ESKAPES: Emerging Pathogens of Concern

ESKAPE Pathogens

ESKAPE Pathogens
ESKAPE Pathogens


Emerging pathogens in healthcare settings are a concern due to their antibiotic resistant nature. Historically, the water treatment providers have been concerned with Legionella, but the danger has now been extended to include Pseudomonas aeruginosa following numerous outbreaks. The ESKAPES pathogens highlight a further five pathogens which should be of concern due to their antibiotic resistant nature.

Main Article:

Those in charge of managing healthcare environments have had guidance on the importance of controlling the presence of Legionella via the Health Technical Memorandum (HTM) 04-01 (1), which provides specific guidance for the control of Legionella in hot and cold water systems in healthcare settings. The addendum to HTM 04-01 (2) was published in March 2013 in response to the threat posed by Pseudomonas aeruginosa to immunocompromised patients in augmented care settings. However, there are other emerging pathogens of concern that should be considered by Healthcare and Water Treatment companies–the ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter, Pseudomonas aeruginosa and Enterobacter) pathogens. These pathogens were first identified by the Infectious Diseases Society of America (IDSA) in 2004 (3, 20).  Recently, Stenotrophomonas maltophila has become more prevalent leading to ALS Environmental producing a specific suite of analysis for the IDSA pathogens plus S. maltophila (4, 5), which are referred to as ESKAPES pathogens:

ESKAPES Pathogen General Description
Enterococcus faecium Belonging to the Enterococci genus, E. faecium are Gram-positive cocci that can grow in both aerobic and anaerobic conditions (6) and in the presence of bile salts or sodium azide, which are inhibitory conditions for the vast majority of Gram-negative bacteria. E. faecium can survive in temperatures up to 44°C.
Staphylococcus aureus Gram-positive, non-motile cocci often found as single cells or clumps of cells. Staphylococcus spp. are anaerobic bacteria. Staphylococci can grow between 30-37°C, found in water and human faeces, spread through air on dust particles, and be resistant to drying (6).


Staphylococcus aureus is oxidase negative, catalase-positive, and coagulase-positive. Three species of StaphylococciS. epidermis, S. haemolyticus, and S. aureus–form part of the normal flora of the skin and mucous membranes of humans and have been linked with human infections.

Klebsiella pneumoniae Part of the Enterobacteriaceae family and coliform group of organisms, Klebsiella spp. are non-motile, Gram-negative bacteria that are capable of growing at 37°C. There are four species of Klebsiella: K. pneumoniae, K. oxytoca and K. terrigena. A large polysaccharide capsule distinguishes Klebsiella from other coliform and Enterobacteriaceae family members.
Acinetobacter baumannii Acinetobacter is a genus of Gram-negative bacteria commonly found in soil, water, and sewage environments. Acinetobacter spp. are part of the natural microbial flora of the skin and occasionally the respiratory tract of healthy individuals (6,9). A. baumannii, a coccobacillus and rapidly emerging pathogen in health care settings, is usually introduced into hospitals by a colonized patient. It has the ability to survive on artificial surfaces and resist desiccation, which allows it to potentially infect new patients.


There are many species of Acinetobacter that can cause human diseases (6) with A. baumannii accounting for about 80% of reported infections. It is hypothesized that A. baumannii growth favours nosocomial settings due to the constant use of antibiotics by patients in the hospital (10).

Pseudomonas aeruginosa An aerobic, Gram-negative bacteria that is a member of the Pseudomonadaceae familu. Pseudomonas frequently colonizes manmade environments, such as water systems, where they can form biofilms. It has been suggested that biofilms provide a breading ground for Legionella bacterial growth.


Pseudomonas aeruginosa forms pigmented (green-brown or reddish-brown) and/or fluorescent colonies on Pseudomonas CN agar after incubation at 37°C for 40-48 hours (2).

Enterobacter Species A common Gram-negative, anaerobic, non-spore-forming bacteria, Enterobacter is a member of the Enterobacteriaceae family, with several pathogenic strains (6).  Three clinically important species from this genus are E. aerogenes, E. cloacae, and E. sakazak. E. sakazakii been found as a contaminant in infant formulas and to be more resistant to osmotic and dry stress than any other members of the Enterobacteriaceae family.


Enterobacter species are biochemically similar to Klebsiella. However, unlike Klebsiella, Enterobacter is ornithine positive.

Stenotrophomonas maltophila S. maltophila is a Gram-negative, opportunistic pathogen that is difficult to treat once in a human host because of its antibiotic resistant nature (4,5, 7, 8, 10). In nosocomial settings, S. maltophila cause cause infections. Although infections are rare, it is difficult to treat and the pathogen has shown the capacity to quickly develop into strains with increased drug resistance/tolerance.

The IDSA report cited the antibiotic resistant nature of ESKAPE pathogens as the main consideration for concern because drug companies are reducing funding into the development of new antibiotics (3). The ESKAPES pathogens are of more prominent concern in healthcare settings due to the high susceptibility of patients (elderly, young, and immunocompromised) to infections.

ESKAPES: Stability Times

All of the ESKAPES pathogens have a sample stability time of 24 h from the point of sampling for accredited analysis. This means that the sample needs to be sampled, transported to the laboratory, registered, and processed within that time period. The guidance for stability times is provided by United Kingdom Accreditation Service (UKAS) (11) with laboratories able to extend stability times with sufficient validated data.

ESKAPES: MALDI-ToF Confirmations

The use of Matrix Assisted Laser Disorption and Ionization by Time of Flight (MALDI-ToF) has been well documented for the rapid identification of bacteriological isolates(21). The innovative approach allows microbiologists to provide an instant confirmation of any isolates from culture media and deliver results back to customers along with Colony Forming Unit (CFU) by volume filtered. The technique generates a mass spectra creating a protein fingerprint unique to each genus and species of bacteria. The protein finger print generated by MALDI-ToF is then compared against an ever expanding bespoke internal library of over 15,000 known species and >1000 bacterial genera.

The HTM 04-01 and addendum both state that the confirmation of Legionella and Pseudomonas aeruginosa should be performed at a UKAS ISO 17025:2005 accredited laboratory using referenced methods. The use of MALDI-ToF to confirm the pathogens protein finger print is in line with all UKAS, HTM, and Drinking Water Testing Specification (DWTS) guidance

ESKAPES Pathogen Incubation Time Traditional Confirmation: MALDI-ToF Confirmation: MALDI-ToF Saving
Enterococcus faecium 2 days 1 day Minutes 1 day
Staphylococcus aureus 2 days 1 day Minutes 1 day
Klebsiella pneumoniae 1 day 1 day Minutes 1 day
Acinetobacter baumannii 1 day 1 day Minutes 1 day
Pseudomonas aeruginosa 2 days 1 day Minutes 1 day
Enterobacter spp. 1 day 1 day Minutes 1 day
Stenotrophomonas maltophilia 1 day 1 day Minutes 1 day

Enterococcus faecium (6, 12,15)


The faeces of warm blooded animals are a common source of Enterococci bacteria and are an indicator of contamination, with Coliforms and E. coli being the primary indicators. Enterococci are more resilient than the primary pathogens to environmental stress and chlorination. Enterococci tend to outlive their counterparts outside of laboratory conditions.

By using the MALDI-ToF confirmation technique, 34 different species of Enterococcus, including 10 different strains of E. faecium, can be identified. This ability to distinguish between the contamination type can therefore potentially identify the source of contamination. E. faecalis, E. faecium, and E. durans are normally present in the faeces of humans and various animals. Streptococcus bovis, Streptococcus equinus, and E. avium have been found to be associated with cattle, horse, and bird faecal contamination, respectively. These organisms are not normally found in human faeces.

Pathogenesis and routes of exposure

E. faecium enters the human body through contact (ingestion, open wounds, etc.) with contaminated sources. Once infected, the patient suffers from septicaemia and the mortality rate can be as high as 50%.

Significance in water

The presence of intestinal Enterococci provides evidence of recent faecal contamination and detection should lead to consideration of further action, which could include further sampling and investigation of potential sources such as inadequate treatment or breaches in distribution system integrity.

Staphylococcus aureus (6, 13)


Staphylococci bacteria can colonize in water and soil. Moreover, the bacteria are part of the human flora and commonly colonize the skin and mucous membranes (e.g. the naso pharyngeal system).


S. aureus can multiply and colonize a patient in a short time frame, especially in immunocompromised individuals because a weakened immune system and normal body temperature provides the ideal conditions for growth. This can result in a range of ailments including boils, skin sepsis, post-operative wound infections, enteric infections, septicaemia, endocarditis, osteomyelitis, pneumonia, impetigo, meningitis, and arthritis.

Routes of exposure

The most common pathway for Staphylococcus infection is via skin to skin contact, with inadequate basic hand hygiene from staff, patients, and visitors being a frequent root cause in nosocomial settings. Food contaminated with S. aureus can provide an ideal breeding ground, especially when left at room temperature. The consumption of foods containing S. aureus toxins can lead to enterotoxin food poisoning within a few hours.

Significance in drinking water

Although S. aureus can be found in drinking-water supplies, there is no evidence of transmission through the consumption of such water. Although Staphylococci are slightly more resistant to chlorine residuals than E. coli, their presence in water is readily controlled by conventional treatment and disinfection processes. ALS Environmental can identify over 40 different species of Staphylococcus. Fourteen of these species are strains of S. aureus; 10 of which are variants on the subspecies S. aureus sub species anaerobius and S. aureus sub species aureus.

Klebsiella pneumoniae (6, 14, 15)


Klebsiella colonization can occur in soil, water, or animal faeces. The bacteria are more prominent in waters that are rich in nutrients. In nosocomial scenarios, Klebsiella has been known to colonize taps and water distribution systems.


Poor hand hygiene of staff, patients, and visitors has been identified as one of the main sources of colonization of Klebsiella spp. in nosocomial settings. The weakened immune systems of the immunocompromised, both young and old, or those with open wounds are highly susceptible to infection from Klebsiella and colonization may lead to an infection. On rare occasions, Klebsiella spp., notably K. pneumoniae and K. oxytoca, may cause serious infections such as destructive pneumonia.

Significance in water

The contamination of drinking water with Klebsiella is not considered as a potential source of gastrointestinal illness. The bacteria are generally present in biofilms and are unlikely to represent a health risk to humans. The pathogen is usually well controlled with chemical disinfection and entry into distribution systems can be prevented by adequate treatment. Raoultella and Klebsiella are very closely related with some Raoultella being reclassified as Klebsiella in some circumstances by older traditional microbiological confirmation techniques. The MALDI-ToF database used and developed by ALS Environmental identifies 13 strains of K. pneumoniae which we are able to uniquely distinguish.

Acinetobacter baumannii (6, 10, 16, 17)


Acinetobacter spp. can survive for an extended period on most surfaces. The bacteria particularly favours nosocomial settings due to the high use of antibiotics, as outlined in the IDSA 2004 and WHO 2011 paper and guidance. MALDI-ToF confirmation can identify 19 different species of the genus Acinetobacter, 15 of which are A. baumannii. The pathogen can colonize soil, water, sewage, and the skin of healthy people. This allows the bacteria to be spread by skin contact from person-to-person or from a colonized surface to another person.

Routes of exposure

Intravenous catheters have been identified as a source of infection in patients with Acinetobacter. The WHO 2011 guidance on Drinking Water lists Acinetobacter as a pathogen which transmission through drinking water has been suggested but not yet conclusively evidenced.


Acinetobacter infections in immunocompromised individuals can include urinary tract infection, pneumonia, meningitis, and wound infection. There are various reports of veterans from the US and UK who were injured while on tours of Iraq or Afghanistan being colonized by A. baumannii.

Significance in water

Acinetobacter spp. are frequently detected in treated drinking-water supplies. However, an association between the presence of Acinetobacter spp. in drinking-water and clinical disease has not been confirmed. There is no evidence of gastrointestinal infection through ingestion of Acinetobacter spp. in drinking-water among the general population. However, transmission of non-gastrointestinal infections by drinking-water may be possible in susceptible individuals, particularly in settings such as healthcare facilities and hospitals.

Pseudomonas aeruginosa (2, 6, 15, 18)


P. aeruginosa is a common environmental organism that can be found in faeces, soil, water, and sewage. It can multiply in water environments and also on the surface of suitable materials in contact with water. P. aeruginosa has been isolated from a range of moist environments such as sinks, water baths, hot water systems, showers, and spa pools.


Principal infections include septicaemia, skin, respiratory, urinary tract, and ear and eye. These infections may occur due to burns, surgery, and open wounds. Cystic fibrosis and immunocompromised patients are prone to colonization with P. aeruginosa, which may lead to serious progressive pulmonary infections.

Routes of exposure

The highly susceptible nosocomial patients include those on breathing machines, premature babies, and patients with wounds from surgery or burns.  In additional, healthy people can develop mild illnesses with P. aeruginosa, especially after exposure to water. Ear infections, especially in children, and more generalized skin rashes may occur after exposure to inadequately chlorinated hot tubs or swimming pools.

Significance in water

P. aeruginosa can be found in drinking water which can lead to potential colonization in production and healthcare facilities. Bottled waters should be examined for this organism. P. aeruginosa can also be found in swimming pools and bathing facilities. This may be a particular concern for augmented care units due to the organisms antibiotic resistant nature. Large numbers of this bacterium growing in polluted waters, swimming pool waters, or spa pool waters may, following immersion, produce ear infections or a follicular dermatitis.

Enterobacter species (6,10, 14, 15)


Enterobacter spp. are present in various environments including soil, water, and sewage. In addition, Enterobacter spp. are present in animals and humans, where it can cause opportunistic infections within the gastrointestinal system.


E. sakazakii has been associated with sporadic cases or small outbreaks of sepsis, meningitis, cerebritis, and necrotizing enterocolitis. Most of the infections are seen in low-birth-weight infants (i.e. less than 2 kg) or infants born prematurely (i.e. less than 37 weeks of gestation). Mortality has been reported to be as high as 50% but has decreased to less than 20% in recent years.

Routes of exposure

Risk factors for nosocomial Enterobacter infections include hospitalization of greater than 2 weeks, invasive procedures in the past 72 h, treatment with antibiotics within the past 30 days, and the presence of a central venous catheter. Specific risk factors for infection with nosocomial multidrug-resistant strains of Enterobacter spp. include the recent use of broad-spectrum cephalosporins or aminoglycosides.

Significance in water

Enterobacter is present in water, although its presence can be prevented with a successful water treatment programme. In particular, E. sakazakii is sensitive to disinfectants and its presence can be prevented by adequate treatment.

Stenotrophomonas maltophilia (4, 5, 6, 7, 8)


S. maltophilia is an organism of low virulence and frequently colonizes fluids used in the hospital setting (e.g. irrigation solutions and intravenous fluids) and patient secretions (e.g. respiratory secretions, urine, and wound exudates). Stenotrophomonas can be found in soils and waters and can colonize drinking water systems. Although there is no natural infection route in humans, S. maltophila infection can be transmitted by contaminated prosthetics such as catheters and intravenous lines.


In severely ill patients, S. maltophila causes a wide range of infections such as bacteremia, pulmonary infections, urinary tract infections, wound infections, meningitis, and endocarditis. The risk of infection can be elevated in healthcare settings and other environments (e.g. in care homes or intensive care units) where immunocompromised patients may be exposed.

Significance in water

The presence of S. maltophilia in the water of nosocomial settings is a concern to infection control due to its opportunistic and antibiotic resistant nature. The risk of S. maltophilia infection comes from its ability to survive on artificial surfaces and be transferred, via medical equipment, to immunocompromised patients. Once an immunocompromised patient is infected with S. maltophila, the bacterial infection can be difficult to treat, which can result in a severe infection.


The ESKAPE pathogens, as identified by the IDSA (3) in 2004, still pose a threat to the immunocompromised and healthcare institutions, with S. maltophilia being considered a further potential risk factor because it is resistance to antibiotics. When analyzing water samples for any pathogenic or bacteriological contamination, time is an important factor. Infection control teams need as much information on any potential risk factors, such as waterborne pathogens, as soon as possible. MALDI-ToF allows for rapid confirmed and identified analytical data–down to both the genus and species of bacteria (and/or fungi).

The risks associated with the ESKAPES pathogens are not going away. As our antibiotic use increases, the health risks posed to patients and general population from these pathogens are also likely to increase. With a stringent Water Safety Group, well-maintained maintenance plans, and routine analytical testing and monitoring, we can help keep patients, staff, and the general public safe from Legionella and ESKAPES pathogens.


Home, C. “HTM04-01: The control of Legionella, hygiene “safe” hot water, cold water and drinking water systems. Part A: Design Installation and Testing (2006)

Ashcroft, P. “HTM04-01 – Addendum: Pseudomonas aeruginosa – advice for augmented care units (2013)

3) Brews C, “Infectious Diseases Society of America. “Bad Bugs, No Drugs. As Antibiotic Discovery Stagnates. A Public Health”. (2004).

4) Denton M, Todd NJ, Kerr KG, Hawkey PM, Littlewood JM. Molecular epidemiology of Stenotrophomonas maltophilia isolated from clinical specimens from patients with cystic fibrosis and associated environmental samples. J Clin Microbiol 1998;36:1953–1958.

5) Stenotrophomonas maltophila. Venkateswara rao, T. Travancore Medical College. (July 2011).

6) World Health Organisation (WHO) Guidelines for Drinking-water Quality, Fourth Edition (2011).

7) Palleroni N, Bradbury J (1993). “Stenotrophomonas, a new bacterial genus for Xanthomonas maltophilia (Hugh 1980) Swings et al. 1983”. Int J Syst Bacteriol 43 (3): 606–609.

8) Yoon, J.-H., Kang, S.-J., Oh, H. W., Oh, T.-K. (2006). “Stenotrophomonas dokdonensis sp. Nov., isolated from soil”. Int J Syst and Evo Micro 56: 1363 – 1367.

9) Centre for Disease Control, Guideline for Disinfection and Sterilization in Healthcare Facilities, (2008).

Seigel, J,D et al. “Management of Multi-drug resistant Organisms in Healthcare Settings” (2006)


12) Environment Agency, The Microbiology of Drinking Water (2012), Part 5 – Methods for the Isolation and enumeration of enterococci – Methods for the Examination of Waters and Associated Materials.

13) Environment Agency, The Microbiology of Recreational and Environmental Waters (2000) -Standing Committee of Analysts – Methods for the Examination of Waters and Associated Materials.

14) Environment Agency, The Microbiology of Drinking Water (2002), Part 4 – Methods for the Isolation and enumeration of coliform bacteria and Escherichia coli -Standing Committee of Analysts – Methods for the Examination of Waters and Associated Materials.

15) Environment Agency, The Microbiology of Drinking Water (2002), Part 1 – Water Quality and Public Health  -Standing Committee of Analysts – Methods for the Examination of Waters and Associated Materials.

16) Al Sehlawi et al, “Isolation and Identification of Acinetobacter baumannii Clinical Isolates using Novel Methods”, Journal of Babylon University/Pure and Applied Sciences/ No.(3)/ Vol.(22): (2014).


18) Environment Agency, The Microbiology of Drinking Water (2002), Part 8 – Methods for the Isolation and enumeration of Aeromonas and Pseudomonas aeruginosa- Methods for the Examination of Waters and Associated Materials.

19) Denton M, Todd NJ, Kerr KG, Hawkey PM, Littlewood JM. Molecular epidemiology of Stenotrophomonas maltophilia isolated from clinical specimens from patients with cystic fibrosis and associated environmental samples. J Clin Microbiol 1998;36:1953–1958

20) “Clinical Relevance of the ESKAPE Pathogens” J N Pendleton JN, Gorman SP , Gilmore BF

21)  “Subtyping of Legionella pneumophila for epidemiological investigations by matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectrometry.” Jung, j., Grob, ß., Weinert, k., Maier, T., Kostrzewa, M., Schubert, S. 2012.

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Nick Barsby
A member of the Water Management Society, Nick is the Sales & Marketing Manager for ALS Environmental. With over 10 years’ experience in the Environmental sector Nick has been involved in several high profile developments at ALS including the development and launch of the MALDI-ToF, overhaul of the company website and growing our bacteriological database for MALDI-ToF confirmations. Nick has presented at various UK conferences and technical working groups on MALDI-ToF, Drinking Water Analysis and ESKAPE pathogens. Having worked for various Environmental laboratories in the UK Nick has a breadth of experience in Water Treatment techniques, contaminated land and waste management
Pervinder Singh Johal
Pervinder Singh Johal MSc CBiol MRSB is the Microbiology and Site Manager at ALS Coventry. With over 20 years of Laboratory operational and technical management of UKAS and DWTS accredited methods in the enironmental sector. Other laboratory management systems include MHRA, ISO 14001, OHSAS 1800. Pervinder in his time as Microbiology operations manager has successfully introduced the following into operations which have included extensive validation data where required - full microbiology automation (first in the UK water market), PCR for Legionella analysis, Cryptosporidium automation and recently MALDI TOF to enhance the service which ALS offers. Pervinder is a Chartered Biologist with membership of the Society of Biology, has membership with the Water Management Society and sits on the expert water and environmental microbiology panel with the Standing Committee of Analysts (SCA) who review and publish updates to microbiology working methods.