Lecture 1.4: Antibiotics and Antibiotic Resistance Lecture

What is the definition of a chemotherapeutic agent?

Chemical compounds used to treat disease.

What is the function of antimicrobials?

To destroy pathogenic microbes or inhibit their growth within a host.

What is the specific target of antibiotics?

Antibiotics destroy or inhibit bacteria.

Most antibiotics are derived from what sources?

Microbial products or their derivatives.

Which bacterial genus is the source of the antibiotic Amphotericin B?

$$Streptomyces\,spp.$$

Which bacterial genus is the source of Chloramphenicol?

$$Streptomyces\,spp.$$

Which bacterial genus is the source of Erythromycin?

$$Streptomyces\,spp.$$

Which bacterial genus is the source of Kanamycin?

$Streptomyces\,spp.$, although Chloramphenicol is also synthetic.

Which genus produces Neomycin?

$$Streptomyces\,spp.$$

Which bacterial genus is the source of Nystatin?

$$Streptomyces\,spp.$$

Which bacterial genus is the source of Rifampin?

$$Streptomyces\,spp.$$

Which bacterial genus is the source of Streptomycin?

$$Streptomyces\,spp.$$

Which bacterial genus is the source of Tetracyclines?

$$Streptomyces\,spp.$$

Which bacterial genus is the source of Vancomycin?

$$Streptomyces\,spp.$$

Which bacterial genus produces Gentamicin?

$$Micromonospora\,spp.$$

Which bacterial genus produces Bacitracin?

$$Bacillus\,spp.$$

Which bacterial genus produces Polymyxins?

$$Bacillus\,spp.$$

Which fungal genus provides the source for Griseofulvin?

$$Penicillium\,spp.$$

Which fungal genus provides the source for Penicillin?

$$Penicillium\,spp.$$

Which fungal genus provides the source for Cephalosporins?

$$Cephalosporium\,spp.$$

Who discovered penicillin and in what year?

Fleming in 1928.

According to the Scientific American article, when was Fleming's original penicillin culture sold at auction?

August 18, 2015.

How much was paid for Fleming's original penicillin culture specimen at the July 2015 auction?

$\pounds 4649$ (approximately $\$7000$).

What is selective toxicity?

The ability of a drug to kill or inhibit a pathogen while damaging the host as little as possible.

Define therapeutic dose.

The drug level required for clinical treatment.

Define toxic dose.

The drug level at which the drug becomes too toxic for the patient (produces side effects).

What is the therapeutic index?

The ratio of the toxic dose to the therapeutic dose.

What are bacteriocidal antibiotics?

Antibiotics that kill bacteria.

What are bacteriostatic antibiotics?

Antibiotics that inhibit the growth of bacteria.

What are broad-spectrum antibiotics?

Antibiotics that attack many different bacteria (both Gram + and Gram -).

What are narrow-spectrum antibiotics?

Antibiotics that attack only a few different bacteria.

What does MIC stand for?

Minimal inhibitory concentration.

Define minimal inhibitory concentration (MIC).

The lowest concentration of a drug that inhibits the growth of a pathogen.

What does MBC stand for?

Minimal bacteriocidal concentration.

Define minimal bacteriocidal concentration (MBC).

The lowest concentration of a drug that kills the pathogen.

How is MIC determined in dilution susceptibility tests?

The broth or agar with the lowest concentration of the drug showing no growth.

How is MBC determined in broth dilution susceptibility tests?

Tubes showing no growth are subcultured into drug-free medium; the broth from which the microbe cannot be recovered is the MBC.

In the provided images, what was the MIC of Clindamycin for Group B Streptococcus?

$>32\,\mu g/ml$, classified as Resistant.

In the provided images, what was the MIC of Penicillin for Group B Streptococcus?

$0.06\,\mu g/ml$, classified as Sensitive.

In the provided images, what was the MIC of Erythromycin for Group B Streptococcus?

$8\,\mu g/ml$, classified as Resistant.

What are disk diffusion tests?

Tests where disks impregnated with specific drugs are placed on inoculated agar plates to establish a concentration gradient and observe clear zones.

What is the Kirby-Bauer method?

A standardized method for carrying out the disk diffusion test.

How is degree of microbial resistance determined in the Kirby-Bauer method?

By using tables that relate zone diameter to the degree of resistance.

What are clear zones around the disks in a diffusion test called?

Zones of inhibition.

What are the general steps of the Kirby-Bauer method?

1. Inoculate plate evenly with bacteria; 2. Place discs containing antibiotics/controls; 3. Incubate for a defined time/temperature; 4. Measure zone of inhibition.

According to the Fluconazole sensitivity images, what is the MIC for the Wild-type strain?

$0.50$ to $0.75$ (units as per the scale shown on the E-test strip).

According to the Fluconazole sensitivity images, what is the MIC for the FLU-sensitive strain?

$0.125$ to $0.19$ (units as per the scale shown on the E-test strip).

What must the concentration of drug at the infection site be to be effective?

It must be $> MIC$.

What types of assays can be used to determine the concentration of drug in the blood?

Microbiological, chemical, immunological, enzymatic, or chromatographic assays.

What three factors influence the effectiveness of antimicrobial drugs?

1. Ability of drug to reach site of infection; 2. Ability of drug to exceed MIC; 3. Susceptibility of pathogen to drug.

The ability of a drug to reach the site of infection depends in part on what?

The mode of administration.

Why can some drugs not be taken orally?

They may be destroyed by stomach acid.

What are parenteral routes of administration?

Nonoral routes of administration.

What anatomical structures/conditions can exclude a drug from an infection site?

Blood clots or necrotic tissue.

What four factors influence the ability of a drug to reach concentrations exceeding the MIC?

1. Amount administered; 2. Route of administration; 3. Speed of uptake; 4. Rate of clearance (elimination).

What two biological factors regarding the pathogen influence drug susceptibility?

1. Whether the drug requires bacterial cell growth to be effective; 2. The speed of action (how many generations it takes to kill or stop growth).

What is the ideal characteristic for a drug's mechanism of action to ensure a high therapeutic index?

The targeted function should be very specific to the pathogen.

List the main classes of antibiotics that disrupt the bacterial cell wall.

$\beta$-lactams, Glycopeptides, and Polypeptides.

Name the four types of $\beta$-lactam antibiotics.

Penicillins, cephalosporins, carbapenems, and monobactams.

Name two glycopeptide antibiotics.

Vancomycin and teichoplanin.

Name two polypeptide antibiotics.

Bacitracin and polymyxins.

List three other anti-Mycobacterial antibiotics besides cycloserine.

Isoniazid, ethionamide, and ethambutol.

What cell wall component is unique to bacteria and found in most pathogens?

Peptidoglycan.

Where does the peptidoglycan repeat unit form in the cell?

In the cytoplasm.

What carrier is used in the formation of the peptidoglycan repeat unit in the cytoplasm?

Uridine diphosphate (UDP).

What molecule transports the peptidoglycan repeat unit across the membrane?

Bactoprenol (a lipid).

What process forms the cross-links in peptidoglycan?

Transpeptidation.

Which antibiotic inhibits the D-Ala-D-Ala formation step in the cytoplasm?

Cycloserine.

Which antibiotic inhibits the recycling of the lipid carrier (bactoprenol) in peptidoglycan synthesis?

Bacitracin.

How many peptide bonds are exchanged during transpeptidation?

One peptide bond is exchanged for another.

In $Escherichia\,coli$, which molecules are involved in the transpeptidation cross-link?

DAP (Diaminopimelic acid) and D-Ala.

In $Staphylococcus\,augureus$, what structure forms the cross-link bridge in peptidoglycan?

A peptide interbridge consisting of five Gly residues (-Gly-Gly-Gly-Gly-Gly-).

What is the specific mechanism by which $\beta$-lactam antibiotics work?

They inhibit the transpeptidation reaction.

What is the structural core of penicillins?

6-aminopenicillanic acid.

Which specific structural feature of penicillins is attacked by penicillinases?

The $\beta$-lactam ring.

What are the characteristics of Penicillin G?

High activity against Gram-positives, low against Gram-negatives, destroyed by acid and penicillinase.

How does Penicillin V differ from Penicillin G?

It is more acid resistant.

Which penicillin is active against Gram-negative bacteria like $Pseudomonas$ and $Proteus$ and is acid stable?

Carbenicillin.

Which penicillin is active against both Gram-positives and Gram-negatives and is acid stable?

Ampicillin.

What are the characteristics of Methicillin?

Penicillinase-resistant, but less active than Penicillin G and acid-labile.

Which penicillin is more active against $Pseudomonas$ than carbenicillin?

Ticarcillin.

What is the nucleus/structural core of cephalosporins?

7-aminocephalosporanic acid.

Give an example of a first-generation cephalosporin.

Cephalothin.

Give an example of a second-generation cephalosporin.

Cefoxitin.

Name two examples of third-generation cephalosporins.

Cefoperazone and Ceftriaxone.

What are carbapenems and monobactams?

Two newer classes of $\beta$-lactam antibiotics.

What is the function of $\beta$-lactamase inhibitors?

They prevent the degradation of $\beta$-lactam antibiotics by $\beta$-lactamases.

Are $\beta$-lactamase inhibitors considered antibiotics?

No, they are used in combination with $\beta$-lactam antibiotics.

Name three examples of $\beta$-lactamase inhibitors.

Clavulanic acid, sulbactam, and tazobactam.

What was the first combination drug of a $\beta$-lactam and a $\beta$-lactamase inhibitor, and what were its components?

Augmentin; composed of amoxicillin and clavulanic acid.

What is the mechanism of action for Vancomycin?

It binds to the terminal D-Ala-D-Ala and sterically inhibits the addition of peptidoglycan subunits and the transpeptidation reaction.

Vancomycin is especially important for treating which types of infections?

Antibiotic resistant staphylococcal and enterococcal infections.

To which class of molecules do Vancomycin and Teichoplanin belong?

Glycopeptides.

What is the specific action of Bacitracin?

It prevents the recycling of the lipid carrier (bactoprenol).

What is the mechanism of action for Polymyxins?

They bind to phospholipids and disrupt the outer and inner membranes of Gram-negative bacteria.

Why are Polymyxins typically used topically?

They have a general mode of action that can be toxic to host cells.

Cycloserine is used as a second-line treatment for which disease?

$Mycobacterium\,tuberculosis$.

Cycloserine is a cyclic analog of which amino acid?

Alanine.

Besides being an antimicrobial, Cycloserine acts as an agonist for which receptor?

NMDA receptor (it also crosses the blood-brain barrier).

How does Isoniazid inhibit Mycobacteria?

By affecting the synthesis of mycolic acid.