Biofilm history and names

Over the years, a considerable amount of research has dealt with the subject of biofilm. The first official report of surface-associated microbial cells was in 1684, when Van Leeuwenhoek informed the Royal Society of London that he had observed an accumulation of microorganisms on tooth surfaces using a simple microscope. But it was not until 1936 that Zobell and Anderson introduced the term periphytes, referring to bacteria associated with surfaces in bottles used to store seawater (Zobell and Anderson, 1936). The examination of biofilms was promoted just after electron microscope development, which allowed high resolution at much higher magnifications than when observed under the light microscope (Donlan, 2002). In the following decades many microbiologists investigated aquatic bacteria, especially for the study and treatment of many serious diseases. Some studies reported that bacteria attached to aquatic surfaces are often 1,000 or 10,000 times greater in number than planktonic (free-floating) bacteria (Costerton et al., 1978;Donland, 2002).

The first word proposed to describe microorganisms attached to aquatic surfaces was ‘aufwuchs’, a German word, meaning surface growth (Ruttner, 1953). But it was not until 1975 that the term biofilm made its first appearance in the scientific literature (Mack et al., 1975). In 1983, Wetzel described biofilms as assemblages of bacteria, algae, fungi, and protozoa within a protective matrix of extracellular polymeric substances and detritus, which colonize submerged surfaces in lakes and rivers (Wetzel, 1983). By this time scientists and engineers used develoing technology to effectively study microbial communities,and biofilm research was progressively established as a relevant scientific topic. Many definitions of aquatic biofilms appeared during the eighties referring to biofilm as an assemblage of autotrophic and heterotrophic microorganisms embedded in a polymeric matrix and developing on wetted surfaces (Lock et al., 1984; Costerton et al., 1987; Characklis and Marshall, 1989). Later, the concept of biofilm evolved by including new knowledge from research on biofilm formation, organization, cell-to-cell communication, interaction between microorganisms, and threedimensional structure. The great development of the research on biofilm formation in medicine and animal health (i.e. cancer processes, transplants or prosthesis) has also helped to improve the knowledge of aquatic biofilms by using relevant techniques such as confocal microscopy and cell-to-cell signalling. The study of biofilm in different systems, where distinct environmental variables and types of substrata are found, led to the appearance of different terms when referring to biofilm such as periphyton and microphytobentos, or other less common ones like benthos, haptobenthos and herpobenthos. Thus, there can be a dilemma when defining the terminology used to describe a biofilm (Wetzel, 1983), and sometimes it becomes a real problem when looking for the appropriate bibliography. In most scientific publications periphyton refers to freshwater ecosystems while microphytobenthos refers to marine ecosystems. Periphyton is defined as an assemblage of freshwater organisms mainly composed of photoautotrophic algae, heterotrophic and chemoautotrophic bacteria, fungi, protozoans, metazoans and viruses which grow upon a benthic substrate (Wetzel, 1983; Larned, 2010). Microphytobenthos are defined as populations of photoautotrophic microorganisms (diatoms, euglenids, crysophyceans, dinoflagelates) that colonize benthic substrata in marine systems, especially in intertidal and lower supra-tidal sediments reached by light (MacIntyre et al., 1996; Jesus et al., 2009; Pan et al., 2013). The term biofilm was initially used in engineering and referred to attached heterotrophic communities (Wetzel,1983), but it has been spread to both natural and anthropogenic aquatic systems as a general term referring to attached-microbial communities.Even though biofilm and periphyton are mostly used synonymously, Saikia (2001) reported slight differences between the terms periphyton and biofilm, indicating that periphyton is linked to nutrient dynamics in ecosystems and especially including photosynthetic organisms. Finally, benthos were initially described as organisms associated to the bottom or solid–liquid interfaces in aquatic systems, haptobenthos as adhered organisms but not penetrating a solid surface, and herpobenthos as organisms penetrating bottom sediments (Wetzel, 1983; Neuswanger et al., 1982).

Additionally, since biofilms are formed in sand, sediment, rocks and cobbles, wood and leaves, and the surface of submerged plants, specific names for each natural substrate have also been used by adding the corresponding adjective (such as epilithic, epixylic, or epipsammic biofilm; see Table 1.1 and Fig. 1.1) (Lock, 1993; Romani, 2010; Vadeboncoeur and Steinman, 2002). The term biofilm is also commonly used in man-made surfaces such as industrial equipment and in water management (water treatment plants and drinking water distribution systems, MacLeod et al., 1990). In these artificial substrates the concept of biofouling (or microbial fouling) is widely used to describe the accumulation process of microorganisms on wet surfaces, especially in shipping equipment and water distribution systems (Melo and Bott, 1997). Similar to biofilms, a special and characteristic form of attached microbial community is a microbial mat.

Microbial mats are organo-sedimentary structures that develop on solid surfaces, formed by the trapping and binding of sediment and/ or the net carbonate-precipitating activities of microorganisms, resulting in a layered structure. They are mainly formed by cyanobacteria, colourless sulfur bacteria, purple sulfur bacteria, and sulfate-reducing bacteria, and can be found in lagoons, marine intertidal and subtidal zones, hypersaline ponds, hot springs, fresh water rivers and lakes (van Gemerden, 1993; Dupraz and Vissche,2005). Ancient microbial mats (stromatolites) were abundant and diverse in shallow zones of the oceans in the Proterozoic (Bottjer et al., 1996).Like the first photosynthetic communities, stromatolites consumed the greenhouse gas CO2, and produced free O2 and H2, playing a crucial role for the early establishment of life (Dupraz and Vissche, 2005). However, in the latter part of the Proterozoic they declined most likely due to metazoan diversification, increased grazing and sediment disturbance (Bottjer et al., 1996).

Within the limits of the biofilm concept, aggregates or granules as suspended microbial flocs have many features in common with classical biofilms, such as the polymeric matrix and the association of diverse groups of microorganisms such as bacteria, algae and protozoans, which interact structurally and functionally (De Beer and Stoodley, 2006). Aggregates play a role in nutrient cycling and organic matter decomposition in lakes, rivers and the sea (Azam and Cho, 1987; Grossart and Simon, 1993).

Source of reading: Aquatic Biofilms Ecology, Water Quality and Wastewater Treatment