Cefepime Resistance
Susceptibility Testing
When possible, antibacterial substances, such as cefepime, 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 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 species as susceptible to cefepime, intermediate, or resistant to cefepime. Adapted from (FDA, 2012). 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 |
|---|---|---|---|
| Microorganisms other than Haemophilus spp. or Streptococcus pneumoniae | ≤8 | 16 | ≥32 |
| Haemophilus spp. | ≤2 | N/A | N/A |
| Streptococcus pneumoniae | ≤0.5 | 1 | ≥2 |
Resistance Mechanism(s)
Cefepime resistance occurs when the antibiotic is not able to treat the infections it is intended to because the bacterial pathogens causing these infections have developed mechanisms to prevent the drug from functioning. As can be seen in Table 5, it takes a relatively low concentration of cefepime to kill susceptible strains that lack these mechanisms, while it would take a much greater concentration of the drug to even stop the growth of the resistant strains. In the case of cefepime resistance, these mechanisms include:
* Reduced permeability to the antibiotic
* Antibiotic inactivation
* Antibiotic efflux
Reduced Permeability to Antibiotic
β-lactam antibiotics, including cefepime, permeate through the outer membrane of gram-negative bacteria and reach their intracellular PBP targets using outer membrane proteins (OMPs), known as porins. However, if the bacterial cell reduces the expression of porins or expresses mutated porins so that it is no longer able to translocate the β-lactam through the membrane, the antibiotic will no longer be able to enter the cell. As a result, the bacterium becomes less susceptible to the antibacterial agent. Examples of resistance causing porins are included in Table 7.
Learn more about porins.
Table 7. Outer membrane Protein (omp) proteins that cause cefepime resistance (CARD, 2017).
| Resistance Cause | Description |
|---|---|
| ompF | in Escherichia coli this porin functions as a nonspecific channel for translocation of small hydrophilic molecules, including β-lactam antibiotics. Mutations in ompF can decrease the diffusion of antibiotics, leading to resistance. |
| Omp36 | in Klebsiella aerogenes mutant versions of this porin reduced permeability to antibiotics. |
| OmpK35 | in β-lactam resistant Klebsiella pneumoniae this porin is often deleted, restricting antibiotic entry into the cell. |
Learn more about porins.
Antibiotic Inactivation
When a β-lactam antibiotic, like cefepime, enters the bacterial cell, in addition to its intended PBP target, it may also bind to β-lactamase. These enzymes inactivate the drug by breaking the amide bond (shown in turquoise in Figure 4), which causes the functional β-lactam ring (shown in pink in Figure 4) 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. Examples of β-lactamase that cause cefepime resistance are included in Table 8.
Learn more about β-lactamases.
|
| Figure 4. A 2D depiction of cefepime inactivation by β-lactamases by breaking the amide bond (in turquoise), causing the β-lactam ring (in pink) to open up. The aminothiazole group that makes cefepime a poor substrate for many β-lactamases is shown in blue. Figure created using ChemDraw. |
Table 8. β-lactamases leading to cefepime resistance (ARO:0000059)
| Resistance cause | Description |
|---|---|
| SCO-1 | This narrow-spectrum β-lactamase isolated from several Acinetobacter spp. isolates and E. coli. |
| LAP | LAP-1 and LAP-2 are Ambler class A β-lactamases is seen in many bacteria e.g., Enterobacter hormaechei, Escherichia coli, Klebsiella pneumoniae. |
| KPC | This β-lactamase is seen in Citrobacter koseri |
| VIM | This β-lactamase is seen in Citrobacter freundii, and Pseudomonas aeruginosa |
| LAQ-1 | This is a class C β-lactamase. |
| ACC-1 | This is a β-lactamase found in Klebsiella pneumoniae |
In general, cefepime is a poor substrate for many β-lactamases due to its aminothiazole group (shown in blue in Figure 4; CARD, 2017). Its low binding affinity to β-lactamases indicates that cefepime is highly resistant to hydrolysis by most β-lactamases. However, some β-lactamases can still alter the chemical structure of the antibiotic, thus inactivating the drug.
Click here to learn more about beta-lactamases.
Antibiotic efflux
A resistance-nodulation-cell division (RND) type antibiotic efflux pump, AxyXY-OprZ, leads to the efflux of aminoglycosides, cefepime, fluoroquinolones, tetracyclines, and erythromycin.
Interestingly, mutations in KpnEF, a small multidrug resistance (SMR) antibiotic efflux pump, increase susceptibility to cefepime, ceftriaxon, colistin, erythromycin, rifampin, tetracycline, and streptomycin.
Learn more about antibiotic efflux pumps.
Mechanisms Against Resistance
Inhibitors of β-lactamases, e.g., tazobactam are used as adjuvants for cefepime. Other adjuvants like taniborbactam, and enmetazobactam may also be used (CARD 2017).
Back to article on cefepime.
