Disinfection to control of pathogenic microbes (that is, those that cause disease) is important in Society and there is currently a concern that such control may be inadequate in hospitals to prevent nosocomial infections.

Denyer and Stewart have reviewed the mechanisms by which disinfectants act. They regard most disinfectants as chemical biocides with a relative lack of selectivity. The vegetative bacterial cell presents three broad regions for biocide interaction: the cell wall, the cytoplasmic membrane and the cytoplasm. The target most frequently cited in the biocide literature is the cytoplasmic membrane.

Disinfection has a special meaning: a reduction in microbial numbers from about 10⁸ cells /ml to 10³ cells /ml or fewer at 20°C for 5 microbial types in a standard test (European Suspension Test). Now when this test is carried out, the result is extremely variable (Bloomfield et al) and so far, no one has given a convincing explanation as to why this is so.

Computer simulations using a cellular automata approach (Rabone et al) suggest that the variability in microbial disinfection may have nothing to do per se with the fact that microbes are alive.

Microbial cell death can result from the catastrophic collapse of the cytoplasmic membrane of the cell as the concentration of biocide is concentrated in that membrane by physico-chemical partition and reaches some critical concentration. To get to the membrane, the biocide must first adsorb at the cell wall and then diffuse through it (Rachid et al). One can imagine that organelles will be themselves modified by such processes.

Chaotic processes caused by positive feedback can result in pseudo-random noise (Vivaldi), which could be the origin of the variability in the determination of disinfection.

Disinfection is a complex process with a catastrophic event: the collapse of the membrane caused by the disinfecting agent. If disinfection is a chaotic process, then it is only possible to estimate the probability that disinfection has taken place! A branch of statistics (logistic regression), not normally covered in basic courses on statistics, is required to analyse binary events (disinfected / not disinfected) and to return a probability for the event (Agresti).

Agresti, A. (1996), An Introduction to Categorical Data Analysis, Wiley Series in Probability and Statistics

Bloomfield, S. F.; Looney, E., (1992), Evaluation of the repeatability and reproducibility of European suspension test methods for antimicrobial activity of disinfectants and antiseptics, J. Applied Bacteriology, 1992, 73, No.1, pp.87-93

Bloomfield, S. F.; Arthur, M.; Klingeren, B. van; Pullen, W; Holah, J. T.; Elton, R. (1994) An evaluation of the repeatability and reproducibility of a surface test for the activity of disinfectants, J. Applied Bacteriology, 1994, 76, No.1, pp.86-94

Denyer, S.P.; Stewart, G.S.A.B. (1998), Mechanisms of action of disinfectants, International Biodeterioration and Biodegradation, 41 (3-4), pp.261-268

European Suspension Test (1996), European Committee for Standardisation: Chemical Disinfection and Antiseptics. Quantitative suspension test for the evaluation of the bactericidal activity of chemical disinfectants and antiseptics used in food, industrial, domestic and institutional areas. Test method and requirements (phase 2 step 1) prEN 1276 CEN/TC 216 N 109

Rabone, K. L.; Rabone, J.K.; Rabone, J.A.L., Unpublished

Rachid, F; Horobin, R.W.; Golding, M.C.H.M.; and Payne, J.N. (1989), Application of a simplistic Chinese-box model to the interaction of cultured cells with fluorescent probes, Inst. Phys. Conf. Ser. No. 98, Chapter 18

Vivaldi, F., An Experiment with Mathematics, "New Scientist" Guide to Chaos Ch.3, Nina Hall (Editor), Penguin Science, 1992