23.09.2016Prions aside, bacterial endospores are generally considered to be the toughest challenge for disinfectants. Consequently, they are used as the ‘gold standard’ for disinfectant testing; if it can kill bacterial endospores, you make the assumption that it can kill everything else besides. However, how do we know which disinfectants are sporicidal?
Bacterial endospores are ‘survival structures’ produced by only a few types of bacteria (including Clostridium difficile, which is the most important spore-former in the context of hospital cleaning and disinfection). Spores are resistant to many of the chemicals used for hand and surface disinfection in hospitals, including alcohol and quaternary ammonium compounds. Therefore, soap and water hand washing is recommended when caring for a patient with C. difficile (or gastro-intestinal symptoms of unknown origin), and a sporicidal disinfectant for surface hygiene (such as peracetic acid or hydrogen peroxide).
To be classified as a sporicide, a disinfectant must meet be able to pass a sporicidal disinfectant test. Whilst a number of different sporicidal testing standards are available, they share core methodology: testing a particular disinfectant formulation against spores of a known quantity, for a set period of contact time, with appropriate neutralisation to prevent ‘over-exposure’ of the spores to the disinfectant, and under controlled laboratory conditions. It is important to note that “non-sporicidal” disinfectants will be able to kill some of the spores in a laboratory test, so if these testing conditions are not controlled appropriately, they may appear to be sporcidial even when they are not. This was illustrated by a fairly recent study, which showed that many wipes that claimed to be sporicidal were not when tested under standardised laboratory conditions. The study also highlighted the importance of the wipe material in preventing the transfer of organisms from surface to surface – highlighting that the Gama sporicidal wipe was the most effective at preventing the transfer of organisms between surfaces. Another recent study highlight the importance of appropriate neutralisation in sporicidal disinfectant testing. Quaternary ammonium compounds when combined with amines appeared to be sporicidal in initial testing, which would be a major development in the range of disinfectant available. However, subsequent testing demonstrated that this was due to inadequate neutralisation of the disinfectant – so it turns out that quaternary ammonium compounds combined with amines are not sporicidal after all!
So, some key questions that you should ask when reviewing whether a sporicidal claim for a disinfectant is legitimate are:
· Did the test match the proposed usage of the product? For example, a laboratory test that evaluates the ability of a liquid disinfectant against a liquid suspension of spores is not representative of a disinfectant-impregnated wipe being used to disinfect surface attached spores!
· Is the contact time representative of in-use recommendations (or practice)? A disinfectant that is sporicidal only following a 60 minute contact time cannot be considered sporicidal in real terms for hospital surface disinfection.
· Has the disinfectant been neutralised appropriately? If not, the laboratory test will over-expose the spores to the disinfectant, and over-estimate the sporicidal capability of the disinfectant.
· Was the testing performed by a reputable laboratory?
Sporicidal wipes offer the potential to reduce reliance on the operative to assure correct formulation of a disinfectant, and are likely to improve compliance cleaning protocols due to the convenience factor. However, we need to be sure.
09.09.2016Biofilms have only fairly recently been identified on hospital surfaces. However, more recent data suggests that biofilms on dry hospital surfaces are the norm rather than an exception. What does this mean in the context of hospital cleaning and disinfection? A recent study by the same Australian group that first discovered biofilms on dry hospital surfaces demonstrate neatly that S. aureus in surface biofilms can survive extended exposure to high concentration sodium hypochlorite, suggesting that biofilms are the ‘cockroaches’ of the microbial world – is there anything that they can’t survive?
The study dried S. aureus into a surface biofilm using a modification of the CDC biofilm reactor, to make the biofilms more representative of hospital surface biofilms. They then exposed them to a hefty dose of sodium hypochlorite (up to 20,000 ppm for 10 minutes). Following exposure, the biofilms were examined visually to measure physical changes, and cultured to determine if any bacteria were present. Visual live/dead staining immediately after sodium hypochlorite exposure suggested the presence of living cells protected in the biofilm, and the S. aureus was able to grow and create new biofilms when cultured.
In one sense, this is no surprise. We know already that biofilms can reduce the susceptibility of microbes to biocides by up to 1000-fold. However, on the other hand, these results are surprising because the exposure to sodium hypochlorite was very, very heavy. Enough to easily inactivate bacterial endospores, which are the yardstick for measuring the most powerful disinfectants. These findings go some way to confirming suspicions that biofilms may play an important role in the defence of pathogens associated with hospital-acquired infection against disinfectants.