Ampicillin Resistance

Susceptibility Testing

When possible, antibacterial substances, such as ampicillin, 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 (Table 6).

Table 6. Minimum inhibitory concentrations (MIC) that would classify the pathogenic bacterial species as susceptible to, intermediate, or resistant to ampicillin. Adapted from (FDA, 2017). 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 ≤8 16 ≥32
Enterococcus spp. ≤8 - ≥16
Haemophilus influenzae and Haemophilus parainfluenzae ≤1 2 ≥4
Streptococcus spp. (β-hemolytic group) ≤0.25 - -
Streptococcus spp. (viridans group) ≤0.25 0.5-4 ≥8
Neisseria meningitidis ≤0.12 0.25-1 ≥2

Resistance Mechanisms

Ampicillin 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 ampicillin specifically, the main mechanisms of bacterial resistance against the drug are:
* Reduced permeability to antibiotic
* Antibiotic efflux
* Antibiotic inactivation

Reduced Permeability to Antibiotic

β-lactam antibiotics, including ampicillin, 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, inhibit PBP activity, and kill the cell (see examples in Table 7).

Table 7. Examples of Porins related to Antibiotic Resistance

Cause of Resistance Description
OmpK35 In Klebsiella pneumoniae this outer membrane porin protein is often deleted, limiting diffusion of the antibiotic into the cell and leading to resistance (CARD 2017).
OmpF In E. coli this porin binds ampicillin in the extracellular and periplasmic parts and blocks ionic current. Disruption of binding increases E. coli's susceptibility to the antibiotics (Figure 5, Ziervogel et al., 2013)
Figure 5: Structure of E. coli OmpF porin in complex with ampicillin (PDB ID 4gcp, Ziervogel et al., 2013). Inset shows the binding of ampicillin to the wall of the OmpF protein.
Figure 5: Structure of E. coli OmpF porin in complex with ampicillin (PDB ID 4gcp, Ziervogel et al., 2013). Inset shows the binding of ampicillin to the wall of the OmpF protein.

Learn more about porins.

Antibiotic Efflux

Some resistant bacterial cells confer resistance against ampicillin by transporting the drug out of the cell through efflux pumps. Examples of resistant bacteria and their efflux pumps are described in Table 8 (CARD, 2017).

Learn more about efflux pumps.

Table 8. Bacterial pumps responsible for antibiotic efflux.

Cause of Resistance Description
MexAB-OprM This resistance-nodulation-cell division (RND) antibiotic efflux pump is expressed in the gram-negative Pseudomonas aeruginosa. MexAB-OprM is associated with resistance to fluoroquinolones, chloramphenicol, erythromycin, azithromycin, novobiocin, and certain β-lactams and lastly over-expression is linked to colistin resistance. Mutations in MexR, CpxR, NalD, and NalC confer resistance to multiple antibiotics.
AcrAB-TolC In gram-negative bacteria, e.g., E. coli the tripartite RND efflux system AcrAB-TolC confers resistance to tetracycline, chloramphenicol, ampicillin, nalidixic acid, and rifampin. The system spans the cell membrane (AcrB) and the outer-membrane (TolC), and is linked together in the periplasm by AcrA. Here the binding of ampicillin to AcrB in complex with a transmembrane helix YajC is shown (Figure 6, Törnroth-Horsefield et al., 2007).
MtrD In Neisseria gonorrhoeae, an obligate human pathogen and causative agent of the sexually transmitted infection (STI) gonorrhea, the multiple transferrable resistance (Mtr) pump mediates resistance to a number of different classes of structurally diverse antimicrobial agents, including clinically used antibiotics (e.g., β-lactams and macrolides), dyes, detergents and host-derived antimicrobials (Lyu et al., 2020).
Figure 6. X-ray crystal structure of AcrB (green) in complex with the transmembrane helix YajC (orange) and ampicillin (PDB ID 2rdd, Törnroth-Horsefield et al., 2007). A closeup of two different modes of ampicillin binding in the pump is shown in purple.
Figure 6. X-ray crystal structure of AcrB (green) in complex with the transmembrane helix YajC (orange) and ampicillin (PDB ID 2rdd, Törnroth-Horsefield et al., 2007). A closeup of two different modes of ampicillin binding in the pump is shown in purple.

Antibiotic Inactivation

When a β-lactam antibiotic, like ampicillin, 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 β-lactam ring (see Figure 2), 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 (Table 9).

Table 9: β-lactamase responsible for ampicillin resistance

Cause of Resistance Description
BlaC Mybocaterium has this highly active nucleophilic serine β-lactamase, allowing it to readily hydrolyze and render most β-lactams non-functional (Tremblay and Blanchard 2011).
TEM series These are a broad-spectrum β-lactamase found in many gram-negative bacteria. Confers resistance to penicillins and first-generation cephalosphorins.
CTX-M series These enzymes were named for their activity against cefotaxime (e.g., ceftazidime, ceftriaxone, or cefepime). They represent examples of plasmid acquisition of β-lactamase genes. For example see Figure 7, (Soeung et al., 2020)
OXA-1 This is a β-lactamase found in E. coli and many other gram-negative bacteria.
Figure 7. Structure of CTX-M-14 β-lactamase in complex with ampicillin (PDB ID 7k2y, Soeung et al., 2020). The inset shows the active site Serine (S70) and various other amino acids that interact with the antibiotic.
Figure 7. Structure of CTX-M-14 β-lactamase in complex with ampicillin (PDB ID 7k2y, Soeung et al., 2020). The inset shows the active site Serine (S70) and various other amino acids that interact with the antibiotic.

Learn more about β-lactamases.

Mechanisms Against Resistance

Ampicillin is often combined with the β-lactamase inhibitor Sulbactam and marketed as Unasyn. Learn more about Sulbactam (DrugBank).

Back to article on ampicillin.

References

Lyu, M., Moseng, M. A., Reimche, J. L., Holley, C. L., Dhulipala, V., Su, C. C., Shafer, W. M., Yu, E. W. (2020) Cryo-EM Structures of a Gonococcal Multidrug Efflux Pump Illuminate a Mechanism of Drug Recognition and Resistance. mBio. 11(3), e00996-20. https://doi.org/10.1128/mbio.00996-20

Soeung, V., Lu, S., Hu, L., Judge, A., Sankaran, B., Prasad, B. V. V., Palzkill, T. (2020) A drug-resistant β-lactamase variant changes the conformation of its active-site proton shuttle to alter substrate specificity and inhibitor potency. J Biol Chem. 295(52):18239-18255. https://doi.org/10.1074/jbc.RA120.016103

Törnroth-Horsefield, S., Gourdon, P., Horsefield, R., Brive, L., Yamamoto, N., Mori, H., Snijder, A., Neutze, R. (2007) Crystal structure of AcrB in complex with a single transmembrane subunit reveals another twist. Structure. 15,1663-73. https://doi.org/10.1016/j.str.2007.09.023

Ziervogel, B. K., Roux, B. (2013) The binding of antibiotics in OmpF porin. Structure. 21, 76-87. https://doi.org/10.1016/j.str.2012.10.014