Many Heater-Cooler Units Suffer Bacterial Contamination

A new study reveals that over a third of heater-cooler units (HCUs) used during open-heart surgery are contaminated with Mycobacterium chimaera, a bacterium associated with fatal infections.

Researchers at Special Pathogens Lab (Pittsburgh, PA, USA) conducted a study to assess 505 water samples from 103 HCUs across the United States and Canada for the presence of non-tuberculous mycobacteria (NTM) colonization (primarily M. chimaera), both before and after decontamination with a weakened acid solution. The samples were plated on selective media, and read weekly for six weeks. Heterotrophic plate count was used to provide an overall count of contamination, and NTMs were identified to specific species by DNA sequencing.


The results revealed that NTMs were isolated from 19% of the HCUs tested, with M. chimaera testing positive in 60% of the contaminated units. Additional Mycobacteriums identified included M. Cookii, M.Gordonae, M.Chelonae, M.Palustre, and others; six units, which negatively tested initially had positive cultures in subsequent tests, and four units were also colonized with Legionella. Of the total samples cultured, 20% were uninterpretable due to high levels of bacterial and fungal contamination. The study was presented at the 44th annual conference of the Association for Professionals in Infection Control and Epidemiology (APIC).

“Our results showed M. chimera in 37% of units tested and is consistent with previous findings. The extent of contamination from such a rare organism in multiple units from all over the country was surprising,” said lead author and study presenter John Rihs, BSc, VP of laboratory services at Special Pathogens Laboratory. “Some devices remained positive for M. chimera for months, indicating that disinfection can be difficult and routine testing is advisable. Beyond M. chimera, we found other NTM species, Legionella, and fungi, indicating these units are capable of supporting a diverse microbial population.”

M. chimaera is often found in soil and water but is rarely associated with infections. However, patients exposed to the bacteria from aerosol sprays released from HCUs during open-heart surgery can develop general and nonspecific symptoms that can often take months to emerge. As a result, diagnosis of these infections can be missed or delayed, sometimes for years, making these infections more difficult to treat.

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