Biofilm

A biofilm is a thick layer of prokaryotic organisms that have aggregated to form a colony. The colony is attached to a surface with a layer of slime that helps protect the micro-organisms. There are a number of reasons why biofilms form, all of which promote the growth and survival of microorganisms. Biofilms are found in almost all environments and can have negative effects.

Biofilm structure

A microbial biofilm consists of many prokaryotic organisms that combine to form a colony. The colony adheres to a surface and is coated with a layer of polysaccharide (or slime layer). The slime consists of many porous layers with channels that allow cells in the centre of the colony to receive nutrients and remove waste products.

A biofilm is formed and maintained through the cell cell-to-cell communication. A biofilm first forms when one or a few cells attach to a surface. These first cells produce proteins that act as signals to nearby cells. The signals are detected by neighbouring cells and essentially recruit new cells into the colony. As nearby cells detect the chemical signals, they aggregate and begin to form the biofilm. These cells then send additional signals, recruiting more cells to the colony and growing the biofilm. The proteins also signal the development of polysaccharides that will form the mucus layer. This slime layer forms on and around the growing colony.

Function of the biofilm

Microorganisms in a biofilm aggregate to form a colony for metabolic cooperation. This cooperative method of growth increases cell survival through improved defence, increased nutrient availability, and better opportunities for cell communication and transfer of genetic material.

Cellular defence is important to combat physical threats such as displacement by a flowing fluid or elimination by the immune system. The polysaccharide coating of the biofilm acts as an adhesive to bind the colony to a surface. This prevents removal of the cells by physical force. It also prevents penetration of the biofilm by the immune system or antibiotics. Biofilms can be difficult to remove and can cause risks to human health. For example, with cystic fibrosis, biofilm can form in the lungs and cause adverse symptoms. Dental plaque is another example of bacterial biofilm; this can lead to tooth decay and gum disease. Several other bacterial conditions can also be caused by biofilms, such as cholera, tuberculosis and Legionnaires’ disease.

Biofilm provides a favourable environment for microorganisms. Cells adhere to a surface with a greater source of nutrients, retaining the cells in a niche optimal. The cells are in close proximity, which facilitates cell communication via signalling molecules. Proximity also provides a greater opportunity for horizontal gene transfer or exchange of genetic material between cells.