Disinfection tests with intact biofilms: combined use of the Modified Robbins Device with impedance detection

Abstract

Many disinfectants are currently evaluated using suspensions of pure bacterial cultures. These tests may be misleading since studies have shown that bacteria colonising surfaces (biofilms) are more resistant to disinfectants than the same cells released into suspension. This demonstrates the importance of maintaining the intact biofilm structure during biocide testing. Using a Malthus 2000 instrument, biofilm activity was measured in situ (without disrupting the biofilm structure) following disinfection of surfaces. These in situ measurements showed a higher number of surviving cells than cell recovery by swabbing or vortexing. This demonstrates the importance of the structural integrity of the biofilm during both disinfection and detection of survivors. A procedure was required for growing biofilms of a standard cell number on multiple discs, which could be placed directly into Malthus tubes following disinfection. The Modified Robbins Device was evaluated for this purpose, and was found to be suitable for growing 25 biofilm discs of a uniform cell number. Finally the use of the Modified Robbins Device and the Malthus were combined in a series of disinfection studies, which proved to give a simple, reliable and realistic method for studying the decontamination of biofilms.