Spore-forming microorganisms (bacteria and fungi) are the most challenging microorganisms for a facility’s contamination control programme. This is because:
- Spores are difficult to destroy using most cleaning and disinfection processes.
- Bacterial endospores are resistant to desiccation, temperature, starvation, ultraviolet and gamma radiation, and chemical disinfectants.
- Spores can be spread over considerable distances, e.g., transferred via trolley wheels, harboured in cardboard packaging, etc. or through fungal dispersal.
- When the environmental conditions are suitable, spores will germinate.
Even with the best designed cleanrooms and supporting procedures, sporicidal disinfection remains a critical component of the contamination control programme. There is an important choice for the cleanroom user to make in terms of the type of sporicide to be selected.
Sporicidal agents
Disinfectants are designed to reduce a population of microorganisms on a surface down to a safe level (microbiocidal action). Many disinfectants, while effective against vegetative cells, are not sporicidal. To kill bacterial and fungal spores the cleanroom needs to deploy a special class of disinfectant called a sporicide.
The majority of sporicides available are either chlorine derived, like sodium hypochlorite, chloramine, chlorine dioxide, and hypochlorous acid; or hydrogen peroxide, peracetic acid, or a blend of the two chemicals. These disinfectants display multiple modes of action against microorganisms, such as penetrating the cell wall, disrupting the cytoplasmic membrane or by destroying intracellular components.
As well as the chemicals and their formulation there are other criteria that contributes to making something a sporicide, Table 1 sets out some basic requirements.
What makes an optimal sporicide?
When the cleanroom manager is considering which sporicidal agent to select, several factors need to be evaluated. The section below reviews these factors, with a summary provided in Table 2 .
Spectrum of activity: This refers to the ability of the disinfectant to kill distinct types of microorganisms and microorganisms which are in different physiological states. In the case of sporicides, this is a necessary indication that the product can kill both bacterial and fungal spores from several representative species.
Validation studies outcomes: The sporicide must meet the requirements of a selected validation standard or internally developed protocol, designed to measure its sporicidal activity against bacterial and fungal spores. The selected concentration and contact time must be assessed against a representative range of surfaces (such as stainless steel, aluminium, glass, epoxy, polyvinyl chloride, acrylic, and plastics) using an appropriate test panel of microorganisms based on the microbiota from the facility. Material compatibility can also be assessed at this stage e.g. a check for corrosion.
Once proven in laboratory tests, the criteria of time, concentration and application must be verified through field trials with the additional criteria of product presentations, application techniques (methods of wiping), cleanroom classes and different operators to establish suitable frequencies and rotation patterns. Field trials require considerable amounts of pre- and post-application environmental monitoring of surface samples. Depending on the material compatibility assessment, additional rinsing, following application, may be required. Ongoing environmental monitoring provides additional evidence on the continued suitability of the sporicide.
Contact time: To be of practical use, the sporicidal disinfectant must be rapid in action, with an ideal contact time of less than 15 minutes. The contact time is the time taken for the disinfectant to bind to the microorganism, traverse the cell wall and membrane and reach its specific target site.
Environmental conditions: Some disinfectants require certain temperature and pH ranges to function correctly. One type of disinfectant, for example, may not be effective in a cold room due to the lower temperature, or it may require an additional contact time (each 10oC decrease in temperature requires a doubling of the contact time based on the validated time).
Detergent compatibility: Prior to the use of disinfectants, it is essential that as much dirt and soil as possible is removed. This may require the application of a detergent. Some disinfectants are not compatible with certain detergents. In such circumstances, detergent residues could neutralise the active ingredient in the disinfectant.
Residue formation: Material compatibility, as mentioned above, becomes a greater concern where the disinfectant leave residues on surfaces. Whilst this can mean a continuation of the antimicrobial activity, it is more common to regard residues as unwanted matter, leading to sticky or slippery surfaces, and the presence of a residue has the potential to trigger the inactivation of another disinfectant in rotation.
Surface coverage and wettability: The amount of coverage that the sporicide can provide (in addition to the time a solution can be used for) needs to be accounted for. This is a factor normally addressed in field trials. Intrinsically associated to coverage is ‘wettability,’ which relates to surface tension. A sporicide with lower surface tension will exhibit better wettability, meaning it will spread more easily and form a thin, continuous film on the surface. The addition of a fatty acid or other surfactant type to the sporicide formulation can help to improve the contact angle of the sporicide on the cleanroom surface.
Stability: The stability of the disinfectant needs to be considered, especially when it is used in manufacturing. Some active ingredients will retain their efficacy for shorter periods than others. With stability, it needs to be known:
- How stable is the sporicide when a concentrate is diluted?
- How stable is the sporicide once a trigger spray is activated or a packet of saturated wipes opened?
- What is the shelf-life?
This evaluation might require an assessment of end-of-use disinfectant efficacy studies.
Occupational health and safety: The disinfectants must be relatively safe to use, in terms of health and safety standards. Here, the main concern is with operator welfare and how the sporicide relates to occupational exposure levels. There is a spectrum of consideration from odours that cleanroom personnel find unpleasant to volatile compounds that require personnel to wear respiratory equipment.
Synthesis
Balancing each of the factors discussed above (and summarised in Table 2), it is likely that an attempt to create an ‘ideal’ sporicide is probably impossible. While each factor is important, there will be decisions to be made that lead to one factor being weighted above another. This is especially so when balancing between occupational safety and biocide efficacy. It is often the case that the more efficacious the chemicals are, the greater the array of safety concerns presented. The cleanroom manager must, therefore, use Table 2 to evaluate various products and to decide which factors should be given the highest weighting based on the specific nature of the facility.
Summary
This article has looked at the requirements for introducing a sporicide, or replacing a sporicide, within a cleanroom facility from the perspective of the cleanroom manager. The article has presented several factors for consideration, and understanding these and seeking optimisation will lead to a more robust and efficacious use of a sporicidal agent.
