Antibiotic Resistance: Mechanisms & Action

Antibiotic Resistance Mechanisms of Action

• Antibiotic resistance mechanisms include limiting antibiotic access, enzymatic inactivation, target modification/protection, and failure to activate the antibiotic.

Restriction of Access to Target

• Bacteria restrict access to the target using outer membrane barriers, selective porins, and efflux pumps.
• Outer membrane porins are selective gatekeepers, allowing small molecules to diffuse through the periplasm.
• Antibiotics like vancomycin and daptomycin are too large to pass through.
• Efflux pumps have been discovered for nearly every class of antibiotic.
• Gram-negative pumps span both membranes and use proton motive force to pump antibiotics from the periplasm.
• Gram-positive efflux pumps are simple anti-porters or ABC transporters.

Enzymatic Inactivation

• Serine β-lactamases are common in gram-negative bacteria and cleave the beta-lactam ring of antibiotics.
• The mechanism is similar to the β-lactam's action on the transpeptidase enzyme.
• β-lactams can be administered with a β-lactamase inhibitor, and clavulanic acid inactivates β-lactamase.
• Zinc β-lactamases (metallo β-lactamases) cleave antibiotics at a different site.
• Clavulanic acid and other inhibitors cannot interact with the zinc form.
• Zinc β-lactamases can cleave a wider range of β-lactams than serine β-lactamases.
• Kanamycin or gentamycin can be inactivated by the addition of modifying groups.
• Modifications disrupt the interaction of antibiotics with their cellular target (30S ribosome via 16S rRNA).

Target Modification or Protection

• Penicillin-binding protein (PBP) resistance is common in gram-positive bacteria (MRSA/VRE).
• Modification prevents the binding of the β-lactam ring and involves modification of existing amino acids.
• Clavulanic acid cannot block this type of resistance.
• Vancomycin resistance occurs when antibiotic normally prevents peptide interbridge formation by binding strongly to D-ala-D-ala residues in the stem peptide, preventing transpeptidase from linking the cross-bridge.
• Vancomycin binds D-Ala-D-Lac about 1000-fold less efficiently than D-Ala-D-Ala, which explains resistance.
• In macrolide resistance, RNA methylases add methyl groups to residue A2058 in the 23S RNA target site of the ribosome.
• The antibiotic can no longer bind to the large subunit and block polypeptide exit.
• In colistin resistance, the mechanism includes modification of LPS.
• Ribosome protection by TetO, TetM, and TetQ mediates tetracycline resistance.
• A new modified version of tetracycline called glycylglycines (tigecycline) overcomes these resistance mechanisms.

Failure to Activate the Antibiotic

• Metronidazole resistance results when the drug, a prodrug, is not activated by flavodoxin.
• Less flavodoxin leads to failure to activate.
• Absence of activation damages DNA by causing nicks.
• Isoniazid resistance occurs when the drug is not activated by KatG (catalase).
• Isoniazid blocks mycolic acid synthesis so no activation results in no inhibition of the cell wall.
• Resistance stems from altered expression or function of KatG.

Regulation of Resistance

• Expression of antibiotic resistance often comes as a fitness tradeoff.
• Efflux pumps derive their energy for transport through the proton motive force, competing with ATPases for ATP production.
• Beta-lactamase production in Staphylococcus aureus and AmpC regulation in E. coli are also regulated processes.
• Translational attenuation is used for Erythromycin resistance

Resistance, Tolerance, and Persisters

• Resistance is when bacteria can still grow in the presence of the antibiotic.
• Tolerance is when bacteria stop growing when the antibiotic is present but are not killed.
• When antibiotic levels fall, the tolerant bacteria can resume growth.
• Persisters are a small sub-population of cells (1%) that temporarily stop dividing playing an important role in biofilm physiology and antibiotic tolerance.
• Toxin-antitoxin (TA) systems induce growth arrest or dormancy.