Methicillin Resistance

Resistance Mechanisms

Methicillin resistance occurs when the antibiotic is not able to treat the infections it is intended to. This may either be because the bacterial strains causing these infections have developed mechanisms to prevent the drug from reaching its target or from functioning. These mechanisms largely include (CARD, 2017):
* Antibiotic target replacement
* Antibiotic inactivation
* Antibiotic efflux

Antibiotic inactivation

When a β-lactam antibiotic, like methicillin, enters the bacterial cell, it may also bind to an enzyme known as a β-lactamase in addition to its intended PBP target. The β-lactamase will then inactivate the drug by breaking the amide bond, which causes the functional β-lactam ring to open up. As a result of this chemical modification, the antibiotic is no longer a structural mimic of the natural D-Ala-D-Ala substrate. The altered antibiotic will not be able to bind to and inhibit its target PBP enzymes, thus losing its antibacterial properties.

Learn more about β-lactamases.

As mentioned earlier, methicillin is a poor substrate for many β-lactamases; however, bacteria have evolved to express some β-lactamases that are able to hydrolyze and inactivate the antibiotic. For example, the structure of methicillin in complex with blaC, a broad-spectrum class A β-lactamase coded in the chromosome of Mycobacterium tuberculosis is shown here (Figure 5, Tremblay and Blanchard, 2011).

Figure 5: Structure of BlaC-E166A covalently bound with methicillin (PDB ID 3ndg). Inset shows a closeup of the interactions stabilizing the antibiotic binding to the β-lactamase

Antibiotic target replacement

The main mechanism of methicillin resistance observed in S. aureus is through the expression of a foreign PBP known as PBP2a (Stapleton and Taylor, 2002). The PBP2a is produced by lateral gene transfer of the genes MecA, MecB, MecC, MecI, or MecD (in Macrococcus caseolyticus) (ARO:3003942). Resistance develops because PBP2a has a much lower binding affinity for β-lactams; therefore, even in the presence of the antibiotic, the enzyme can still function normally and cross-link peptidoglycan strands, producing a rigid bacterial cell wall.

Learn more about the structure of methicillin bound to PBP2a.

Antibiotic efflux

Bacterial cells can actively pump the antibiotic back out using cellular machinery known as efflux pumps. As resistant bacteria continue to quickly remove the drug out of their cells, the antibiotic becomes ineffective against the pathogens. One such example of an efflux pump system that confers methicillin resistance by extruding the antibiotic out of the cell is the abcA multidrug resistant ATP-binding cassette (ABC) transporter (ARO:3003942).

Learn more about efflux pumps.

Back to the article on methicillin.

References