Lecture 1: The mode of action of antibiotics Flashcards
What is biofilm?
Assemblies of bacteria attached to surfaces and encapsulated in a matrix (extracellular polymeric substance)
What types of surfaces are biofilms found on?
Living tissues, medical devices (e.g., artificial valves), or environmental surfaces.
Is it true that the biofilm found on medical devices is less complex?
Yes, it is made up of: a single, coccoid organism and the associated (EPS) matrix.
How does biofilm form?
- 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. Spores are also formed.
Biofilms in the environment:
Contain multiple species of bacteria, diatoms, corrosion products, and debris.
Biofilms in hosts:
Usually simpler, with fewer bacterial species and blood components.
How does antibiotic resistance occur in biofilms
- 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.
Can all microbes form biofilm?
No, only some
What are antibiotics?
Any natural, semisynthetic, synthetic product that has the ability to kill or inhibit the growth of bacteria.
What are antimicrobials?
Any product that has the ability to kill or inhibit the growth of microorganism including: bacteria, viruses, fungi
What is the key difference between antibiotics and antimicrobials?
All antibiotics are antimicrobials, but not all antimicrobials are antibiotics.
What are the different types of antibiotics?
Bacteriostatic Antibiotic:
Bactericidal Antibiotic:
What are Bacteriostatic Antibiotics?
- 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.
Is it true that bacteriostatic antibitoics are effective for most infections, but not in immune-compromised sites like the brain or bones?
Yes
What do bactericidal antibiotics do?
- They actively kill the susceptible bacteria
- This is irreversible
- Preferred for conditions like meningitis, endocarditis, and osteomyelitis, where the immune response may be insufficient.
What can antibiotics act as?
- Cell all synthesis inhibitors ( β-lactam and Glycopeptides in the cell wall)
- Nucleic acid synthesis inhibitors (quinolones
) - Protein synthesis inhibitors:
- Folate synthesis inhibitors
What do the antibitoics β-lactams include?
penicillins, cephalosporins, carbapenems, monobactams
What do the antibitoics glycopeptides include?
Vancomycin
What part of a bacterial cell to beta lactams and glycopeptides target?
The bacterial cell wall
How do beta lactams work?
- They inhibit penicillin-binding proteins (PBPs), blocking peptidoglycan cross-linking.
- This results in weak cell walls and eventually, cell lysis
What are penicillin binding proteins?
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.
Give an exmample of a Quinolone antibiotic
Ciprofloxacin
Describe quinolone antibiotics
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.
How do quinolones work?
- 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
What do topoisomerases do?
They regulate supercoiling in DNA synthesis and RNA synthesis
What are examples of 30S Ribosomal Subunit Inhibitors?
Aminoglycosides and Tetracyclines
Describe how Aminoglycosides work?
- 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
Describe aminoglycosides
An aminoglycoside antibiotic contains amino-sugar structures
What do Tetracyclines do?
- Block tRNA entry into the A site of the ribosome, locking 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.
What are Amphenicols?
- 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.
What do 50S Ribosomal Subunit Inhibitors include ?
Chloramphenicols and Macrolides (and amphenicols)
Give an example of a macrolide
Erythromycin
Describe how Chloramphenicols work?
- They prevent peptide bond formation between amino acids
- Their effect is bacteriostatic
Describe how Macrolides work?
- They block translocation of the peptide chain, causing premature termination.
- Their effect is bacteriostatic
What do folate synthesis inhibitors include?
Sulfonamides and Trimethoprims
What do Sulfonamides and Trimethoprims do?
- Inhibit enzymes in the folic acid synthesis pathway.
- Humans obtain folic acid from diet, making this pathway a specific bacterial target.
How do gram negative cell walls differ to gram positive cell walls?
- 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.
Describe resistance to β-lactam antibiotics
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
For the exams
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).
You say ‘main mode’ of action. so we don’t need to go into specifics?
What is the modified Hodge test used for?
To identify bacterial strains that produce carbapenemases
What is the primary mechanism of methicillin resistance in Staphylococcus aureus?
Acquisition of a new penicillin-binding protein (PBP2’
What role does PBP2’ (encoded by mecA) play in methicillin-resistant S. aureus?
It is responsible for cross-liniking peptidoglycan chains with low affinity for methicillin.
How does vancomycin inhibit bacterial cell wall synthesis?
By binding the terminal Ala-Ala motif of peptidoglycan peptide chains
What is the primary purpose of the coagulase test?
To differentiate Staphylococcus aureus from other Staphylococci
What does the enzyme coagulase do?
Converts fibrinogen into fibrin, leading to clot formation
How can the coagulase test be performed?
By adding colonies to plasma and observing clot formation
What is the purpose of the catalase test?
To differentiate Staphylococcus from other Gram-positive cocci
What indicates a positive catalase test?
Gas bubble formation when H₂O₂ is added to a colony
What is serotyping primarily used for?
Strain discrimination based on antibody binding to cell surface antigens
What is the estimated range of inappropriate antibiotic prescriptions for respiratory tract infections (RTIs) in developed countries?
40%-90%
What is the difference between intrinsic and acquired resistance in bacteria?
Intrinsic resistance involves existing genes, while acquired resistance involves gaining new genetic material.
Why are Gram-negative bacteria intrinsically resistant to many antibiotics?
They have a double-membrane structure that makes their cell envelope relatively impermeable to many antibiotics
Which structural changes in Gram-positive bacteria contribute to reduced permeability to antibiotics?
Porin loss
What is the basis of phage-typing in bacteria?
Resistance or susceptibility to specific bacteriophages.
Which of the followings is considered anthropologic driver of antibiotic resistance?
spraying fruits and vegetables with antibiotics
Using antibiotics as growth promoters in animals
prescribing antibiotics for infectious diseases in animals
All of the above!!
