Formation & composition of Biofilm

TVSP Ltd is indebted to Dr. P. Simpson for the content of this section.

A Biofilm forms when certain microorganisms adhere to a surface in a moist environment and begin to reproduce.

Biofilm formation on surfaces usually starts with phototrophic organisms (algae, cyanobacteria) which use carbon dioxide (CO2) from the atmosphere and sunlight as their carbon and energy source.

Heterotrophic organisms (most bacteria and all fungi) need some organic source for their growth, and this is provided by the metabolites of phototrophic organisms or by airborne deposition.

The biofilm community is therefore sometimes formed by a single microbial species, but in nature biofilms

almost always consist of mixtures of many species of bacteria, as well as fungi, algae, yeasts, protozoa, and other microorganisms, along with non-living debris and corrosion products.

All biofilm forming microorganisms may cause biodeterioration and degrade stone or render mechanically, chemically and aesthetically through the metabolic activities and biomineralization process in these biofilms.

The porosity of the surface, it's mineral composition, alkalinity and it's ability to retain air borne sea salts creates an environment favourable to the settlement of microrganisms.

Studies on biofilms, along with their potential damage to a range of substrates, has led to investigations of a range of building substrates, coatings etc.

It is apparent that some species, in particular pigment-producing fungi (Alternaria, Penicillium, Aspergillus), can lead to deterioration of appearance which, even after removal of the species itself, can be difficult to remove.

Other species such as algae and lichens are also very brightly coloured and can rapidly develop and spoil the appearance of surfaces.

Black fungi

Black excretions from a filamentous fungi

Green algae

Green algae

Red algae

Red algae : One of the 5 common species of Trentepohlia

Filamentous cyanobacteria

Filamentous Cyanobacteria

Livewort

Livewort

White & yellow lichen on tiles colonised by a black fungi

White & yellow lichen on tiles colonised by a black fungi

Food requirements of biological growth on exterior building materials

Source: BRE Digest 370, March 1992.

Organism Requirements
for food
Requirements
for light
Appearance Remarks
Algae Mineral salts Necessary Green, red or brown areas consisting of powdery deposits or filaments which may be slimey under wet conditions Found on all types of substrates.
Lichens Mineral salts Necessary May consist of leathery incrustations on the surface or may be embedded in the substrate. Usually orange, green, grey or black. Found on all types of substrates.
Mosses Mineral salts Necessary Typically consists of green cushion of spiky tufts but may also be low and spreading. Usually brown when dry. Usually found on surfaces where salts,soil and dirt have accumulated. Commonly occur on roofing materials.
Liverworts Mineral salts Necessary Typically leafy, close growing, green coloured plants but some occur as a leathery tissue and may resemble lichens. Usually found on surfaces where soil and dirt have accumulated. Commonly occur on stone walls.
Moulds Organic material Unnecessary Moulds appear as spots or patches which may spread to form a furry layer on the surface: Grey, green, brown or black. They may also grow within or behind a paint causing a stain, usually pink or purple, but the mould itself may not be visible. Flaking can occur. Commonly found on painted surfaces and on surfaces where dirt and dust accumulates. Also occur on and under wood stains, glazing, putty, building sealants and plastics.
Bacteria Various Unnecessary Not visible individually to the naked eye. Capable of causing discolourations. Certain bacteria cause deterioration of stone and corrosion of metals.