ESKAPE

Enterococcus faecium is a Gram-positive sphereically shaped (coccus) bacteria that tends to occur in pairs or chains, most commonly involved in HAI in immunocompromised patients. It often exhibits a resistance to β-lactam antibiotics including penicillin and other last resort antibiotics.[13] There has also been a rise in vancomycin resistant enterococci (VRE) strains, including an increase in E. faecium resistance to vancomycin, particularly vancomycin-A.[13] These vancomycin-resistant strains display a profound ability to develop and share their resistance through horizontal gene transfer, as well as code for virulence factors that control phenotypes. These virulence phenotypes range from thicker biofilms to allowing them to grow in a variety of environments including medical devices such as urinary catheters and prosthetic heart valves within the body.[14] The thicker biofilms act as a “mechanical and biochemical shield” that protects the bacteria from the antibiotics and are the most effective protective mechanism that bacteria have against treatment.[4]

Staphylococcus aureus is a Gram-positive round-shaped (coccus) bacteria that is commonly found as a part of the human skin microbiota and is typically not harmful in humans with non-compromised immune systems in these environments. However, S. aureus has the ability to cause infections when it enters parts of the body that it does not typically inhabit, such as wounds. Similar to E. faecium, S. aureus can also cause infections on implanted medical devices and form biofilms that make treatment with antibiotics more difficult.[13] Additionally, approximately 25% of S. aureus strains secrete the TSST-1 exotoxin responsible for causing toxic shock syndrome.[13] Methicillin-resistant S. aureus, or MRSA, includes strains distinct from other strains of S. aureus in the fact that they have developed resistance to β-lactam antibiotics. Some also express an exotoxin that has been known to cause “necrotic hemorrhagic pneumonia” in those with an infection.[13] Vancomycin and similar antibiotics are typically the first choices for treatment of MRSA infections, however from this vancomycin-resistant S. aureus, or VRSA (VISA for those with intermediate resistance) strains have emerged.[13]

Klebsiella pneumoniae is a Gram-negative rod-shaped (bacillus) bacteria that is particularly adept to accepting resistance genes in horizontal gene transfer. It is commonly also resistant to phagocyte treatment due to its thick biofilm with strong adhesion to neighboring cells.[13] Certain strains have also developed β-lactamases that allow them to be resistant many of the commonly used antibiotics, including carbapenems, which has led to the creation of carbapenem-resistant K. pneumoniae (CRKP), for which there are very few antibiotics in development that can treat infection.[13]

Acinetobacter baumannii is most common in hospitals, which has allowed for the development of resistance to all known antimicrobials. The Gram-negative short-rod-shaped (coccobacillus) A. baumannii thrives in a number of unaccommodating environments due to its tolerance to a variety of temperatures, pHs, nutrient levels, as well as dry environments.[13] The Gram-negative aspects of the membrane surface of A. baumannii, including the efflux pump and outer membrane, affords it a wider range of antibiotic resistance.[13] Additionally, some problematic A. baumannii strains are able to acquire families of efflux pumps from other species, and commonly first to develop new β-lactamases to improve β-lactam resistance.[13]

The Gram-negative, rod-shaped (bacillus) bacteria Pseudomonas aeurginosa is ubiquitous hydrocarbon degrader that is able to survive in extreme environments as well as in soil and many more common environments. Because of this versatility, it survives quite well in the lungs of patients with late-stage cystic fibrosis (CF).[13] It also benefits from the same previously mentioned Gram-negative resistance factors as A. baumannii. Mutants of P. aeurginosa with upregulated efflux pumps also exist that make finding an effective antibiotic or detergent incredibly difficult.[13] There are also some multi-drug resistant (MDR) strains of P. aeruginosa that express β-lactamases as well as upregulated efflux pumps which can make treatment particularly difficult.[13]

Enterobacter encompasses a family of Gram-negative, rod-shaped (bacillus) species of bacteria. Some strains cause urinary tract (UTI) and blood infections and are resistant to multiple drug therapies, which therefore puts the human population in critical need for the development of novel and effective antibiotic treatments.[15] Colistin and tigecycline are two of the only antibiotics currently used for treatment, and there are seemingly no other viable antibiotics in development.[13] In some Enterobacter species, a 5–300-fold increase in minimum inhibitory concentration was observed when exposed to several gradually increasing concentrations of benzalkonium chloride (BAC).[16] Other Gram-negative bacteria (including Enterobacter, but also Acinetobacter, Pseudomonas, Klebsiella species, and more) also displayed a similar ability to adapt to the disinfectant BAC.[16]