ESKAPE Pathogens

ESKAPE Pathogens is an acronym coined in 2008 by Louis B. Rice to designate the six leading opportunistic bacterial taxa responsible for the majority of multidrug-resistant (MDR), extensively drug-resistant (XDR), and pandrug-resistant (PDR) health-care-associated infections worldwide: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species (most notably the Enterobacter cloacae complex). Together, these two Gram-positive cocci and four Gram-negative bacilli account for over 40% of intensive care unit (ICU) bloodstream infections, ventilator-associated pneumonias (VAP), catheter-associated urinary tract infections (CAUTI), and surgical-site infections, contributing to an estimated 1.27 million attributable deaths annually from bacterial antimicrobial resistance.

The mnemonic ESKAPE reflects the capacity of these six lineages to "escape" the bactericidal action of nearly all conventional classes of clinical antimicrobials—including β-lactams, carbapenems, glycopeptides, fluoroquinolones, aminoglycosides, macrolides, and polymyxins. In the World Health Organization (WHO) Bacterial Priority Pathogens List, carbapenem-resistant Acinetobacter baumannii (CRAB), carbapenem-resistant and third-generation-cephalosporin-resistant Enterobacterales (Klebsiella pneumoniae and Enterobacter spp.), and carbapenem-resistant Pseudomonas aeruginosa (CRPA) occupy the highest priority tiers, alongside vancomycin-resistant Enterococcus faecium (VRE) and methicillin-resistant Staphylococcus aureus (MRSA). Their clinical persistence arises from the convergence of intrinsic cell-envelope barriers, inducible multi-substrate efflux pumps, biofilm persister dormancy, and a shared pool of mobile genetic elements (MGEs) propagated through horizontal gene transfer.

Gram-Positive ESKAPE Lineages: Enterococcus faecium and Staphylococcus aureus

The two Gram-positive members of the ESKAPE consortium overcome cell-wall-active β-lactams and glycopeptides through enzymatic target remodeling and low-affinity transpeptidase acquisition. Nosocomial Enterococcus faecium isolates belonging to the hospital-adapted clade Clonal Complex 17 (CC17) exhibit intrinsic low-level resistance to cephalosporins and aminoglycosides and acquire high-level ampicillin resistance via overproduction and active-site substitution of the chromosomal Penicillin-Binding Protein 5 (PBP5-R). In vancomycin-resistant E. faecium (VRE), acquisition of the Tn1546 transposon carrying the vanA operon (vanR–vanS–vanH–vanA–vanX–vanY–vanZ) on conjugative plasmids replaces the canonical D-alanyl-D-alanine (D-Ala-D-Ala) C-terminal dipeptide of peptidoglycan muramyl pentapeptide precursors with the depsipeptide D-alanyl-D-lactate (D-Ala-D-Lac). Replacing the amide NH group with an ester oxygen removes a hydrogen bond required for vancomycin binding, lowering glycopeptide binding affinity by 1,000-fold and elevating the vancomycin minimum inhibitory concentration (MIC) from ≤2 μg/mL to >256 μg/mL.

In Staphylococcus aureus, methicillin resistance (MRSA) is driven by the Staphylococcal Cassette Chromosome mec (SCCmec, a 21–67 kb mobile chromosomal island integrated at the 3′ end of the rlmH/orfX locus). SCCmec carries the mecA (or mecC) gene encoding Penicillin-Binding Protein 2a (PBP2a), a 76-kDa transpeptidase whose distorted active-site cleft exhibits micromolar dissociation constants for penicillins, antistaphylococcal isoxazolyl penicillins (oxacillin, nafcillin), cephalosporins (excluding fifth-generation ceftaroline), and carbapenems. While native staphylococcal PBPs (PBP1–PBP4) are acylated and inactivated by β-lactams, PBP2a continues cross-linking pentaglycine bridges in the cell wall in cooperation with the native transglycosylase domain of PBP2.

Gram-Negative ESKAPE Lineages: Klebsiella, Acinetobacter, Pseudomonas, and Enterobacter

The four Gram-negative ESKAPE pathogens possess an asymmetric diderm cell envelope in which an outer membrane (OM) composed of lipopolysaccharide (LPS) in the outer leaflet and glycerophospholipids in the inner leaflet acts as a selective permeability barrier. Hydrophilic β-lactams and fluoroquinolones must traverse water-filled outer-membrane β-barrel porins (OmpK35/OmpK36 in Klebsiella pneumoniae, Omp35/Omp36 in Enterobacter cloacae, CarO/OmpA in Acinetobacter baumannii, and OprD in Pseudomonas aeruginosa), while hydrophobic macrolides and rifamycins are excluded by the tightly packed, Mg²⁺- and Ca²⁺-bridged lipid A core.

When outer-membrane porin loss or down-regulation is paired with active extrusion by tripartite Resistance-Nodulation-Division (RND) efflux systems (AcrAB-TolC in Klebsiella and Enterobacter, AdeABC/AdeIJK in A. baumannii, and MexAB-OprM/MexXY in P. aeruginosa) and periplasmic β-lactamases, intracellular drug accumulation drops below the inhibitory threshold. Across these four genera, carbapenem resistance is mediated by transferable Ambler Class A serine carbapenemases (Klebsiella pneumoniae Carbapenemase (blaKPC-2, blaKPC-3)), Class B1 dizinc metallo-β-lactamases (New Delhi Metallo-Beta-Lactamase (blaNDM-1, blaNDM-5), blaVIM-2, blaIMP-1), and Class D oxacillinases (blaOXA-48 in Enterobacterales; blaOXA-23, blaOXA-24/40, and blaOXA-58 in A. baumannii).

Cross-Species Plasmid, Integron, and Transposon Networks

Whole-genome genomics and metagenomics tracking of intensive care unit outbreaks demonstrate that the ESKAPE pathogens do not evolve resistance in isolation; instead, they participate in an interconnected nosocomial resistome network. Self-transmissible broad-host-range plasmid replicons (IncX3, IncFII, IncL/M, IncA/C2, and IncHI2), integrative and conjugative elements (ICEs), and composite transposons move antimicrobial resistance cassettes across species and genus boundaries within patient gastrointestinal tracts and hospital plumbing reservoirs (sink P-traps, aerators, and wastewater biofilms).

At the center of this horizontal exchange are Class 1 integrons (intI1–qacEΔ1–sul1) and IS26 composite transposons. For example, the blaNDM-1–bleMBL–trpF module—originally assembled inside Tn125 (ISAba125) in Acinetobacter—regularly transfers onto IncX3 and IncFII plasmids or integrates into chromosomal tRNA islands in Klebsiella pneumoniae, Enterobacter hormaechei, and Pseudomonas aeruginosa. In chromosomal and plasmid resistome scans, these horizontally acquired resistance islands exhibit a pronounced tetranucleotide Kullback–Leibler divergence (D_KL) anomaly (7.4× above host chromosomal baseline, Z > +5.0 SD), allowing bioinformatic surveillance to trace cross-species cassette sharing even when standard single-species multilocus sequence typing (MLST) indicates unrelated clonal backgrounds.

Biofilm Persister Tolerance and Multi-Target Synergy

Beyond heritable genetic resistance, all six ESKAPE pathogens form surface-adherent biofilms on endotracheal tubes, central venous catheters, prosthetic joints, and chronic wound beds. Encased in an extracellular polymeric substance (EPS) matrix composed of exopolysaccharides (PIA/PNAG in staphylococci; alginate, Pel, and Psl in P. aeruginosa; capsular polysaccharide in K. pneumoniae), extracellular DNA (eDNA), and amyloid fibers, biofilm-embedded cells experience steep radial gradients of dissolved O₂, nutrients, and pH.

Within the nutrient-depleted interior of the biofilm, a subpopulation of cells (typically 10⁻⁴ to 10⁻² of the total population) enters a metabolically dormant persister state governed by stringent-response alarmones ((p)ppGpp synthesized by RelA/SpoT) and toxin–antitoxin (TA) modules (HipAB, MazEF, RelBE). Because bactericidal antibiotics require active peptidyl transferase center translation (aminoglycosides), active DNA gyrase supercoiling during DNA replication (fluoroquinolones), or nascent peptidoglycan synthesis (β-lactams) to corrupt cellular targets, dormant persisters survive antibiotic concentrations 100-fold to 1,000-fold above the planktonic MIC and re-seed infection follows treatment cessation, following a biphasic time-kill decay curve (N(t) = N_0 exp(-k_1 t) + P_0 exp(-k_2 t) where k_2 << k_1).

Exploiting Collateral Sensitivity and Evolutionary Dead-Ends

To overcome multidrug resistance in ESKAPE infections, contemporary infectious-disease pharmacology exploits epistasis, collateral sensitivity, and dual-target synergy:

1. Rational β-Lactam / β-Lactamase Inhibitor Combinations: Pairing diazabicyclooctane (avibactam, relebactam, durlobactam) or cyclic boronate (vaborbactam, taniborbactam, xeruborbactam) inhibitors with partner β-lactams restores efficacy against Class A (KPC), Class C (AmpC), and Class D (OXA) enzymes. Against blaNDM-1-producing Enterobacterales, aztreonam–avibactam pairs an MBL-stable monobactam with a serine β-lactamase shield, while sulbactam–durlobactam inhibits both Class D OXA-23/OXA-24 carbapenemases and PBP1/PBP3 in Acinetobacter baumannii.
2. Epistatic Mutational Constraints in Ribosomal and Topoisomerase Targets: In the 23S rRNA peptidyl transferase center (G2058–C2611 base pair) and DNA gyrase (GyrA/ParC), single resistance mutation events impose a thermodynamic fitness penalty (ΔE ≈ +14.7 kcal/mol in uncompensated rRNA stems) unless rescued by specific secondary compensatory substitutions. Combination regimens that simultaneously target orthogonal cellular pathways (or exploit negative collateral sensitivity between aminoglycoside uptake driven by proton-motive force and RND efflux pump overexpression) force the pathogen into an evolutionary dead-end where simultaneous double resistance occurs at frequencies below 10⁻¹⁴.

Why was the ESKAPE acronym created, and what is the ESKAPEE extension?

Infectious disease specialist Louis B. Rice introduced the acronym ESKAPE in 2008 to highlight the six hospital-acquired bacterial pathogens that most frequently evade conventional antibiotic therapy and lack robust pipelines of new therapeutic agents. Many clinical epidemiologists and the World Health Organization now expand the acronym to ESKAPEE by adding Escherichia coli (specifically sequence type ST131 and carbapenemase- or CTX-M-15-producing extraintestinal pathogenic E. coli), because E. coli and Klebsiella pneumoniae together represent the largest global burden of fatal bloodstream and urinary tract infections caused by resistant Gram-negative bacteria.

Why are Gram-negative ESKAPE pathogens harder to treat than Gram-positive ESKAPE pathogens?

The four Gram-negative ESKAPE pathogens (Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) possess a two-membrane diderm cell envelope. Their outer membrane features an asymmetric outer leaflet of negatively charged lipopolysaccharide (LPS) that blocks large hydrophobic or amphipathic antibiotics (such as vancomycin, daptomycin, and linezolid, which have molecular masses >600–1,400 Da) and restricts small hydrophilic drugs to narrow β-barrel porin channels. When these bacteria combine porin loss with inner-membrane-spanning RND tripartite efflux pumps that actively expel drugs directly across both membranes into the extracellular medium, very few antibiotic classes can achieve bactericidal periplasmic or cytoplasmic concentrations.

How can hospital sinks and ICU plumbing spread resistance across different ESKAPE species?

Hospital sink drains and P-traps maintain continuous polymicrobial biofilms exposed to sub-inhibitory concentrations of excreted antibiotics, disinfectants, and quaternary ammonium compounds (QACs). Within these moist biofilms, Klebsiella pneumoniae, Enterobacter cloacae, and Pseudomonas aeruginosa exchange broad-host-range conjugative plasmids (IncX3, IncFII) and Class 1 integrons carrying both quaternary ammonium resistance (qacEΔ1) and carbapenemase genes (blaNDM-1, blaKPC-2). Pressurized faucet water striking the drain grid generates contaminated droplet aerosols that colonize nearby ICU patients and medical equipment, driving multi-species outbreaks of the identical resistance cassette.

1. Rice, L. B. (2008). "Federal funding for the study of antimicrobial resistance in nosocomial pathogens: no ESKAPE." The Journal of Infectious Diseases, 197(8), 1079–1081. https://doi.org/10.1086/533452
2. De Oliveira, D. M. P., Forde, B. M., Kidd, T. J., Harris, P. N. A., Schembri, M. A., Beatson, S. A., Paterson, D. L., & Walker, M. J. (2020). "Antimicrobial resistance in ESKAPE pathogens." Clinical Microbiology Reviews, 33(3), e00181-19. https://doi.org/10.1128/CMR.00181-19
3. Antimicrobial Resistance Collaborators (2022). "Global burden of bacterial antimicrobial resistance in 2019: a systematic analysis." The Lancet, 399(10325), 629–655. https://doi.org/10.1016/S0140-6736(21)02724-002724-0)
4. Pendleton, J. N., Gorman, S. P., & Gilmore, B. F. (2013). "Clinical relevance of the ESKAPE pathogens." Expert Review of Anti-infective Therapy, 11(3), 297–308. https://doi.org/10.1586/eri.13.12