Table 3.

Factors influencing the antimicrobial activity of biocides

Factorsa	Comments	Relevance and consequence in practice
Factors inherent to the biocide
Concentration	Understand the concentration exponent (ie, the effect of dilution upon activity)	Appropriate staff training required
Contact time	Longer contact time often associated with increased activity	Appropriate staff training required
Organic load	Quench the activity of a biocide or protect microorganisms	Combination of physical (cleaning) and chemical action required
Formulation	Possible inactivation of biocide	Understand the nature of the active agent
Temperature	Important for some devices (eg, endoscope washer)	Important to understand that adequate staff training is required with certain types of equipment
pH	Affect both the biocide (stability and ionisation) and the microorganism (growth and electric charge)	Probably not as important in the healthcare environment
Factors inherent to the cell
Presence of biofilm	Dormant “persister” cells difficult to eradicate. Likely to be present on equipment, certain surfaces	Combination of physical (cleaning) and chemical action required
Type of microorganisms	Will affect the choice of the agent to use. Bacterial spores: the most resistant; envelope viruses: the least resistant	Evaluation of the possible type of biocide needed
Number of microorganisms	High number more difficult to eradicate	Biocides often used in high (ie, excess) concentration. High number of cells might not be a problem

^aFactors listed in order of importance.