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Antibiotics – Microbiology Online Lecture Notes

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Antibiotics Online Lecture Notes

I. History of Antibiotics

A. “Magic Bullet” concept (early 1900’s)

1. Paul Ehrlich proposed the idea of using a drug that would selectively find and kill

the pathogen, but not harm human cells (early 1900’s)

2. Basis of antimicrobial chemotherapy (drug therapy)


B. Discovery of Penicillin (1928)

1. Alexander Fleming noticed that the growth of Staphylococcus aureus was inhibited in

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the area surrounding a colony of mold that contaminated a Petri plate.

2. The compound was later isolated from the mold and named penicillin

3. 1st Antibiotic

4. Antibiotic – a substance produced by one organism that inhibits another organism


C. Sulfanilamide compound in dyes was used (Sulfa Drugs, 1932)

1. Technically an antimicrobial and not an antibiotic, because it is not produced by an

organism, i.e. synthetic drug

2. Resurrected interest in penicillin


D. Florey and Chain (1940, WWII)

1. 1st Successful clinical trial with penicillin

2. Fleming, Chain, and Florey won the Nobel Prize


II. Antibiotic – Production and Range

A. Production

1. Naturally occurring – produced entirely by bacteria or fungi (true antibiotic)

2. Semi-synthetic antibiotics – part natural and part designed in lab

3. Synthetic antibiotics – designed in lab

B. Narrow Spectrum Antibiotic

1. Affect narrow range of bacteria, such as only Gram+


C. Broad Spectrum Antibiotic

1. Affect large range of bacteria, such as Gram+ and Gram-

2. Benefit vs. Disadvantage?

3. Superinfection

a) Kill good bacteria and allow opportunistic organism to flourish, such as

Candida albicans in a yeast infection

b) Also, occurs if the strain causing the infection gains antibiotic resistance to the

antibiotic being used to treat the infection


III. Modes of Action, i.e. How do Antimicrobial Drugs Work?

A. Target differences between our cells and pathogens; selective toxicity

B. Modes of Action

1. Inhibit Cell Wall Synthesis

a. Peptidoglycan is only in bacteria cell walls

2. Inhibit Protein Synthesis – Translation at Ribosome

a. Differences in ribosomes allows for selective toxicity

b. Eukaryotes have 80S (60S and 40S) ribosomes, and prokaryotes have 70S (50S & 30S) ribosomes

*NOTE: Mitochondria contain 70S ribosomes; can affect liver and bone marrow cells

*Translation occurs at ribosomes, mRNA to protein


DNA (gene) —–transcription———-mRNA ——translation——- protein

3. Injure Plasma Membrane

4. Inhibit DNA Replication & Transcription

*NOTE: Transcription produces mRNA from gene on DNA

5. Inhibit Synthesis of Essential Metabolites

C. Drugs Used Against

1. Bacteria

2. Fungi

3. Viruses

4. Protozoans

5. Helminths


IV. Antibacterial Antibiotics


A. Inhibit Cell Wall Synthesis

1. Penicillin – prevents peptidoglycan cross-linking

a. 50 chemically related antibiotics

b. Common core of ß-lactam ring

c. Natural Penicillins

1) Produced from mold Penicillium

2) Penicillin G (injection) and Penicillin V (oral)

3) Against Gram+ (staphylococci, streptococci)

d. Semi-synthetic Penicillins

1) Part produced by mold, part produced in lab

2) Ampicillin – broad spectrum (against Gram+ and Gram-)

3) Amoxicillin and Imipenim – broad spectrum


e. Resistance to Penicillin

1) Penicillinase (a.k.a. ß-lactamase)

a) Breaks ß-lactam ring; inactivates penicillin

2) Staphyloccocus infections resistant to penicillin

a) ß-lactamase gene on plasmid

f. Methicillin (semi-synthetic penicillin) was introduced

1) Initially resistant to penicillinase, but resistance soon appeared

2) Methicillin no longer used


g. Methicillin-Resistant Staphylococcus aureus (MRSA)

1) Pronounced “mersa”

2) Now resistant to most penicillins and cephalosporins

2. Cephalosporins

a. Most common prescribed antibiotics

b. ß-lactam ring; similar mechanism to penicillins

c. 4 Generations

1. Each generation more effective against Gram- (broader spectrum)

d. Examples

1) 1st Generation – cephalexin (Keflex®), Cefazolin (Ancef®)

2) 2nd Generation – cephaclor (Ceclor®)

3) 3rd Generation – cephtriaxone (Rocephin®)

4) 4th Generation – cefepime (Maxipime®)


3. Bacitracin

a. Isolated from Bacillus from a girl named Tracy

b. Topical, against Gram+ staphylococci and streptococci

c. Neosporin®

4. Vancomycin

a. Used to treat MRSA

b. It’s overuse led to Vancomycin-resistant enterococci (VRE)

c. VRE is a Gram+ pathogen


5. Antimycobacterial Antibiotics – inhibits mycolic acid synthesis

a. Mycolic acid is cell wall component of mycobacteria

a. Isoniazid (INH)

1) Against Mycobacterium tuberculosis



B. Inhibitors of Protein Synthesis

1. Aminoglycosides – changes 30S ribsosome, mRNA cannot be read properly

a. Streptomycin – 1st antibiotic agains Gram- bacteria (1944)

b. Neomycin – topical

c. Gentamicin – Pseudomonas infections


2. Tetracyclines

a. Broad spectrum antibiotic; produced by Streptomyces

b. Interfere with attachment of tRNA at 30S ribosome; interrupts protein synthesis

c. Against Gram+, Gram-, and intracellular rickettsias and chlamydias

d. Treat UTI’s

e. Broad spectrum nature leads to suppression of intestinal flora, leading to GI

upsets. Also, superinfections by Candida albicans

f. Can discolor teeth in children and cause liver damage in pregnant woman

g. Doxycycline is semi-synthetic tetracycline


3. Macrolides

a. Interferes with growing peptide at 50S ribosome

b. Erythromycin

1) Spectrum of activity similar to penicillin G

2) Cannot penetrate Gram- bacilli

3) Alternate to penicillin

4) Treat streptococcal and staphylococcal infections

c. Azithromycin (Zithromax®) and clarithromycin (Biaxin®)

1) Macrolides with broder spectrum and tissue penetration

2) Used to treat intracellular bacteria such as Chlamydia

4. Oxazolidinones (ox-a-zō-lēd-i-nōnes)

a. Interferes with interaction between mRNA and ribosomes

b. Completely synthetic

c. Used in response of VRE and MRSA

d. linezolid (Zyvox®)


C. Injury to Plasma Membrane

1. Triclosan – interrupts fatty acid synthesis

*household antimicrobial


D. Inhibit Nucleic Acid (DNA and RNA) Synthesis

1. Fluoroquinolones

a. Ciprofloxacin – Cipro®


E. Inhibit Synthesis of Essential Metabolites

1. Sulfonamides, a.k.a. sulfa drugs

2. Competitive Inhibition; binds to active site of enzymes

3. Structural analog of PABA; interferes with folic acid synthesis

4. Trimethoprim-sulfamethoxazole (TMP-SMZ)

a. Synergistic effect – much more effective when used in combination


V. Antifungal Drugs

A. Affect fungal sterols in plasma membrane; ergosterol

1. Sterol in our plasma membranes is cholesterol


B. Azoles

1. Clotrimazole and miconazole

a. Treat athlete’s foot and vaginal yeast infections

C. Tolnaftate

1. Treat athlete’s foot

2. Mechanism unknown



V. Antiviral, Antiprotozoan, and Antihelminthic Drugs

1. Will cover in future lectures



VI. Considerations When Choosing an Antimicrobial Drug


A. Identify Infectious Agent (pathogen causing disease)

1. Physicians direct examination; using experience will prescribe antibiotic

2. Culture and send to lab

3. Antibiotic is prescribed and if lab results show resistance to (or wrong) antibiotic

prescribed, then there will be patient follow up to get them on the right antibiotic.

4. Also, even if lab (in vitro) supports that correct antibiotic was prescribed, it may not

work in patient (in vivo)

a. Patient may have a few resistant cells that did not show up in lab or there could                           be one than one pathogen, some of which may be resistant to drug




B. Determining Pathogen Sensitivity to Drug (in lab)

1. Expose pure culture of pathogen to several types of drugs in vitro

2. Kirby-Bauer technique

a. Agar disc diffusion test

b. Look for zone of inhibition

3. Etest®

a. Used to determine Minimum Inhibitory Concentration (MIC)

b. MIC is lowest antibiotic concentration (ug/ml) that inhibits growth

4. Broth Dilution Test

a. Used to determine MIC and Minimal Bactericidal Concentration (MBC)

b. MIC and MBC minimize excessive use to decrease chance of tissue toxicity

c. Microdilution plates used in test


C. Determining Drug Safety

1. Therapeutic Index (TI)

a. Ratio of dose toxic to humans compared to minimum therapeutic dose

b. TI = Toxic Dose / MIC


(10 ug/ml)  / (9 ug/ml)  = TI = 1.1


(10 ug/ml)  / (1ug/ml)  = TI = 10 *much better choice


D. Patient Factors

1. Take careful history of patient (check for drug contraindication)

a. Preexisting medical conditions

1) Kidney or liver diseases – metabolizing and excreting drug

b. Allergies to medications (penicillins)

c. Elderly – decreased GI absorption

d. Pregnancy – drugs can cross placenta and affect fetus (tetracycline)

e. Other medications currently being used (Drug interaction)

1) Antacids reduce absorption of isoniazid

2) Tetracycline reduces effect of oral contraceptives


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