Canonical Question
Micro – Resistance
Master answer
Gram Stain
Method of differentiated bacteria according to their cell wall characteristics, using
staining with crystal violet dye
- Gram positive bacteria have a thick outer cell wall composed of peptidoglycan,
which stains positive with crystal violet dye - Gram negative bacteria have an outer cell membrane enclosing a thinner
peptidoglycan cell wall, which has decreased affinity to the crystal violet dye
Classification of bacteria
Examples of Antimicrobials + MoA
| Group | Antimicrobials | Mechanism of Action |
|---|---|---|
| Gram Positive Cocci | Penicillin | β lactam antibiotic Binds to penicillin binding protein (transpeptidase) and prevents crosslinking of bacterial peptidoglycan impairs cell wall synthesis |
| Vancomycin | Inhibits Glycopeptide synthetase prevents peptidoglycan formation in bacterial cell well (Unlike penicillins, prevents the transfer and addition of the muramylpentapeptide building blocks that make up the peptidoglycan molecule itself.) May also alter membrane permeability and selectively inhibit RNA synthesis. | |
| Gram Positive Bacilli | Clindamycin | Inhibitor of bacterial 50S ribosomal subunit Prevents protein synthesis |
| Erythromycin | Inhibits 50s subunit to prevent protein synthesis Bacteriocidal/static | |
| Gram Negative Cocci | Ceftriaxone | Beta lactam ring binds to multiple penicillin binding proteins (carboxy/endo/ transpeptidase) and prevents crosslinking of bacterial peptidoglycan inhibits cell wall synthesis BROADER SPECTRUM Bacteria eventually lyse due to ongoing activity of cell wall autolytic enzymes (autolysins and murein hydrolases) while cell wall assembly is arrested Configuration: Stability against Betalactamases |
| Ciprofloxacin | Bacteriocidal antimicrobials that block DNA replication by blocking tropoisomerase enzymes, which are essential for the supercoiling, replication and separation of circular bacterial DNA | |
| Gram Negative Bacilli | Gentamicin | bactericidal Binds to the bacterial 30S ribosomal subunit to inhibit protein synthesis and thus bacterial growth |
| Meropenem | Binds to several penicillin binding protein and prevents crosslinking of bacterial peptidoglycan inhibits cell wall synthesis BROADEST SPECTRUM Bacteria eventually lyse due to ongoing activity of cell wall autolytic enzymes (autolysins and murein hydrolases) while cell wall assembly is arrested Configuration: Stability against Betalactamases and ESBLs |
Gladwin / Sakurai / JC 2020
MECHANISMS OF ANTIBIOTIC RESISTANCE
- Mechanisms of Antibiotic Resistance can be classified broadly:
- Efflux Pumps
- Blocked Penetration / Alteration in access to target site
- Target Modification
- Modification of Drug or pathways
- There might be multiple resistance mechanisms at play in the same organism
| Efflux Pumps: Increased efficiency or expression of efflux pumps (inner membrane proteins). | |
| Removed from cell | Active transport of ciprofloxacin out of the bacterial cell |
| E.Coli to tetracycline. | |
| Trapped between cell wall layers | glycopeptide resistance in VRSA. |
| Blocked Penetration / Alteration in access to target site | |
| Narrowing of porin channels | Streptococcal resistance to penicillins typically occurs by reducing access to PBPs. |
| Alteration of porins in gram negative outer membranes | downregulation of Outer Membrane Proteins (eg enterobacteriaceae, pseudomonas vs penems/cephs) |
| increased selectivity of Outer Membrane Proteins (eg klebsiella outbreaks) | |
| Loss of non-essential transporter channels | Anaerobes have no oxygen-transport channel which prevents penetration by aminoglycosides (Gentamicin) |
| Reduced binding of the antibiotic | VanA and VanB vancomycin resistance involves a gene mutation leading to decreased affinity of vancomycin for the binding sites of peptidoglycan precursors |
| Changes to the DNA-binding surface of DNA supergyrase infers resistance against ciprofloxacin | |
| Target Modification | |
| Modifying the enzyme that the antibiotic inhibits | Rifampicin resistance by point mutations, insertions, or deletions in RNA polymerase gene |
| Production of an alternative enzyme for that which the antibiotic inhibits | MRSA develop or acquire the gene mecA which synthesizes an additional penicillin binding protein that enables it to continue cell wall synthesis in the presence of a beta lactam drug |
| Overproduction of the target of the antibiotic | Trimethoprim DHFR enzyme overproduction in Escherichia coli and Haemophilus influenzae. |
| Synthesis of target-protecting proteins | Ribosomal protection proteins against Tetracyclines in multiple Gram positive and gram negative bacteria |
| Modification of Drug or pathways | |
| Development of metabolic pathways to bypass site of action of antibiotic | Resistance to TMP-SMX by allowing bacteria to synthesize or absorb folic acid. |
| Enzymes produced to Metabolize the drug | β-lactamases and cephalosporinases hydrolyse β-lactam rings |
| Enzymes produced that add a chemical group to antibiotic to inhibit its activity | Aminoglycoside resistance by Staphylococcus aureus or Pseudomonas (Multiple enzymes acetyltransferase, adenyltransferase, and phosphotransferase) |
Spread of Bacterial Resistance
- Selective pressure selects for favourable mutations of resistance
- mosaic genes (from other bacteria eg strep pneumo from strep mitus) or uptake of DNA from environment
- transfer of resistant bacteria from person to person
- horizontal gene transfer;
- transduction – acquisition of bacterial DNA from a phage (a virus that propagates in bacteria);
- transformation – uptake and incorporation of free DNA released into the environment by other bacterial cells;
- conjugation – gene transfer (usually on plasmids), by direct cell-to-cell contact through a bridge.
Sources:
Microbiology Lippincott Williams & Wilkins
https://courses.lumenlearning.com/microbiology/chapter/drug-resistance/
https://www.encyclopedie-environnement.org/en/health/antibiotics-antibiotic-resistance-and-environment/
Gladwin / Sakurai / JC 2019
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2024B Q07 | (a) Categorise bacteria by their gram stain appearance and shape. Give TWO examples of bacteria for each group (30% of marks). (b) Provide TWO examples of antimicrobials used against each group and outline their mechanism of action (20% of marks). (c) Outline the mechanisms of bacterial resistance with ONE example for each (50% of marks). | historical_member | — |