Avoiding disinfectant resistance

Published: 2-Feb-2023

At the margins of disinfected areas, microorganisms are exposed to lower disinfectant concentrations enabling less susceptible strains to survive. This can lead to a devastating problem as Adrian Gee-Turner explains

The pandemic forced cleaning businesses to expand their range of services to fulfil a vital role in the fight against COVID-19.

Many cleaning companies already offered disinfection, and when the pandemic hit, the industry ramped up its services in response to overwhelming demand. However, now that the pandemic has subsided, this is a good time to step back and review our disinfection procedures.

Are we using the right disinfectants and are they being applied correctly?

Disinfectant resistance

An issue that is starting to emerge is the microbial development of resistance to disinfectants, which may arise for a number of reasons.

Microorganisms may reside in dirt, in difficult-to-access places and in biofilms, where disinfectants cannot reach easily. Here, at the margins of the disinfected area, microorganisms are exposed to lower disinfectant concentrations enabling less susceptible strains to survive.

Cleaning operations should therefore take place before disinfection.

Equally, if disinfectants are not applied properly, spray droplets that are not fully covering a surface for example, the margins with low concentrations are dramatically increased. Also, some disinfectants are persistent in the environment; presenting microbes with ineffective concentrations that encourage resistance.

The ideal disinfectant is therefore one which is highly effective at the point of application, is applied correctly, and quickly loses all of its efficacy in the environment, before reaching sewers, drains and surface waters.

To ensure that a disinfectant is highly effective, we recommend that it should be certified to deliver a Log-4 kill of viruses. This highlights the importance of choosing the right disinfectant.

At the margins of the disinfected area, microorganisms are exposed to lower disinfectant concentrations enabling less susceptible strains to survive

Bleach is effective at killing bacteria, fungus and viruses, but as an irritant to skin, it can cause burns and serious damage to eyes. Inhalation of its fumes can harm the respiratory tract, and the correct concentration of bleach must be used to achieve effective disinfection.

Most bleach manufacturers claim 99.9% kill (Log-3) of harmful microorganisms, so a Log-6 disinfectant is one thousand times more effective. This means that bleach could be expected (in the right conditions) to reduce 1 million colony-forming bacterial units (CFUs) to 1,000 CFUs whereas a Log-6 disinfectant would be expected to reduce the same size colony down to just one single CFU. Bleach is therefore mainly suitable for the low-cost disinfection of non-porous, unpainted surfaces such as floors and toilets.

The expression ‘99.9% kill’ is used by many manufacturers, but as highlighted above, whilst this might sound effective, it isn’t. Unfortunately, this situation is further complicated by the microbe to which the Log number relates. This is because some microbes are easier to kill than others, so beware of manufacturers that only offer 99.9% (Log-3) for bacteria.

For comparison, the highly effective Nemesis eH2O (HOCl) delivers Log-6 effectivity against bacteria, Log-4 against viruses and Log-3 against spore-forming bacteria. A further advantage of HOCl (and of bleach, incidentally) is that it rapidly degrades to salt and water after application, so it is not persistent in the environment.

Better healthcare disinfection required

Avoiding disinfectant resistance is important is many places, but no where more so than in healthcare settings. The threat from hospital-acquired infections (HAI’s) remains, and of course, this now includes COVID.

According to the World Health Organization (WHO), out of every 100 patients in acute-care hospitals, seven patients in high-income countries and 15 patients in low- and middle-income countries will acquire at least one HAI during their hospital stay, and on average, 1 in every 10 affected patients will die from their HAI.

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The COVID-19 pandemic highlighted the extent to which health care settings can contribute to the spread of infections; harming patients, health workers and visitors if insufficient attention is paid to infection prevention and control (IPC). In a recent report, the WHO showed that where good IPC practices are followed, 70% of those infections can be prevented.

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