Dodging disinfection: biofilm-forming skin bacteria can resist disinfectants
What is this research about?
Occasionally, blood components become contaminated with bacteria. The source of contamination is often found to be bacteria that normally live on the skin. Human skin is home to millions of microorganisms. Skin ‘microflora’ includes many different types of bacteria, which may enter the blood unit during the donation process. If bacteria enter a unit, some may survive and even thrive during storage. The risk is greatest with platelet components. Stored at room temperature, platelets provide a particularly good environment for bacteria to grow. Although harmless on the skin of healthy people, if enough bacteria enter the blood stream of a recipient they can cause severe and even fatal infections, especially in patients who are already ill.
Skin bacteria that form biofilms – dense communities of surfaceattached bacteria - can resist disinfectants. This explains in part why skin disinfection is not fully effective and why bacterial contamination of blood is still a risk.
The first line of defence against contamination is to thoroughly disinfect the donor’s skin before donation. Skin disinfection helps keep contamination rates very low. However, contamination still occurs, suggesting that disinfection is not fully effective. Several recent discoveries may help explain why. While most of the skin microflora are free-floating, about 20% form communities of bacteria on healthy skin. These communities are called biofilms. Biofilms are dense populations of bacteria attached to a surface. Bacteria in biofilms secrete a matrix that surrounds and protects them, helping the bacteria survive and resist disinfection. The researchers recently found that common skin bacteria form biofilms in contaminated platelet components. They examined how effective skin disinfectants are against both biofilm-forming and free-floating forms of these bacteria.
What did the researchers do?
The main method used at Canadian Blood Services to disinfect the donation site on a donor’s arm contains two disinfectants: chlorhexidine and isopropyl alcohol. These are both general disinfectants that inactivate many different types of bacteria.
First, the researchers looked at how effective chlorhexidine alone, isopropyl alcohol alone or both disinfectants together were against free-floating and biofilm forms of two common skin bacteria: Staphylococcus epidermis and Staphylococcus capitis.
Next, the researchers examined how effective the disinfectants were against ‘dual-species’ biofilms containing both S. epidermidis and S. capitis. This more closely reflects how bacteria live on the skin, which is generally in biofilms containing different types of bacteria.
Finally, the researchers tested how well disinfectant-resistant biofilms of these bacteria survive and grow in platelet components.
What did the researchers find?
Bacteria that form biofilms are much more resistant than the free-floating bacteria to disinfection. For both S. epidermidis and S. capitis, the minimum concentration of chlorhexidine needed to inactivate bacteria in biofilms was at least 64 times higher than the concentration that inactivated free-floating bacteria.
There were differences in how the bacteria responded to the disinfectants. For example, S. epidermidis was more sensitive to isopropyl alcohol than S. capitis.
Being in a dual-species biofilm changed the bacteria’s sensitivity to disinfectants - S. epidermidis was more sensitive to chlorhexidine and S. capitis was more sensitive to all disinfectants.
Both S. epidermidis and S. capitis biofilm bacteria that survived disinfection recovered quickly and grew in platelet components. However, S. capitis recovery was not as efficient and they didn’t grow as well as S. epidermidis. This may explain why S. epidermidis is the predominant platelet contaminant.
In platelet components, disinfectant-resistant, dual-species biofilms were less able to grow and proliferate than biofilms containing either S. epidermidis or S. capitis alone.
How can you use this research?
The findings show that biofilm-forming skin bacteria have greater resistance than free-floating bacteria to disinfectants. Bacteria that resist disinfection do survive and grow in platelet components. If it survives disinfection, S. epidermidis, the most frequently isolated species from contaminated units, grows particularly well. These findings explain, at least in part, why bacterial contamination of platelet components remains a problem. How the bacteria in biofilms resist disinfection was not directly investigated, but it may be that the matrix the bacteria in biofilms produce acts as a physical or chemical barrier to disinfection.
Disinfecting the skin of blood donors is an important step to decrease blood contamination by skin microflora. The disinfectants tested in this study are used by Canadian Blood Services and many other blood services worldwide. These results showed that using these disinfectants in combination - as is done at Canadian Blood Services - is the most effective way to kill bacteria in biofilms. However it is not 100 % effective. Further research is needed to investigate how to increase the effectiveness of disinfectants or to find other, more effective disinfectants. Disinfectants are applied to the skin surface.
One way to improve their action is to improve their skin penetration, as bacteria also live in deeper layers of skin. The researchers hope to explore this in the future. It is important that future studies of bacteria and disinfectants should consider bacteria in mixed species, biofilm-forming populations, as this more closely mimics how bacteria live in the skin.
About the research team
Mariam Taha is a PhD student in Dr. Sandra Ramirez-Arcos’ laboratory at Canadian Blood Services, Ottawa, ON. Dr. Ramirez-Arcos is a development scientist with the product and process development group at the Canadian Blood Services Centre for Innovation, Ottawa, ON. Dr. Qi-Long Yi and Dr. Valerie Greco-Stewart work with Canadian Blood Services in Ottawa, ON. Dr. Carey Landry is a former employee with Canadian Blood Services and currently works for the Ottawa Hospital. The research was done in collaboration with Dr. Miloslav Kalab, an honorary research scientist at Agriculture and Agri-Food Canada, Ottawa, ON, Dr. Ann Karen Brassinga at the University of Manitoba, Winnipeg, MB and Dr. Costi D. Sifri at the University of Virginia, Charlottesville, VA.
This research unit is derived from the following publication(s)
Acknowledgements: M. Taha is a recipient of a graduate fellowship awarded by Canadian Blood Services and Health Canada. This research received financial support from Health Canada and Canadian Blood Services (funded by the provincial and territorial Ministries of Health).