Piperacillin Resistance

Susceptibility Testing

When possible, antibacterial substances, such as piperacillin, are tested for their effectiveness against various infectious pathogens. These test results allow clinicians to choose the antibiotic likely to result in the most effective treatment of a particular bacterial infection. For instance, one such susceptibility test provides minimum inhibitory concentration (MIC) values that can then be used to identify a pathogenic bacterial strain as susceptible, intermediate, or resistant to a certain antibiotic (Table 6).

Table 6. Minimum inhibitory concentrations (MIC) that would classify the pathogenic bacterial strain as susceptible to, intermediate, or resistant to piperacillin. Adapted from (FDA, 2017). These values may not be the latest approved by the US FDA.

Resistance Mechanism(s)

Piperacillin resistance occurs when the antibiotic is not able to treat certain bacterial infections because the pathogens causing these infections have developed mechanisms to prevent the drug from functioning. For piperacillin specifically, the main mechanisms of bacterial resistance against the drug are:
* Antibiotic efflux
* Antibiotic inactivation

Antibiotic Efflux

In Klebsiella pneumoniae, KpnGH-TolC is a major facilitator superfamily (MFS) antibiotic efflux pump that exports drugs such as azithromycin, ceftazidime, ciprofloxacin, ertapenem, erythromycin, gentamicin, imipenem, ticarcillin, norfloxacin, polymyxin-B, piperacillin, spectinomycin, tobramycin, and streptomycin, out of the cell (CARD, 2017). This allows bacteria to confer resistance to these drugs. Disrupting the components of the KpnGH complex significantly decreases bacterial resistance.

Learn more about efflux pumps.

Antibiotic Inactivation

When a β-lactam antibiotic, like piperacillin, 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 of the functional β-lactam ring to open up. As a result of this chemical modification, the antibiotic is no longer able to bind to and inhibit its target PBP enzymes, thus losing its antibacterial properties.

AmpC is a class C β-lactamase found in Acinetobacter baumannii that hydrolyzes piperacillin to allow the bacteria to develop resistance. Other bacterial species have similar mechanisms of resistance, including OXA-1 in E. coli, ACO-1 in Acinetobacter spp. isolates, and blaS1 in Mycobacterium smegmatis (CARD, 2017).

Learn more about β-lactamases.

Here we see the structure of piperacillin bound to the β-lactamase CTX-M-9 with a mutation in the active site (S70G) preventing the formation of a covalent complex (Leyssne, et al., 2011). However, non-covalent interactions of the antibiotic with other amino acids in the enzyme are shown in Figure 6.

Figure 6. Structure of CTX-M-9 S70G in complex with piperacillin (PDB ID 3q07, Leyssne, et al., 2011). The inset shows a closeup of the antibiotic piperacillin binding to the enzyme. The mutated active site residue S70G is shown in magenta. Side chains forming direct hydrogen bonds with the antibiotic are shown in ball and stick representation.

Mechanisms Against Resistance

Piperacillin is often used in combination with a β-lactamase inhibitor, tazobactam, to prevent the drug from being degraded by β-lactamases and increase its antibacterial properties. Click here to learn more about tazobactam.

Back to the article on piperacillin.

References

Leyssne, D., Delmas, J., Coignoux, A., Robin, F., Bonnet, R. (2011) CTX-M-9 S70G mutant in complex with piperacillin. https://doi.org/10.2210/pdb3Q07/pdb