Chapter 14 - Antimicrobial Drugs

use of antimicrobials in ancient societies

• there is evidence that humans have been exposed to antimicrobial compounds for millennia, not just in the last century

• antimicrobial properties of certain plants were also recognized by various cultures

first antimicrobial drugs - early 1900s

Paul Ehrlich and his assistant Sahachiro Hata

Paul Ehrlich and his assistant Sahachiro Hata

found compound 606 (which killed Treponema pallidum), sold under the name Salvarsan

first antimicrobial drugs - 1928

Alexander Fleming

Alexander Fleming

discovered penicillin, the first natural antibiotic

first antimicrobial drugs - 1930s

Klarer, Mietzsch, and Domagk

Klarer, Mietzsch, and Domagk

discovered prontosil - killed streptococcal and staphylococcal infections

• the active breakdown product of prontosil is sulfanilamide

• sulfanilamide was the first synthetic antimicrobial created

first antimicrobial drugs - early 1940s

Dorothy Hodkin

Dorothy Hodkin

determined the structure of penicillin using X-rays

• scientists could then modify it to produce semisynthetic penicillins

first antimicrobial drugs - 1940s

Selman Waksman

Selman Waksman

his research team discovered several antimicrobials produced by soil microorganisms

chemotherapeutic agent or drug

any chemical agent used in medical practice

antibiotic agent

usually considered to be a chemical substance made by a microorganism that can inhibit the growth of or kill microorganisms

antimicrobic or antimicrobial agent

a chemical substance similar to an antibiotic, but may be synthetic

antibiotic

usually one bacterial target, e.g. a key bacterial enzyme is blocked

antimicrobial

a broad term but can often mean multiple targets, e.g. membranes and DNA

selective toxicity

harms microbes but does not damage the host

chemotherapeutic index

maximum tolerable dose per kg of body weight / minimum dose per kg of body weight which cures the disease

no single chemotherapeutic agent affects

all microbes

antimicrobial drugs are classified based on

the type of organism they affect (ex. antibacterial, antifungal, etc)

narrow spectrum of activity

targets only specific subsets of bacterial pathogens

broad spectrum of activity

targets a wide variety of bacterial pathogens, including both Gram-positive and Gram-negative species

3 steps in the development of superinfections

1.) Normal microbiota keeps opportunistic pathogens in check

2.) Broad-spectrum antibiotics kill non resistant cells

3.) Drug-resistant pathogens proliferate and can cause a superinfection

3 types of antibiotic activity

1.) Bacteriostatic

2.) Bactericidal

3.) Bacteriolytic

bacteriostatic

inhibits bacterial growth and reproduction without killing the bacteria

bactericidal

kills bacteria directly

bacteriolytic

kills bacteria by lysing (breaking open) their cell walls

in vitro

outside of a living organism

the in vitro effectiveness of an agent is determined by

how little of it is needed to stop growth

minimal inhibitory concentration (MIC)

the lowest concentration of the drug that will prevent the growth of an organism

the Kirby-Bauer assay uses

a series of round filter paper disks impregnated with different antibiotics

a Kirby-Bauer dispenser delivers up to

12 disks simultaneously to the surface of an agar plate covered by a bacterial lawn

standard medium used in a Kirby-Bauer test

Mueller-Hinton agar

Kirby-Bauer test - during incubation, the drugs

diffuse away from the disks into the surrounding agar and the diameter of the zone of inhibition can be measured to determine drug susceptibility

E-test (AB Biodisk)

a commercially prepared strip that produces a gradient of antibiotic concentration (ug/ml) when placed on an agar plate

E-test (AB Biodisk) - the MIC corresponds to

the point where bacterial growth crosses the numbered strip

neither the MIC test (E-test) nor the Kirby-Bauer test can

distinguish whether the drug is bacteriostatic or bactericidal

the minimum bactericidal concentration (MBC) is determined by

using a tube dilution test and removing the antibiotic

• if cells grow in the fresh medium without antibiotic, the drug is bacteriostatic

• if cells do not grow, the drug is bactericidal

8 attributes of an ideal antimicrobial

1.) Solubility of body fluids

2.) Selective toxicity

3.) Toxicity not easily altered

4.) Non-allergenic (no side effects)

5.) Stability

6.) Resistance by microorganisms not easily acquired

7.) Long shelf-life

8.) Reasonable cost (affordable)

dosage

amount of medication given during a certain time interval

dosage in children

based upon the patient’s weight

dosage in adults

a standard dosage is used, independent of weight

for dosage - need to take into consideration the

half-life of the antibiotic

half-life

rate at which 50% of a drug is eliminated from the plasma

3 routes of administration

1.) Oral

2.) IM (intramuscular)

3.) IV (intravenous)

one of the most important decisions a clinician must take when treating an infection is

which antibiotic to prescribe

3 things a clinician prescribing an antibiotic needs to keep in mind

1.) Whether the organism is susceptible to the antibiotic

2.) Whether the attainable tissue level of the antibiotic is higher than the MIC

3.) The understanding of the relationship between the therapeutic dose and the toxic dose of the drug

therapeutic dose

the minimum dose per kg of body weight that stops pathogen growth

toxic dose

the maximum dose tolerated by the patient

chemotherapeutic index

the ratio of the toxic dose to therapeutic dose

combinations of antibiotics can be either

synergistic or antagonistic

synergistic drugs

may work poorly when they are given individually, but very well when combined (combined effect is greater than additive effect)

example of synergistic drugs

aminoglycoside and vancomycin

antagonistic drugs

mechanisms of action interfere with each other and diminish their effectiveness

example of antagonistic drugs

penicillin + macrolides

how do antibiotics work?

antibiotics exhibit selective toxicity because they disturb enzymes or substrates unique to the target cell

6 mechanisms targeted by antibiotics

1.) Cell wall synthesis

2.) Cell membrane integrity

3.) DNA synthesis

4.) RNA synthesis

5.) Protein synthesis

6.) Metabolism

3 classes of antibiotics that target cell wall synthesis

1.) Beta-lactams

2.) Glycopeptides

3.) Bacitracin

4 examples of beta lactams that target cell wall synthesis

1.) Penicillins

2.) Cephalosporins

3.) Monobactams

4.) Carbapenems

example of glycopeptide that targets cell wall synthesis

vancomycin

2 classes of antibiotics that target plasma membrane integrity

1.) Polymyxins

2.) Lipopeptide

2 examples of polymyxins that target plasma membrane integrity

1.) Polymyxin B

2.) Colistin

example of lipopeptide that targets plasma membrane integrity

daptomycin

class of antibiotics that targets DNA synthesis

fluoroquinolones

3 examples of fluoroquinolones that target DNA synthesis

1.) Ciprofloxacin

2.) Levofloxacin

3.) Moxifloxacin

class of antibiotics that targets RNA synthesis

rifamycins

example of rifamycin that targets RNA synthesis

rifampin

2 classes of antibiotics that target the 30S subunit of ribosomes

1.) Aminoglycosides

2.) Tetracyclines

4 classes of antibiotics that target the 50S subunit of ribosomes

1.) Macrolides

2.) Lincosamides

3.) Chloramphenicol

4.) Oxazolidinones

2 metabolic pathways targeted by antibiotics

1.) Folic acid synthesis

2.) Mycolic acid synthesis

2 classes of antibiotics and 1 antibiotic that targets folic acid synthesis

1.) Sulfonamides (class)

2.) Sulfones (class)

3.) Trimethoprim (antibiotic)

example of antibiotic that targets mycolic acid synthesis

isoniazid

penicillin-binding proteins (PBPs)

the enzymes that attach the disaccharide units to preexisting peptidoglycan and produce peptide cross links

penicillin

a bactericidal drug → without an intact cell wall, the growing cell eventually bursts due to osmotic effects

cephalosporins

beta-lactam antibiotic originally discovered in nature but modified in the laboratory - a type of semisynthetic drug

chemists have modified the basic structure of cephalosporin in ways that

improve the drug’s effectiveness against penicillin-resistant pathogens → each modification is a new “generation” of cephalosporins

5 generations of cephalosporins

• 1st generation = cephalexin

• 2nd generation = cefoxitin

• 3rd generation = ceftriaxone

• 4th generation = cefepime

• 5th generation = ceftaroline

2 polypeptide antibiotics that inhibit cell wall synthesis

1.) Bacitracin

2.) Vancomycin

bacitracin

topical application, against Gram-positives

vancomycin

glycopeptide, important “last line” against antibiotic resistant S. aureus

2 antimycobacterial antibiotics that inhibit cell wall synthesis

1.) Isoniazid (INH)

2.) Ethambutol

isoniazid (INH)

inhibits mycolic acid synthesis

ethambutol

inhibits incorporation of mycolic acid

4 antibiotics that target the bacterial membrane

1.) Polymyxin

2.) Tyrocidine

3.) Platensimycin

4.) Gramicidin (cyclic peptide)

polymyxin, tyrocidine, and platensimycin

• act as detergents and disrupt the structure of the cell membrane by binding to the phospholipids

• highly toxic

mode of action of polymyxin, tyrocidine, and platensimycin

they interact with LPS in the outer membrane of Gram-negative bacteria, killing the cell through the eventual disruption of the outer membrane and cytoplasmic membrane

mode of action of gramicidin

inserts into the cytoplasmic membrane of Gram-positive bacteria, disrupting the membrane and killing the cell (pokes holes)

3 antibiotics that affect DNA synthesis and integrity

1.) Metronidazole

2.) Sulfonamides

3.) Quinolones

metronidazole

• aka flagyl

• activated after being metabolized by microbial protein cofactors ferredoxin found in anaerobic and microaerophilic bacteria such as Bacteroides and Fusobacterium

• aerobic microbes are resistant because they do not possess the electron transport proteins capable of reducing metronidazole

sulfonamides

• sulfonamide (sulfa) drugs act to inhibit the synthesis of nucleic acids by preventing the synthesis of folic acid, an important cofactor in the synthesis of nucleic acid precursors

• all organisms use folic acid to synthesize nucleic acids

all organisms use folic acid to synthesize nucleic acids

• bacteria make folic acid from the combination of PABA, glutamic acid, and pteridine

• mammals do not synthesize folic acid and must get it from the diet or microbes

quinolones

• DNA gyrase bound to and inactivated by a quinolone will block progression of a DNA replication fork

• quinolone antibiotics will not affect mammalian DNA replication because bacterial DNA gyrases are structurally different from mammalian

rifampin

• RNA synthesis inhibitor

• best-known member of the rifamycin family of antibiotics that selectively binds to bacterial DNA polymerase and prevents transcription

• also used to treat tuberculosis and meningococcal meningitis

mode of action of antibiotics that inhibit protein synthesis

the major classes of protein synthesis inhibitor target the 30S or 50S subunits of cytoplasmic ribosomes

3 drugs that affect the 30S ribosomal subunit

1.) Aminoglycosides

2.) Tetracyclines

3.) Glycylcyclines

aminoglycosides

• streptomycin, gentamicin, tobramycin

• cause misreading of mRNA and inhibit peptidyl-tRNA translocation

tetracyclines

• doxycycline, minocycline

• bind to the 30S subunit and prevent tRNAs carrying amino acids from entering the A site

glycylcyclines

• tigecycline

• bind to 30S subunit and inhibit the entry of aminoacyl-tRNA into the A site; able to function in tetracycline resistant cells

5 drugs that affect the 50S ribosomal subunit

1.) Chloramphenicol

2.) Macrolides

3.) Lincosamides

4.) Oxazolidinones

5.) Streptogramins

chloramphenicol

prevents peptide bond formation by inhibiting peptidyl transferase in the 50S subunit