Phenomena of biocide resistance in microorganisms

Intrinsic tolerance of microorganisms to different biocides varies considerably and may be clustered according to its biological variance in 3 groups: 1. The variance is very small, i.e. the majority (> 90%) of strains of a certain species has the same m.i.c. 2. The tolerance is binomially distributed with variance factors between 5-50. 3. The tolerance is bimodally distributed, i.e. there are 2 m.i.c.-maxima. There is a problem to define exactly where the natural variance of tolerance should end and a resistance begins. There is another problem to separate true resistance from pseudo-resistance which is mainly caused by mistakes in the application of disinfectants. True resistance still is a rather rare event while pseudo-resistance is frequent. Resistance may be acquired under natural or practical conditions or may be artificially increased under laboratory conditions. Adaptation is an intermediate resistance caused by the physiological conditions of the biotope. Adaptive resistance is lost rapidly as soon as these physiological conditions change. Known mechanisms of true resistance are: 1. Exclusion of the active agent from the cellular loci of lethal destruction. 2. Secretion or enzymatic detoxification of the active agents. Bacterial resistance to biocides is genetically determined by plasmids or by the chromosome. It has also been demonstrated by in vitro experiments that formaldehyde resistance is transferable. QAC-resistance plasmids in staphylococcal strains isolated from meat processing industry could also be found in Staphylococci from other food branches. This gives first evidence for the possibility of an epidemiological development of biocide resistance. (C) 1998 Elsevier Science Ltd. All rights reserved.