identification of m. chimaera infection in 6 patients prompted an outbreak investigation at the university hospital zurich , a 900-bed tertiary - care hospital in zurich , switzerland , that performs 700 open - heart surgeries that use extracorporeal circulation per year . surveillance cultures of hcu water from the cardioplegia and patient circuits and airflow samples from running hcus at 23 m distance were gathered in monthly intervals . failure to eradicate m.  chimaera and other ntm from older hcus prompted the acquisition of 5 factory - new hcus ( model 3 t ; sorin [ now livanova , london , uk ] ) during 2014 : 2 in january , 1 in april , and 2 in september . mycobacterial cultures were performed according to standard methods by using the mycobacteria growth indicator tube system ( mgit 960 ; becton dickinson , sparks , md , usa ) or middlebrook 7h11 agar plates ( bd difco mycobacteria 7h11 agar ; becton dickinson ) that were incubated at 37c for 7 wks or until positive . air specimens were gathered with a microbiologic air sampler ( mas-100 nt , mbv , stfa , switzerland ) running for 2.5 min at a rate of 100 l / min by using middlebrook 7h11 agar plates . mycobacterial species were identified by 16s rrna gene sequencing , as described ( 6 ) . before mid - april 2014 , hcus were serviced according to the manufacturer s recommendations . before first use and every 3 months thereafter , a disinfection cycle was performed by adding 200 ml of 3% sodium hypochlorite ( maranon h ; ecolab , northwich , uk ) to the hcu water tanks filled with filtered tap water ( pall - aquasafe water filter aq14f1s ; pall , portsmouth , uk ) to a final concentration 0.045% sodium hypochlorite in the water tank and circuits . water was changed every 14 days ; 100 ml of 3% hydrogen peroxide was added to the initial water filling ( final concentration 0.02% hydrogen peroxide in water tank and circuits ) ; an additional 50 ml of 3% hydrogen peroxide was added every 5 days ( 7 ) . in mid - april 2014 , an intensified in - house cleaning and disinfection procedure was implemented , consisting of daily water changes with filtered tap water ( pall ) and additions of 100 ml of 3% hydrogen peroxide combined with biweekly disinfection using sodium hypochlorite ( maranon h ) . in february 2015 , with lack of availability of 3% sodium hypochlorite and in line with the manufacturer s recommendations , the disinfection solution was changed to a combination of peracetic acid and hydrogen peroxide ( an additional 450 ml puristeril 340 ; fresenius medical care , hamburg , germany ) in filled water tanks every 2 weeks . also , stainless steel housings were custom built around the hcus to ensure strict air separation between the exhaust air of the hcus and the operating room air ( figure 1 ) . custom - made stainless steel housing for heater  cooler units ( model 3 t ; sorin [ now livanova , london , uk ] ) used at the university hospital zurich , zurich , switzerland . a ) front view shows the fine dust filter f7 over the air inlet ( arrow ) . b ) side view shows the half - open back door with the rectangular opening ( arrow ) , through which a duct connects the housing to the operating room ventilation exit . a total of 134 water samples were obtained from the study hcus , 127 after implementation of the intensified protocol . the first water sample tested positive for m.  chimaera in august 2014 , originating from a study hcu introduced 7 months earlier ( figure 2 ) . of all samples , 90 ( 67.2% ) remained sterile for ntm ; 6 ( 4.5% ) were contaminated by bacterial overgrowth ; and 38 ( 28.4% ) yielded ntm . of these ntm , 22 ( 57.9% ) were m. chimaera ; 12 ( 31.6% ) were m. gordonae ; 1 ( 2.6% ) was m. chelonae ; 1 ( 2.6% ) was m. paragordonae ; and 2 ( 5.3% ) were a combination of m. chimaera and m. gordonae . results of heater  cooler unit ( hcu ) water surveillance cultures by year and month , university hospital zurich , zurich , switzerland . the dashed vertical line shows the date of implementation of the intensified protocol ( i.e. , mid - april 2014 ) . the vertical arrows indicate the start of use of each factory - new hcu in the operating room . hcu 3 , hcu 4 , and hcu 5 were serviced with the intensified in - house maintenance from the time of delivery . no indicates a negative culture for nontuberculous mycobacteria ( ntm ) , indicated by x. yes indicates a positive culture for ntm . as per date indicated , cultures were reported as positive if 1 water sample ( 1 sample from cardioplegia and 1 sample from patient circuit were gathered at each date ) grew ntm . for missing data points ( i.e. , no negative or positive results shown ) , mycobacterial cultures could not be tested because of bacterial overgrowth or lack of availability of the hcu . no statistically significant different results for water samples from cardioplegia and patient circuit ( fisher exact test , p = 0.554 ) . six ( 16.2% ) samples had bacterial overgrowth . of 91 air samples , 90 ( 98.9% ) one sample grew m. chelonae , although no mycobacterium was detected simultaneously in the corresponding hcu water . ntm growth was recorded after a median of 174 ( range 158358 ) days in hcu water samples . one of 5 hcus remained permanently without growth of m. chimaera ; 4 grew m. chimaera after a median of 250 ( range 158358 ) days . hcus seem to provide favorable environmental conditions for growth of ntm , in particular m. chimaera . an intensified cleaning and disinfection protocol failed to prevent growth of ntm entirely but succeeded in preventing detectable aerosolization of m. chimaera . the contamination status of hcus seems to be influenced by the intensity of maintenance , especially frequency of water changes . the consistently negative air cultures for m. chimaera and the only intermittently positive water cultures support the benefits of our intensified protocol . our study design did not elucidate the origin of m. chimaera and other ntm in hcus . the hcus might have been already contaminated at time of delivery in a concentration below the detection threshold of mycobacterial cultures . a recent investigation confirmed the presence of environmental mycobacteria , including m. chimaera , in factory - new hcus ( 8,9 ) . in our study , 1 hcu ( hcu 4 , figure 2 ) tested positive for m. chimaera for the first time after being returned from repair at the manufacturer . previous studies indicated the durability of mycobacteria against several disinfectants , likely because of the organisms complex cell wall ( 10 ) . m. avium complex isolates were shown to have a high level of resistance against chlorine when grown in water ( 11 ) . corrosion , certain material characteristics , and dead - end spaces can favor biofilm formation and mycobacterial growth . killing of ntm with heat may be promising ; older studies reported a high efficacy with temperature exposure at 70c ( 10,12 ) . a recent report indicated complete suppression of m. chimaera in hcus by intensified maintenance after a complex decontamination regimen , including replacement of plastic tubing ; however , follow - up was limited to 3 months ( 13 ) . first , some study hcus were temporarily maintained according to the manufacturer s standard before the hospital adopted the intensified protocol . second , we did not include a control hcu with ongoing maintenance according to the manufacturer s recommendations . third , the detection threshold of m. chimaera in water and air cultures remains to be identified . our findings challenge the effectiveness of the hcu manufacturer s maintenance recommendations , which were recently changed to disinfection with sodium hypochlorite before first use and every 14 days and water changes with all - bacteria  filtered tap water plus 150 ml 3% hydrogen peroxide every 7 days ( 14 ) . to ensure patient safety until safe hcu technology is available nevertheless , with this measure , the airflow must be restrained from diffusion back into the or ( 15 ) . because the maximum allowed length of water circuit tubing and the architectural layout prohibited this solution at our hospital , we produced airtight housings for the hcus ; however , this solution has less flexible placement of the hcus within the or .
invasive mycobacterium chimaera infections after open - heart surgery have been reported internationally . 
 these devastating infections result from aerosols generated by contaminated heater  cooler units used with extracorporeal circulation during surgery . despite intensified cleaning and disinfection , surveillance samples from factory - new units acquired during 2014 grew nontuberculous mycobacteria after a median of 174 days .