Heater-cooler units (HCUs) used in cardiopulmonary bypass and extracorporeal membrane oxygenation (ECMO) can generate infectious aerosols containing Mycobacterium chimaera, a slow-growing nontuberculous mycobacterium (NTM) associated with disseminated infection. Since the identification of M. chimaera infective endocarditis in 2013, many more cases of deep-seated infections with M. chimaera have been identified and linked to the use of contaminated Stöckert 3TLivaNova (London, United Kingdom) HCUs.Reference Van Ingen, Kohl and Kranzer1 Few studies have analyzed the water contamination of HCUs used in ECMO.Reference Trudzinski, Schlotthauer and Kamp2 In this study, we aimed to ascertain whether HICO-Variotherm units (Chalice Medical, Worksop, UK) used in ECMO were colonized with Mycobacterium spp and to assess the associated risk of aerosolization into the critical care environment.
Methods
Study setting
This investigation was conducted in the cardiothoracic critical care unit (CTCCU) of Wythenshawe hospital (Manchester, UK) which is a conventionally ventilated unit (ie, no HEPA filters). At the time of study (November 2017), 3 HICO-Variotherm HCUs (Chalice Medical) were in use for patients undergoing veno-venous ECMO for 7–15 days for severe respiratory failure (Fig. 1).
Fig. 1. A heater cooler device.
Water samples
In the CTCCU, filtered tap water is used for the HCUs and water testing is not routinely performed. A serological pipette was used to transfer a sample (100 mL) of the water circulating through each HCU to a sterile sample container. Each water sample was cultured on tryptone soya agar (TSA; Oxoid, Basingstoke, UK), and potential marker organisms were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). To culture for Mycobacterium spp, a 50-mL aliquot of each water sample was decontaminated and neutralized (BBL Mycoprep system, Becton Dickinson, Oxford, UK) before being passed through a 0.2-µm membrane filter. Each filter was transferred to a Middlebrooks 7H11 agar plate and incubated at 37°C for up to 6 weeks. To provide a more rapid indication of the presence or absence of Mycobacterium spp, each water sample was also assayed using an in-house quantitative polymerase chain reaction (qPCR) assay incorporating previously published primers.Reference Bruijnesteijn van Coopenraet, Lindeboom and Prins3 A qPCR assay was also used to detect the presence of Legionella spp using a method validated in a previous study.Reference Collins, Jorgensen and Willis4
Air samples
Air samples were collected immediately adjacent to each HCU and at the bedside (∼1 m from the unit). We used two types of air samplers. First, we used an impaction sampler (AirIdeal, bioMérieux UK Limited, Basingstoke, UK), in which airborne microorganisms are impacted on the surface of an agar plate. At each position, 3 consecutive samples were taken, each incorporating a different culture media: TSA, cetrimide agar (for Pseudomonas aeruginosa), or Middlebrooks 7H11. On each occasion, the sampler was operated at 100 L per minute for 5 minutes (sample volume, 0.5 m3 of air).
Second, we used a liquid impinger (Coriolis µ, Bertin Instruments, France), in which airborne microorganisms are concentrated in a collecting fluid. The sampler was operated once (300 L per minute for 10 minutes or 3 m3 of air) at each position, and the collecting fluid (phosphate buffered saline + 0.01% Tween 20) was cultured (in duplicate) on TSA, cetrimide agar, and Middlebrook 7H11 agar plates. Each plate was assayed for the presence or absence of Mycobacterium spp and Legionella spp using qPCR.
Water and air samples were obtained on the same day. The presence of organisms in both would imply contamination of the HCU with associated aerosolization into the surrounding clinical environment.
Results
Water samples
A single water sample was taken from each of the 3 ECMO units. The number of bacteria cultured (on TSA) ranged from 5.7 × 106 CFU/L to 3.8 × 107 CFU/L (n = 3). In all cases, the predominant organism was identified as Ralstonia spp (ie, picketti or insidiosa). No NTM were cultured from the water samples. However, when analyzed via qPCR Mycobacterium spp were detected in water taken from 2 of the 3 ECMO units at a level of 10.2 GU/L (the theoretical equivalent of 3.4 × 103 GU/L water) and 14.6 GU/3mL (∼4.8 × 103 GU/L water) (Table 1). A Legionella spp was detected in water taken from 2 of the 3 ECMO units at a mean concentration of 80.9 GU/3mL (∼3.4 × 104 GU/L water; n = 2) and 1,029 GU/3mL (∼3.4 × 105 GU/L water; n = 2). Legionella pneumophila was not detected.
Table 1. Organisms Cultured From Water Collected From All 3 ECMO Units
Note. ECMO, extracorporeal membrane oxygenation; qPCR, quantitative polymerase chain reaction.
Air samples
Neither Ralstonia spp nor Mycobacterium spp were cultured from any of the 12 air samples (Table 1). Mycobacterium spp was not detected in any of the 6 air samples analyzed via PCR. Legionella pneumophila was detected in 1 air sample (86 GU/m3). The organism could not be cultured from the sample, so viability (or source) could not be confirmed.
Discussion
Mycobacteria chimaera produces biofilm that enables it to persist in water systems and its hydrophobicity favors aerosolization.Reference Walker, Moore and Collins5 In our study, we detected Mycobacteria spp in 2 of 3 ECMO water samples. However, unlike previous studies on HCUs used for cardiothoracic bypass, aerosolization into the environment was not demonstrated.
The Ralstonia genus is a group of gram-negative nonfermenters that are well adapted to surviving in low nutrient conditions, which allows them to persist in various water supplies.Reference Safni, Cleenwerck and De Vos6 We found high numbers of Ralstonia spp in the water circulating the ECMO units but none in associated air samples.
The ECMO machines are air-tight and closed systems in contrast to the HCUs used in cardiothoracic surgery, which may have precluded the release of aerosols.Reference Struelens and Plachouras7Mycobacteria chimaera contamination of ECMO devices was reported by Trudzinki et al.Reference Trudzinski, Schlotthauer and Kamp2 As in our study, they did not find any mycobacteria in the 9 room air samples or other environmental samples. Although transmission of M. chimaera from an ECMO device to a patient is yet to be described, the theoretical risk of aerosolization remains when machines are decontaminated or emptied or when circuits are broken during use. Regular and effective decontamination of the HCUs and microbiological surveillance are vital steps in mitigating the risk of infection due to M. chimera and other opportunistic pathogens.
Acknowledgments
We are grateful to the staff of the Extracorporeal Membrane Oxygenation (ECMO) unit of Wythenshawe Hospital for supporting this research.
Financial support
No financial support was provided relevant to this article.
Conflicts of interest
All authors report no conflicts of interest relevant to this article.
