Biofilms

The instant a metallic surface is immersed in water, a biofilm begins to form. A biofilms a microbial mass composed of aquatic bacteria, algae and other microorganisms.

1. Development of a biofilm

many species of bacteria, algae and fungi bind various metals and form biofilm. The development of that film occurs in four stages.

• Conditioning : instantaneous chemical adsorption of organics, organisms not derectly involved.
• Adhension by ‘pioneer’ bacterial species : bacterial epiphytes adhere to submerged surfaces in a matter of hours.
• Colonization of other microorganisms : other bacteria and fungi become associated with the surface following colonization by the pioneering species over a matter of day.
• Accumulation ; entrapment of particles, dead cells and cremation of heavy metals from water (both as corrosion products and ions in bulk solution).

Figure 1.1 is a sketch of a biofilm formed on a metal surface. A biofilm begins with the adsorption of organic matter on the metal surface from the bulk environment (to adsorb means to surface). The turbulent flow transports microbes to the surface, and the microorganisms attach and the grow, using nutrients from the water. These steps are shown in fig. 1.2 Geesey’s work shows the development of biofilms on several metal surfaces in fig. 1.3

Microbiological cells are usually either sessile (attached within a biofilm, usually to something solid and immobile) or planktonic (move with flow of water). These terms describe a generalized water system, in which some microorganisms are attached to the walls, and some float freely in the bulk environment.

Microbiological cells are usually found in three states : planktonic ; sessile; and a subset of the others termed fragments, often called sessile particulates. Planktonic cells are dispersed in the aqueous phase, while sessile cells are immobilized with respect to the surrounding environment.

In soils and water, most microorganisms are found in the sessile rather that the planktonic state. Sessile microorganisms usually form a biofilm through the elaborationof extracellular polymer. The polymers bond the cells together and protect them from any hostile conditions that may arise in the environment. Biofilms retain water, accumulate nutrients and from a polymeric matrix.

source : Corrosion Handbook