Aztreonam Resistance

Susceptibility Testing

When possible, antibacterial substances, such as aztreonam, 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 (see Table 6):

Table 6. Minimum inhibitory concentrations (MIC) that would classify the pathogenic bacterial strain as susceptible to aztreonam, intermediate, or resistant to aztreonam (FDA, 2013). These values may not be the latest approved by the US FDA.

Pathogen MIC (µg/mL) for Susceptible (S) strains MIC (µg/mL) for Intermediate (I) strains MIC (µg/mL) Resistant (R) strains
Enterobacteriaceae ≤4 8 ≥16
Haemophilus influenzae ≤2 - -
Pseudomonas aeruginosa ≤8 16 ≥32

Resistance Mechanism(s)

Aztreonam 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 aztreonam specifically, the main mechanisms of bacterial resistance are:
* Antibiotic efflux
* Antibiotic inactivation

Antibiotic Efflux

Some resistant bacterial cells confer resistance against aztreonam by transporting the drug out of the cell through efflux pumps. Selected examples of resistant bacteria and their efflux pumps are described in Table 7 (ARO:3000386, CARD).

Learn more about antibiotic efflux.

Table 7. Bacterial pumps responsible for antibiotic efflux.

Cause of Resistance Description
MexAB-OprM MexAB-OprM is a multidrug efflux protein expressed in the Gram-negative Pseudomonas aeruginosa. It is known to cause resistance to fluoroquinolones, chloramphenicol, erythromycin, azithromycin, novobiocin, and certain β-lactams and lastly, over-expression is linked to colistin resistance. Learn more about MexAB-OprM.
MuxABC-OpmB MuxABC-OpmB is an RND-type multidrug efflux pump in Pseudomonas aeruginosa. that confers resistance to aztreonam, novobiocin, tetracycline, erythromycin, kitasamycin, and rokitamycin.

Antibiotic Inactivation

In bacterial cells, in addition to binding to the intended PBP target, aztreonam may also bind to β-lactamases. These enzymes can inactivate the drug by breaking the amide bond of the β-lactam ring. As a result of this chemical modification, the antibiotic is no longer a structural mimic of the natural D-Ala-D-Ala substrate and is not able to bind to and inhibit its target PBP enzymes, thus losing its antibacterial properties.

Learn more about antibiotic inactivation by breaking the β-lactam ring.

Various β-lactamases bind to and destroy aztreonam conferring resistance against the drug - e.g., VCC-1, a Class A carbapenemase isolated from Vibrio cholerae; ACC-1 a β-lactamase found in Klebsiella pneumoniae; ACC-4, a β-lactamase found in Escherichia coli; and TEM-61 an extended-spectrum β-lactamase found in clinical isolates. Here we show the covalent binding of aztreonam to a β-lactamase from Citrobacter freundii (Figure 5), where the antibiotic's β-lactam ring is opened.

Figure 5: Structure of a β-lactamase from Citrobacter freundii bound to aztreonam. The inset shows a closeup of the covalent binding of aztreonam to Ser64 and various non-covalent interactions with other amino acids in the enzyme. (PDB ID 1fr6, Oefner et al., 1990)
Figure 5: Structure of a β-lactamase from Citrobacter freundii bound to aztreonam. The inset shows a closeup of the covalent binding of aztreonam to Ser64 and various non-covalent interactions with other amino acids in the enzyme. (PDB ID 1fr6, Oefner et al., 1990)

Mechanisms Against Resistance

Aztreonam is often used in combination with β-lactamase inhibitors clavulanate, tazobactam, and avibactam to prevent the drug from being degraded by β-lactamases and increase its antibacterial properties. Learn more about clavulanate, tazobactam, and avibactam.

Back to the article on aztreonam.

References

Oefner, C., D'Arcy, A., Daly, J. J., Gubernator, K., Charnas, R. L., Heinze, I., Hubschwerlen, C., Winkler, F. K. (1990) Refined crystal structure of beta-lactamase from Citrobacter freundii indicates a mechanism for beta-lactam hydrolysis. Nature. 343, 284-8. https://doi.org/10.1038/343284a0