Challenges of Studying Naturally Occurring Biofilms in the Laboratory
The natural environment is a “tough neighborhood” and microorganisms have evolved entire armamentaria of mechanisms to deal with the various threats imposed by nature. Natural selection pressure maintains and hones the effectiveness of these mechanisms and effectively weeds out organisms, which fail to compete. Free-living bacteria in order to survive must avoid predation by protozoa and metazoans, avoid being washed away, resist toxic chemicals, and persist in low nutrient conditions. Pathogens produce adhesive substances by which they adhere to host tissues, they form a protective matrix which in part protects them from elements of the humoral and cellular immune system, they synthesize virulence factors which overwhelm host defenses and maintain multiple mechanisms to resist the action of antibiotics.
Bringing a biofilm-producing organism into typical laboratory culture results in a relaxation of many selection pressures, while introducing a new set of growth conditions which emphasize maximum growth rate. Nutrition is usually plentiful, adherence is unnecessary and predation is absent. In this permissive environment “wild type” organisms rapidly loose many of the characteristics, which enabled their survival in nature.
Fux et al. (2005) have shown that after as few as ten laboratory passages some reference strains have lost much of their adhesion “equipment” and perhaps as much as 37% of their genes!
This implies that using strains kept in laboratory culture for long periods of time in investigating the properties of biofilm formation is unwise as these strains may be decidedly biofilm defective.
