg/ml 25 µg/ml

Bacterial colonies No colonies No colonies

Drug-free media

Resistance to antimicrobial drugs

• Drug resistance limits use of ALL known antimicrobials

• Penicillin G: first introduced, only 3% of bacteria resistant

• Now, over 90% are resistant

Slowing the emergence and spread of antimicrobial resistance

1. Responsibilities of Physicians: must work to identify microbe and prescribe suitable antimicrobials, must educate patients

2. Responsibilities of Patients: need to carefully follow instructions

Slowing the emergence and spread of antimicrobial resistance

3. Educate Public: must understand appropriateness and limitations of antibiotics ; antibiotics not effective against viruses

4. Global Impacts: organism that is resistant can quickly travel to another country

- in some countries antibiotics available on non-prescription basis

- antibiotics fed to animals can select for drug- resistant organisms

New Approaches to Antibiotic Therapy Are Needed

• Scientists work to find new antibiotic targets in pathogens

• Discovery of new and unique antibiotics is necessary