A Novel Technique for Evaluating the Activity of Biocides Against Biofilm Bacteria

Nalepa, Christopher J. (Albemarle Corp) | Ceri, Howard (The Biofilm Research Group) | Stremick, Carol A. (The Biofilm Research Group)


The control of bacteria attached to surfaces - commonly known as biofilm - is becoming recognized as perhaps the most important function of an industrial microbiological program. We describe here a versatile laboratory method for the generation of reproducible biofilms and demonstrate its utility in assessing the biofilm eradication ability of several common industrial oxidizing and non-oxidizing biocides.

A major function of a good industrial biocide program is to control bacteria and other microorganisms in the bulk water and on system surfaces. Control of bacteria on surfaces - commonly referred to as biofilm - is becoming recognized as perhaps the most important function of an industrial biocide. This is because biofihn development can significantly impact the economics, service life, and safety of recirculating and once-through water systems.

Biofilms pose numerous problems in industrial water systems. Biofilm growth on heat exchanger surfaces can dramatically decrease heat transfer efficiency and lead to increased energy consumption and operating costs. 1 Biofilm deposits collect on cooling tower film fill surfaces - high efficiency film fill types are particularly prone to this - and can compromise cooling tower efficiency to the point of clogging or even collapsing the entire fill structure. 2'3 Biofilms lead to under-deposit corrosion through the formation of differential aeration cells and through the process of biomineralization which concentrates corrosive inorganic chemical species on the metal surface. 4 In addition, biofilms create a low-oxygen environment for undesired anaerobic organisms such as sulfate reducing bacteria which cause accelerated microbiologically induced corrosion (MIC) and pitting. 5 Finally, biofilms can provide a food source for protozoa and other single-cell organisms which essentially function as bioamplifiers for Legionella pneumophila, the bacteria responsible for Legionnaire's disease. 6'7

It is apparent that good control of biofilm is a critical requirement for an effective industrial water treatment program. Conventional techniques for biofilm growth and evaluation include continuous bacterial culture methods using the Robbins device and similar devices, 8j° annular reactors, ~ flow-through tubes, 12 as well as batch-type methods employing coupons, slides, or disks mounted in jars, flasks, or aquaria/TM Recently, a non-destructive method for biofilm growth and evaluation was disclosed based on real time monitoring of changes in heat transfer resistance, dissolved oxygen, and pH.~7 Deposition monitors and electrochemical probes provide additional dynamic methods to assess biofilm formation and the action of biocides. ~8 In general, it has proved difficult to reproducibly grow a number of biofilms in the laboratory and determine the effects of various biocide programs without investing in elaborate equipment and resorting to rather tedious and time-consuming microbiological evaluation methods such as most probable number and standard plate count methods.

We report here a new device for growing number of reproducible biofilms together with a simple method for determining biofilm viability after biocide challenge. Results are obtained quickly - within twenty four hours - which enables rapid changes in formulations. This new technique was used to study the effects of biocide concentration, biocide contact time, and biofilm age on biofilm eradication for a series of oxidizing biocides. It was also used to compare the biofilm eradication properties of several commercial biocide systems used in industrial and recreational water treatment. The results obtained with the new technique are compared to some of tho