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Biofilm Formation

Biofilm Concept

A biofilm is a structured community of bacterial cells enclosed in a self-produced extracellularpolymeric matrix (EPS) and adherent to inert or living surfaces (Costerton, 1999). However,eukaryotes are also known to reside inside biofilms: Candida albicans, a common oral and

genital pathogen, is a diploid fungus. Protozoans also commonly found inside stream biofilms1.

Biofilms are generally resistant

to antibiotics, disinfectants and cleaning fluids2.

Biofilms are considered the predominant state in which microbes are found in their natural

environment.

There are beneficial biofilms like Pseudomonas fluorescens biofilms that grow on the surface of

plant roots prevent the growth of fungal pathogens. Biofilms that grow on ingested plant materialwithin the rumen aid efficient breakdown of the plant which in turn provides the ruminant with

energy which is converted to meat and milk.

Cells within a biofilm are commonly phenotypically and genotypically distinct to their

planktonic counterparts e.g. E.coli do not possess fimbriae within biofilms but do when in the

planktonic phase. Indeed there are many genes which are expressed differently within thebiofilm and planktonic modes of growth.

Biofilm Distribution

Biofilms are dominant communitiesmore than half of Earths biomass is biofilm

3

. Biofilmcommunities are ubiquitous.

Biofilm Development

Biofilms develop in five stages:

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1. Initial (reversible) attachment of cells to the surface by adhesins, receptors and non-specific mechanisms that rely on physical-chemical forces such as van-der Waals forces.

2. Irreversible attachment by production of EPS resulting in more firmly adhered.3. Maturation I. Early development of biofilm architecture.4. Maturation II. Maturation of biofilm architecture, attachment of other organisms

competition, organization to create pores, channels.

5. Dispersion of single cells from the biofilm.Fist, cells attach to the substrate surface (human tissue, glass, rocks, etc.). During this initial

step bacteria are often seen spinning, vibrating or even moving across the substrate surface.Spinning occurs when the body of bacteria is attached to by flagella, as these extensions are still

rotating. Vibration is seen when the water molecules are striking the loosely attached cells

(Brownian motion). As the number of cells increases, they move toward one another and form

aggregations. This allows for testing the micro-environment they are going to colonise before

irreversibly attaching.

Secondary colonizers co-adhere with organisms that first attached to the surface which results in

multi-species biofilm. Recruitment may be random or specific. Water is progressively removed

from the spaces in-between cells. Attachment causes the quorum sensing induction of genes

responsible for the production of compounds found in the matrix of developing biofilms. and thecells enclose themselves in a polymeric matrix that they produce. This stage is irreversible.

Mature biofilms are a complex uneven structure of dormant and actively growing bacteriacolonies of different species. The overall structure is formed along with further enzymes,

excretory products and small channels, as well as accumulated non-living material.

Planktonic cell will leave the biofilm and colonise a different surface. Biofilm extracellular

matrix degrading enzymes, such as dispersin B and deoxyribonuclease, may play a role in

biofilm dispersal. Dispersal may be achieved by means of abrasion and sloughing off.

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Quorum Sensing

Bacteria use quorum sensing to coordinate certain behaviours based on the local density of the

bacterial population. Different species use different molecules for communication; some sense

only their own molecules, others sense their own and other species molecules:

Autoinducer-2 (Al-2) in both Gram-positive and Gram-negative bacteria, N-acyl Homoserine lactones (AHL) in Gram-negative bacteria, and oligopeptides in Gram-positive bacteria

Revision

A biofilm is a complex aggregation of microorganisms found inside, or associated with, apolymeric matrix. Or simply, communities of microorganisms attached to a surface.

Organisms undergo major changes during the transition from planktonic (free-swimming)organisms to biofilm cells.

Biofilms are everywhere. Most microorganisms live in biofilms.

Five stages of biofilm development: 1. Initial reversible attachment; 2. Irreversibleattachment; 3. Early development of biofilm architecture; 4. Maturation of biofilm

architecture; 5. Dispersion.

Quorum sensing is a density-dependent mechanism allowing bacteria to co-ordinate theirgene expression.