Biofilms in marine environments; implications for aquaculture, coral reefs, marine aquariums, and the human pathogen Vibrio cholerae Part 4


In the first sections we looked at a survey of the attributes of biofilms as survival mechanisms and as generalized virulence attributes. In this section, we will be examining the molecular mechanisms that control biofilm formation, and the role of these mechanisms in the pathogenesis of Vibrio species bacteria. Vibrios are found in nearly every marine aquarium, and are opportunistic pathogens of many ornamental fishes. It is also becoming increasingly evident that Vibrios are responsible for a variety of coral diseases, and are also potential pathogens of human reefkeepers. As such, their effects in aquarium settings are being studied in great detail. Some bits of this section do tend to get a bit thick, but I will attempt to sum all of this up at the end.

The genus Vibrio is a diverse group of bacteria found in myriad microcosms within the marine environment. Vibrios have been associated with disease in intensive aquaculture systems, coral bleaching, they are even found in the light organs of marine fishes and are a resident of the teeth of the great white shark. The best-known member of this group is Vibrio cholerae, the causative agent of human cholera. This is a classic disease of mankind and continues to be a significant cause of morbidity and mortality in developing nations.

Unlike other pathogens that utilize biofilms to evade the host immune response, biofilms do not appear to play a role in the disease cholera. This infection is defined by toxin production and does generally have a carrier or chronic state. Instead, toxin production causes fluid production causing copious diarrhea and associated release of the bacteria back into the environment. As a self-limiting disease, cholera would have little use for biofilm genes in the host. However, these bacteria are strongly associated with biofilm development in the marine environment.

Vibrio cholerae can assume two forms when plated on laboratory media. The most common form is the smooth or luminescent (L) form. Occasionally colonies assume a wrinkled or rugose (R) form. It appears that the rugose form is actually a biofilm form. The smooth form is typically isolated from patients infected with Vibrio cholerae. When grown on media that stresses the bacteria, L colonies become R colonies at a high rate. When these R colonies are placed back on rich media, they revert to an L form at a rate of 1.5X10-5. When these bacteria were compared using fingerprint analysis, they were found to be genetically identical. This indicates that the bacteria are not distinct genotypes, but rather undergo a phase variation triggered by stress through an unidentified mechanism.