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
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 |
|---|---|---|---|
| 2008B Q16 | Classify bacteria according to the Gram stain system and the shape of the bacteria, and give two examples for each classification (40% of marks). Outline the different mechanisms of bacterial antibiotic resistance and an antibiotic for which that mechanism may apply (60% of marks). | historical_member | — |
| 2014A Q10 | Classify bacteria according to the Gram stain system and their shape. Give two examples for each classification. (40% of marks) List with examples the mechanisms of bacterial antibiotic resistance. (60% of marks) | historical_member | — |