1-  Biofilms

Comprise extracellular polysaccharide and protein matrices constructed by mono- or multi-species bacterial and/or yeast communities on surfaces and at fluid/gas interfaces.

Biofilm matrix presents a diffusion barrier and a neutralizing environment for some biocides. Biofilm is potentially of great significance in infrequently cleaned and disinfected environments such as animal housing. This protective effect varies with substrate surface, age of biofilm, hydration, fluid shear conditions during biofilm formation, and the mix of microbe species in the biofilm

Biofilm-embedded organisms often show less biocide susceptibility than counterparts dried-on to surfaces they are markedly less susceptible than suspended cells. 

In field situations, gross organic soil, biofilm, and post-cleaning residual hydrophobic organic material on surfaces can protect persistent resident pathogens despite regular cleaning and disinfection (C&D), for example in the case of Salmonella in hatcheries.

2-Cleaning agents (usually detergents)

may also show substantial microbicidal activity and may variously interfere or synergies with the action of a subsequently applied disinfectant

3-Drying downtime

One or more days may add substantially to the observed microbicidal effect of C&D procedures, at least for materials where effective drying occurs. Furthermore, drying between applications of cleaner and of disinfectant will allow porous surfaces to absorb more disinfectant in the latter stage