Why Use Neutralisers in your Environmental Monitoring Plates?

By Gill Power

For those of you working to the orange guide and/or the guidance in the European Pharmacopoeia you will notice the absence of specific guidance on how to monitor your cleanrooms. The USP <797> however does give specific advice on what media to use and specifies that surface sampling must be supplemented with additives to neutralise the effects of disinfectants.To be more precise the residue left by disinfectants post cleaning affects the efficacy of the growth medium possibly masking the presence of viable micro-organisms, especially those on surfaces.

At Cherwell Laboratories our Redipor range of irradiated media can be purchased with several different combinations of neutralisers. The question is which neutralisers shall I use? Although the European Pharmacopoeia has little information on environmental monitoring it does specify in another section (2.6.12 Microbial examination of non-sterile products: Microbial enumeration tests) which neutralisers to use with specific interfering substances. Taken from the EP table 2.6.12 – Common neutralising agents for interfering substances:

Interfering Substance Potential neutralising method
Gluteraldehyde, Mercuials Sodium hydrogensulphite (Sodum bisulphite)
Phenolics, alcohol, aldyhdes, sorbate Diluton
Aldehydes Glycine
Quaternary Ammonium Compounds (QAC’s), parahydroxybenzonates (parabens), bis-biguanides Lecithin
QAC’s, iodine, parabens Polysorbate (Tween)
Mercurials Thioglycollate
Mercurials, halogens, aldehydes Thiosulphate
EDTA Mg2+ or Ca2+ ions


Popular neutraliser combinations

From the above list you will more than likely recognise what type of disinfectant you use. The most popular combinations are TSA or SDA with Lecithin and Tween 80, or a combination of four neutralisers which are Lecithin, Tween 80, Sodium Thiosulphate and L-Histidine which we call neutraliser No 4. Of the popular disinfectants, the Lecithin and Tween combination will neutralise those who use QAC’s and chlorine, neutraliser no.4 will neutralise additionally, disinfectants such as formaldehyde, amphoterics and iodine.

Not mentioned in the table is Hydrogen Peroxide.  In Isolators, after fogging with VHP there will be residual H2O2 after the fogging cycle is complete.  there is good evidence that the addition of Sodium Pyruvate eliminates Hydrogen Peroxide and it’s toxic effects. There is also evidence that the chemicals in neutraliser no.4 are effective as well.

If you have any technical questions regarding the use of neutralisers in growth media or have any other questions regarding our Redipor irradiated media please contact us.

 Contact Cherwell Laboratories


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