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Pseudomonas, Mycobacteria and Legionella: Why HCU Process Water Must Be Consistently Controlled

In this blog article, Pseudomonas, mycobacteria and Legionella show why HCU process water must be consistently controlled. Contaminated process water is not an option in hospitals. This is exactly where our WCD2 comes in: it cleans and disinfects HCU process water using boron-doped diamond electrode technology. In this way, WCD2 helps to maintain germ-free process water permanently under defined operating conditions.

 

Heater-cooler units (HCUs) are essential for many cardiac surgery procedures. They provide temperature-controlled water that supplies heat exchangers or warming blankets through closed circuits. This water does not come into direct contact with the patient’s blood. However, this is precisely where an important technical detail lies: even without direct contact, contaminated water can pose a risk if microorganisms grow inside the device, form biofilms or enter the surrounding environment via aerosols. The FDA (Food & Drug Administration, USA) explicitly describes the risk that contaminated fluid or aerosols generated from it may enter the surrounding environment and potentially the operating room area through openings, vents or exhausts in water-based HCUs.[1] 


The main focus here is on opportunistic waterborne microorganisms. These include Pseudomonas aeruginosa, nontuberculous mycobacteria including Mycobacterium chimaera, and Legionella pneumophila. These pathogens are not exotic exceptions. They are among the microorganisms that play a particularly important role in technical water systems, biofilms and humid environments. The CDC (Centers for Disease Control & Prevention, USA) explicitly lists Pseudomonas aeruginosa, Legionella pneumophila and nontuberculous mycobacteria, including M. chimaera, as opportunistic pathogens in building water systems.[2]

 


Water Is Not a Passive Medium in Clinical Environments

When hospital water enters a building, it generally meets defined quality standards. However, this does not mean that every downstream technical system automatically remains unproblematic in the long term. The age and design of the installation, stagnation, temperature ranges, loss of disinfectant effect, dead spaces and biofilms can all change the microbial situation. Biofilms consist of microorganisms that adhere to wet surfaces and can persist there for long periods of time. The CDC describes them as difficult or, in some cases, practically impossible to remove completely.


For HCUs, this is particularly relevant because they not only store water, but actively circulate, heat, cool and move it technically. In such systems, it is not sufficient to look at water quality only at isolated points. What matters is a reproducible routine that addresses the entire process water.



Pseudomonas aeruginosa: Robust, Water-Loving and Clinically Relevant


Pseudomonas aeruginosa

Pseudomonas aeruginosa is an environmental microorganism commonly found in water and soil. At the same time, it is one of the most important opportunistic pathogens in clinical environments. The CDC describes infections with P. aeruginosa as typically occurring in healthcare settings and potentially affecting the bloodstream, lungs, urinary tract or postoperative areas.[3] 

Patients with ventilators, catheters, open wounds, burns or a weakened general condition are particularly at risk. Antibiotic resistance is an additional problem. The CDC points out that P. aeruginosa can be resistant to antibiotics, including multidrug-resistant variants, making infections more difficult to treat.[4]

For HCU process water, this means that Pseudomonas aeruginosa is not just a laboratory parameter. The microorganism represents the broader question of whether a water-bearing technical system remains microbiologically stable under real operating conditions.



Mykobakterien and Mycobacterium chimaera: Rare Cases, Serious Consequences


Mycobacterium chimaera

Nontuberculous mycobacteria (NTM) occur in the environment, including in soil and water. Mycobacterium chimaera belongs to the Mycobacterium avium complex and became particularly relevant internationally in connection with HCUs. In 2016, the CDC reported invasive M. chimaera infections after open-heart surgery in which contaminated heater-cooler devices had been used during extracorporeal circulation.[5] 




What makes M. chimaera special is the combination of environmental occurrence, slow growth and difficult clinical detection. The CDC describes that affected patients sometimes became ill long after surgery. Aerosolisation from contaminated devices was also discussed as a plausible transmission route, even though HCU water does not come into direct contact with the patient’s blood.


The FDA also considers water-based HCUs to be a relevant risk technology in this context. It describes that NTM can grow in water tanks, that water movement and air bubbles can promote aerosol formation, and that such aerosols can enter the operating room through fans or openings.[6] 


In Germany, an investigation during an outbreak found M. chimaera in used HCUs from several countries, in new HCUs and in the environment of a manufacturing site. The authors concluded that at least some infections may have been caused by contamination of HCUs at the manufacturing site.[7] 


The conclusion is sober: even rare events can be highly relevant in cardiotechnology if they are difficult to detect, potentially serious and technically preventable.



Legionella pneumophila: Aerosols as the Central Issue


Legionella pneumophila

Legionella pneumophila is best known as the causative agent of Legionnaires’ disease, a severe form of pneumonia. People typically become infected by inhaling water mist containing Legionella. The CDC emphasises that the disease is usually not transmitted from person to person, but through contaminated aerosols from water systems .[8] 



For prevention, therefore, not only the water quality itself is decisive, but also whether a device or system can generate or distribute aerosols. The CDC names water management programmes as a central measure for reducing Legionella growth and spread.


For HCUs, the scientific situation is differentiated. A study in Pathogens describes that HCUs and heater units can potentially generate infectious aerosols containing opportunistic pathogens such as M. chimaera, other NTM, Pseudomonas aeruginosa and Legionella spp.[9] In this study, Legionella were detected in 65.7% of the devices examined. The authors therefore recommend regular microbiological monitoring as a precautionary principle. At the same time, they state that consistent evidence of Legionella transmission from HCUs in the operating room context has not yet been clearly demonstrated.


This is exactly the kind of differentiation that matters in clinical risk management: a pathogen does not have to have caused documented infections in every scenario to deserve technical attention. The decisive question is whether conditions exist that enable growth, persistence or aerosol formation.

 


Why Routines Matter More Than Individual Measures

The shared characteristic of these three pathogen groups is not that they are equally dangerous. They differ biologically, clinically and epidemiologically. What they do have in common, however, is their relevance in water-bearing technical systems. They can persist in biofilms, survive in humid environments and become difficult to control under unfavourable conditions.


Conventional HCU routines often rely on recurring chemical disinfection of the process water. This can be effective, but it also brings practical disadvantages: staff time, device downtime, documentation effort, handling of aggressive chemicals and possible material stress. On the homepage, we describe exactly this operational pressure: recurring cleaning and disinfection processes with strong chemical agents increase workload, reduce device availability and can place long-term stress on components.[10]

 


The WCD2 Approach: Keeping HCU-Process Water Germ-Free


WCD2 product picture

WCD2 follows a different technical approach: electrochemical treatment of process water using boron-doped diamond electrodes. The scientific evidence for this includes, among other sources, a study conducted at University Hospital Regensburg. Between December 2019 and November 2020, a total of 60 microbiological sampling series were taken from a conventional HCU treated with a diamond electrode method. During the investigation period, no contamination with Pseudomonas aeruginosa or Legionella was detected. M. chimaera was detected in one sample; the source was assigned to the sampling area and was not detected again after a technical modification.


The study also describes that Legionella in the HCU water samples remained below the technical detection limit of 2 CFU/100 ml and that no Pseudomonas aeruginosa contamination was detected throughout the entire study period.[11] 

 


Conclusion: Process Water Quality Is a Technical Safety Feature

Pseudomonas aeruginosa, Mycobacterium chimaera and Legionella pneumophila represent three different microbiological challenges in water-bearing systems: opportunistic infections, slow-growing environmental mycobacteria and aerosol-relevant pathogens. In HCUs, these challenges meet a technical environment that is indispensable for cardiac surgery.


Process water should therefore not be viewed as a secondary medium, but as a component of the technical infrastructure that requires control. The decisive point is routine: clear baseline conditions, reproducible treatment, documented procedures and regular microbiological monitoring.


WCD2 is positioned precisely in this field: as an automated system that treats HCU process water with boron-doped diamond electrode technology and helps to maintain germ-free process water under defined operating conditions. For hospitals, this primarily means one thing: fewer improvised individual measures and more predictable process water quality.



 
 
 

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