Introduction to biofilm

Abstract

The idea that bacteria grow preferentially on surfaces has come to the fore at regular intervals, for more than 150 years, whenever microbiologists have used direct methods to examine natural populations of these organisms growing in real ecosystems. In 1947, Antonie van Leuwenhoek used his primitive but effective microscope to describe aggregates of ‘animalcules’ that he had scraped from human tooth surfaces. Almost 100 years later, in 1934, Claude Zobell examined natural marine populations by direct microscopy, and concluded that these bacteria are attracted to the surfaces to which they sometimes adhere, to form sessile populations. Steadily, throughout the history of microbiology, a very small proportion of microbiologists have chosen to examine their subjects directly, by microscopy, and have found that bacteria grow differently after they adhere to a surface and initiate biofilm formation. Between 1935 and 1978, microbiologists at the Forsyth Dental Center, notably Ron Gibbons and van Houte, examined the microbial biofilms that constitute dental plaque and form macroscopic accretions on tooth surfaces. In 1964, Ralph Mitchell and Kevin Marshall examined the first stages of biofilm formation by bacteria in pure cultures, and distinguished between the reversible adsorption of bacteria to surfaces and the subsequent irreversible attachment that constitutes the first stage of biofilm formation. All of these bellwether studies of the association of bacterial cells with surfaces meet the criteria of the recent report of the taskforce of the American Academy of Microbiology (AAM). This AAM taskforce has taken the position that the discipline of microbiology has been seriously misled by its tendency to extrapolate from studies of planktonic cells in monospecies cultures, and these early studies that discovered biofilms constitute one of its most persuasive examples. The idea that some bacteria, in some natural ecosystems, lived preferentially on surfaces has been present consistently since the beginning of this relatively ‘young’ scientific discipline. However, partly because the subjects of this arcane science are rarely visible to the unaided eye, most practitioners have removed bacteria from their native ecosystems and have studied them in pure cultures, where they grow predominantly as floating ‘planktonic’ cells that are not, by definition, associated with surfaces.