Effect of biofilms on biocides

Biofilms exist everywhere, whether it be on a pebble in the sea, plaque on your teeth, within the pipework of a hot tub or a cooling system, within your showers and taps or in many water based systems. Whilst sometimes this is not a problem from the perspective of Legionella and Pseudomonas bacteria this is very definitely a problem.

This is because both these bacteria can thrive in a biofilm and then can lead to health problems such as Legionnaires’ disease or Hot Tub Folliculitis.

So knowing that a biofilm is a problem surely biocides can be added that will penetrate the biofilm and kill these problematic bacteria.

Studies have shown that biocides can have difficulty penetrating biofilms leading to bacteria surviving the addition of a biocide to a system such as a hot tub, cooling system, domestic hot and cold water system or dental unit water lines.

Why is this the case? In a scientific paper, Stewart PS. 2015. Antimicrobial tolerance in biofilms. Microbiol Spectrum 3(3):MB-0010-2014, Stewart investigated the properties of a biofilm that can lead to biocides being ineffective.

In his paper he explained why the chemistry of a biocide does not allow us to predict its relative efficacy against biofilm. He also explained how the substratum material on which a biofilm grows is most likely to influence biofilm susceptibility when it leaches or corrodes. Thus metal corrosion can effect the efficacy of the biocide.

Cell density within the biofilm also has a role to play as he showed that there is also an important dependence of a biofilm’s biocidal tolerance on the cell density of the biofilm. Further, biofilms films tend to become less susceptible as they age. Biofilm age and biofilm cell density are usually strongly correlated. This suggests that there is something that happens during biofilm maturation, either physical or physiological, that is essential for full biofilm tolerance.

One often forgotten mechanism of biofilm protection is the effective dosage rate of the biocide in use. The biocide will be consumed as it reacts with materials in the water and planktonic bacteria which means that there may not be sufficient biocide available once it reaches the biofilm. It is therefore important to have the right amount of biocide available throughout the system to ensure there is enough to allow it to penetrate the biofilm.

The extent of biocidal penetration into a biofilm is expected to depend on biofilm thickness, effective diffusivity of the biocide in the biofilm, reactivity of the biocide in the biofilm, the sorptive capacity of the biofilm for the biocide, the dose concentration and dose duration, and external mass transfer properties.

Penetration times of a biocide into a biofilm can range from a few minutes to a full day so it is important to keep the required amount of biocide available at the right dosage rate for sufficient time. This can explain why some remedial cleaning processes, which can be for as little as one hour, may not be effective with unacceptable bacterial levels found after a clean has been carried out.

The retarded penetration of these biocides into the biofilm derives from the reaction or sorption of the biocide in the biofilm as it diffuses. Halogens react with uncharacterized components of biomass and are neutralized. Hydrogen peroxide is destroyed by the action of catalase. Biocides with a positive charge likely bind to negatively charged polymers or to cell surfaces, delaying penetration.

An important consideration is microorganisms in the stationary phase of a batch planktonic culture, which may be slow-growing or non-growing and may be less metabolically active than growing cells, can be less sensitive to killing by biocides. Thus the stage in the life cycle of a particular bacterium can have an effect on whether the biocide is effective, depending on the mode of kill of the biocide.

Unfortunately biofilms will contain bacteria at all stages of their life cycle and hence this can be part of the reason why a biocide will not kill all bacteria present.

It can be appreciated from the above that killing bacteria in a biofilm can be complex and sometimes ineffective. Hence the need for a multi-faceted approach which will involve ensuring biofilms are removed, perhaps with biodispersants where they are allowed, by ensuring the right amount of biocide is required for the right amount of time and by understanding the limitations of some types of biocides.

There is no panacea to killing bacteria within a biofilm but an understanding of the issues that are involved can help with the approach used to minimise the effect of harmful bacteria within a water system.

If you would like to discuss the issues raised above with an expert in water treatment then contact Collaton Consultancy for advice and support. Email us on general@collatonconsultancy.com

Collaton Consultancy Limited are a specialist consultancy offering technical advice on Legionella, Pseudomonas and water treatment. We also act as Authorising Engineer (Water) in healthcare sites and as Expert Witnesses. We also offer both online and face to face training in a host of subjects.

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