Lecture 1: The mode of action of antibiotics

Question 1

What is biofilm?

Answer 1

Assemblies of bacteria attached to surfaces and encapsulated in a matrix (extracellular polymeric substance)

Question 2

What types of surfaces are biofilms found on?

Answer 2

• Living tissues
• medical devices (e.g., artificial valves)
• environmental surfaces.

Question 3

Is it true that the biofilm found on medical devices is less complex?

Answer 3

Yes, it is made up of: a single, coccoid organism and the associated (EPS) matrix.

Question 4

How does biofilm form?

Answer 4

• Bacteria initially exist in a planktonic (free-floating) form, until motile cells attach to a surface.
• The cells consequently lose their motility
• The bacterial cells secrete polysaccharides, forming a protective matrix.
• The bacteria grow within the matrix. They will grow and aggregate.
• Some bacteria revert to the planktonic form and disperse to form new biofilms.

Question 5

Biofilms in the environment:

Answer 5

Contain multiple species of bacteria, diatoms, corrosion products, and debris.

Question 6

Biofilms in hosts:

Answer 6

Usually simpler, with fewer bacterial species and blood components.

Question 7

How does antibiotic resistance occur in biofilms

Answer 7

• Protective Matrix: Acts as a barrier, slowing diffusion of antibiotics and disinfectants.
• Sublethal Exposure: Bacteria within biofilms are exposed to reduced antibiotic concentrations, allowing resistance to develop.
• Selection of Resistant Mutants: Mutant strains adapt and may transfer resistance genes via plasmids to other bacteria.

Question 8

Can all microbes form biofilm?

Answer 8

No, only some

Question 9

What are antibiotics?

Answer 9

Any natural, semisynthetic, synthetic product that has the ability to kill or inhibit the growth of bacteria.

Question 10

What are antimicrobials?

Answer 10

Any product that has the ability to kill or inhibit the growth of microorganism including: bacteria, viruses, fungi.

Question 11

What is the key difference between antibiotics and antimicrobials?

Answer 11

All antibiotics are antimicrobials, but not all antimicrobials are antibiotics.

Question 12

What are the different types of antibiotics?

Answer 12

Bacteriostatic Antibiotic:

Bactericidal Antibiotic:

Question 13

What are Bacteriostatic Antibiotics?

Answer 13

• Antibiotics that reversibly inhibit the growth of bacteria
• Used when the duration of the therapy is sufficient to allow cellular and humoral defence mechanisms to eradicate the bacteria.

Question 14

Is it true that bacteriostatic antibitoics are effective for most infections, but not in immune-compromised sites like the brain or bones?

Answer 14

Yes

Question 15

What do bactericidal antibiotics do?

Answer 15

• They actively kill the susceptible bacteria
• This is irreversible
• Preferred for conditions like meningitis, endocarditis, and osteomyelitis, where the immune response may be insufficient.

Question 16

What can antibiotics act as?

Answer 16

• Cell wall synthesis inhibitors ( β-lactam and Glycopeptides in the cell wall)
• Nucleic acid synthesis inhibitors (quinolones)
• Protein synthesis inhibitors:
• Folate synthesis inhibitors

Question 17

What do the antibitoics β-lactams include?

Answer 17

• penicillins
• cephalosporins
• carbapenems
• monobactams

Question 18

What do the antibitoics glycopeptides include?

Answer 18

Vancomycin

Question 19

What part of a bacterial cell do beta lactams and glycopeptides target?

Answer 19

The bacterial cell wall

Question 20

How do beta lactams work?

Answer 20

• They inhibit penicillin-binding proteins (PBPs), blocking peptidoglycan cross-linking.
• This results in weak cell walls and eventually, cell lysis

Question 21

What are penicillin binding proteins?

Answer 21

• Penicillin-binding proteins (PBPs) are transpeptidases that make cross-links (peptide bonds) between D-amino acid residues in pentapeptides in the bacterial cell walls.
• PBPs are essential for bacterial cell wall synthesis.

Question 22

Give an exmample of a Quinolone antibiotic

Answer 22

Ciprofloxacin

Question 23

Describe quinolone antibiotics

Answer 23

A quinolone antibiotic is a member of a large group of broad-spectrum bacteriocidals that share a bicyclic core structure related to the compound 4-quinolone.

Question 24

How do quinolones work?

Answer 24

• Target bacterial DNA gyrase (topoisomerase II).
• Prevent DNA unwinding needed for replication and transcription.
• Quinolones convert heir targets, topoisomerases (gyrase), into toxic enzymes that fragment the bacterial chromosome.
• Cause double-strand breaks, leading to bacterial death.
• They are specific, so only target bacterial gyrase, sparing human cells.
• Bactericidal

Question 25

What do topoisomerases do?

Answer 25

They regulate supercoiling in DNA synthesis and RNA synthesis

Question 26

What are examples of 30S Ribosomal Subunit Inhibitors?

Answer 26

Aminoglycosides and Tetracyclines

Question 27

Describe how Aminoglycosides work?

Answer 27

- Bind to the decoding region, causing mRNA misreading and faulty protein production. - Faulty proteins compromise the bacterial membrane, leading to cell death. - Bactericidal activity against gram negative aerobes - Their synergy with beta-lactams enhances effectiveness

Question 28

Describe aminoglycosides

Answer 28

An aminoglycoside antibiotic contains amino-sugar structures

Question 29

What do Tetracyclines do?

Answer 29

- Block tRNA entry into the A site of the ribosome, blocking entry of aminoacyl tRNA molecules into the A-site of the ribosome, thus preventing introduction of new amino acids to the growing peptide chain. - They have a Bacteriostatic effect, as this action is usually inhibitory and reversible upon withdrawal of the drug.

Question 30

What are Amphenicols?

Answer 30

- 50S subunit inhibitors - bind to the 50S ribosomal subunit and inhibit the formation of peptide bonds. - Bacteriostatic: as this action is usually inhibitory and reversible upon withdrawal of the drug.

Question 31

What do 50S Ribosomal Subunit Inhibitors include ?

Answer 31

Chloramphenicols and Macrolides (and amphenicols)

Question 32

Give an example of a macrolide

Answer 32

Erythromycin

Question 33

Describe how Chloramphenicols work?

Answer 33

- They prevent peptide bond formation between amino acids - Their effect is bacteriostatic

Question 34

Describe how Macrolides work?

Answer 34

- They block translocation of the peptide chain, causing premature termination. - Their effect is bacteriostatic

Question 35

What do folate synthesis inhibitors include?

Answer 35

Sulfonamides and Trimethoprims

Question 36

What do Sulfonamides and Trimethoprims do?

Answer 36

- Inhibit enzymes in the folic acid synthesis pathway. - Humans obtain folic acid from diet, making this pathway a specific bacterial target.

Question 37

How do gram negative cell walls differ to gram positive cell walls?

Answer 37

- Gram negatives have an extra outer membrane consisting of LPS. Between the LPS and the inner membrane is the periplasmic space. A thin peptidoglycan layer is found within this periplasmic space. - Instead, the cell wall of gram positives consists of a thick outer layer of peptidoglycan.

Question 38

Describe resistance to β-lactam antibiotics

Answer 38

① mutation of PBP, lowering the affinity for penicillins, etc ② down-regulation of porins in Gram-negative bacteria; ③ acquisition of β-lactamase- ESBL (extended spectrum β-lactamase); ④ up-regulation of efflux pumps, which flush out antibiotics

Question 39

For the exams

Answer 39

Revise: - Biofilms and their role in the development of antibiotic resistance. - The difference between bacteriostatic and bactericidal antibiotics. - The main mode of action of each class of antibiotic (Structure of specific antibiotics are not required).

Answer 40

You say 'main mode' of action. so we don't need to go into specifics?

Question 41

What is the modified Hodge test used for?

Answer 41

To identify bacterial strains that produce carbapenemases

Question 42

What is the primary mechanism of methicillin resistance in Staphylococcus aureus?

Answer 42

Acquisition of a new penicillin-binding protein (PBP2')

Question 43

What role does PBP2’ (encoded by mecA) play in methicillin-resistant S. aureus?

Answer 43

It is responsible for cross-liniking peptidoglycan chains with low affinity for methicillin.

Question 44

How does vancomycin inhibit bacterial cell wall synthesis?

Answer 44

By binding the terminal Ala-Ala motif of peptidoglycan peptide chains

Question 45

What is the primary purpose of the coagulase test?

Answer 45

To differentiate Staphylococcus aureus from other Staphylococci

Question 46

What does the enzyme coagulase do?

Answer 46

Converts fibrinogen into fibrin, leading to clot formation

Question 47

How can the coagulase test be performed?

Answer 47

By adding colonies to plasma and observing clot formation

Question 48

What is the purpose of the catalase test?

Answer 48

To differentiate Staphylococcus from other Gram-positive cocci

Question 49

What indicates a positive catalase test?

Answer 49

Gas bubble formation when H₂O₂ is added to a colony

Question 50

What is serotyping primarily used for?

Answer 50

Strain discrimination based on antibody binding to cell surface antigens

Question 51

What is the estimated range of inappropriate antibiotic prescriptions for respiratory tract infections (RTIs) in developed countries?

Answer 51

40%-90%

Question 52

What is the difference between intrinsic and acquired resistance in bacteria?

Answer 52

Intrinsic resistance involves existing genes, while acquired resistance involves gaining new genetic material.

Question 53

Why are Gram-negative bacteria intrinsically resistant to many antibiotics?

Answer 53

They have a double-membrane structure that makes their cell envelope relatively impermeable to many antibiotics

Question 54

Which structural changes in Gram-positive bacteria contribute to reduced permeability to antibiotics?

Answer 54

Porin loss

Question 55

What is the basis of phage-typing in bacteria?

Answer 55

Resistance or susceptibility to specific bacteriophages.

Question 56

Which of the followings is considered anthropologic driver of antibiotic resistance?

Answer 56

- spraying fruits and vegetables with antibiotics - Using antibiotics as growth promoters in animals - prescribing antibiotics for infectious diseases in animals All of the above!!