Innate resistance to sporicides and potential failure to decontaminate

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Summary

Bacterial spores are frequently intrinsically resistant to biocides and only a number of alkylating and oxidising biocides are sporicidal under certain conditions. Activity against spores is affected by several key factors such as concentration, exposure time, soiling, and the types of surface to be treated. Sporicidal efficacy is usually achieved after an exposure time of several minutes with a high concentration of a biocide. Failure to understand these factors will result in decreased sporicide activity and spore survival. Sporicides in healthcare settings are used for surface disinfection and for the high level disinfection of certain medical devices (e.g. endoscopes). With efficacy data in mind, sporicidal activity should be achieved for the disinfection of medical devices where both high concentration and long exposure time occur. However, for the disinfection of environmental surfaces, high concentration is not recommended, nor is long exposure time achievable. In this case, sporicidal activity is severely reduced and spore survival following treatment is to be expected and contributes to the explanation of spore persistence on surfaces.

Introduction

Bacterial endospores are among the most resistant life forms to biocidal processes including disinfection.1 They are thus used as biological indicators for sterilisation processes.2 Their resistance properties to different sterilisation processes, such as heat and radiation sterilisation, are owed primarily to their particular structure, dehydration and certain proteins (i.e. small acid-soluble proteins; SASPs) unique to spores.3 The mechanisms of spore resistance to chemical disinfection have been particularly well-studied in the genus Bacillus.4, 5 However, there is a lack of information in terms of susceptibility to disinfection and mechanisms of resistance in other genera, notably in Clostridium. Such paucity of information is particularly significant for the control of Clostridium difficile outbreaks.6, 7, 8 The widespread view that spores of C. difficile are more susceptible than spores of B. subtilis cannot be traced to any robust scientific evidence, and thus such a concept is not accurate and might provide a false sense of security when biocides are used. This particular aspect will be developed in this paper. In the healthcare environment, the presence of bacterial endospores on surfaces and on medical devices is expected to be controlled by chemical disinfection/cleaning. Despite the lack of evidence of the sporicidal efficacy of biocides in situ, current knowledge of sporicide efficacy, factors affecting sporicidal activity and documented spore contamination/persistence on surfaces following a disinfection regimen provide clear indications about the efficacy of chemical biocides in practice.

This paper endeavours to provide data from peer-reviewed articles on the susceptibility of bacterial endospores to disinfection by chemical biocides and provides some reflections as to the risks associated with sporicide failure in the healthcare environment.

Section snippets

Sporicidal activity

There is only a limited number of chemical biocides that possess sporicidal activity. These biocides are all highly reactive and they can be divided into alkylating and oxidising agents.

The alkylating agents for which a sporicidal activity has been described are glutaraldehyde, formaldehyde, ortho-phthalaldehyde (OPA) and ethylene oxide (Table I). Formaldehyde has been used empirically for fumigation, although evidence of its sporicidal efficacy is scarce and relates exclusively to Bacillus

Conflict of interest statement

None declared.

Funding source

None.

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