This blog is about biofilms, a very important term as far as public health is concerned. Today’s post is more about the formation of biofilm.
To start with, the biofilm is an association of bacteria encased by extracellular polymeric substances (EPS) produced by the bacteria themselves. The biofilm may consist of the bacteria belonging to the same species or different species together. Dental plaque is one of the common example of the biofilm (Read more here)
The EPS consists of polysaccharides, proteins, nucleic acids, glycoproteins, lipids and large amount of water (upto 97%). The polysaccharides may be neutral, negatively charged (Gram negative bacteria) or positively charged (Gram positive bacteria). The protein part of the EPS may includes secreted proteins, cell surface adhesins and flagella and pili subunits. (Read more on composition of EPS here).
The Establishment of a Biofilm:
The biofilm formation involves the transformation of planktonic or free living bacteria to a sessile or attached one. This transformation needs expression of specific set of genes.
Biofilm formation can be divided as 4 different stages, they are:
- Attachment
- Microcolony formation
- Three dimensional Structure Formation and Maturation
- Detachment or Dispersal
Let’s see what happens in each stage:
1. Attachment:
This is the initial stage, wherein a free living or a planktonic bacterial cell makes a connection with a new substratum or an already adhered microbial cell. Initially the attachment is reversible and the cell is loosely attached through a single pole. The cell can get detached due to movement of the bacteria (like Brownian movement or spinning).
The irreversible attachment is made with help of adhesins like pili and flagella. These structures help bacteria to overcome frictional and repulsive forces in the environment and aid attachment.
The best interface for the establishment of biofilm is solid- liquid interface. Also good temperature and good movement of nutrients favours biofilm formation.
2. Microcolony Formation:
After attachment the bacteria divides to form a microcolony. Microcolony can be said to be the is the structural unit of the biofilm. On attachment, the bacterial cells express certain set of genes (more on genetic control of biofilm here), leading to the production of the extracellular polymeric substance (EPS), which envelopes the dividing cells.
3. Maturation or Three-dimensional Structure formation:
As the microcolony formation begins, expression of biofilm related genes take place. The products of these genes lead to more production of the EPS. With the increasing colony size, the structure becomes more complex with formation of water filled channels. The water channels help in transporting nutrients and waste materials from the micro colonies. Eventually, the colony becomes very complicated and heterogenous, with a number of micro-environments within the biofilm. These micro-environments differ in pH, oxygen conditions, nutrients and cell density. Finally the colony becomes a mushroom or a tower like structure (~50μm).
4. Detachment or Dispersal:
This is the process by which the bacterial cells leave the biofilm. The process can be either passive or active.
Active Dispersal
This mode of dispersal is more natural and intended. Here the biofilm cells undergo phenotypic changes (planktonic) to purposely leave the biofilm. This phase involves signals for the production of various “polysaccharidase” (such as alginate lyase, the glycosyl hydrolase) which cleaves the polysaccharides in the matrix, hence making way for the daughter cell to escape.
Active dispersal can be driven either naturally as a part of life cycle or induced by the changes in the environmental conditions.
a. Native Dispersion: can be considered as the terminal stage in biofilm development, also known as seeding dispersal. It involves certain self- synthesized signaling molecules. There is translocation of bacteria to new sites for colonization.
The cells in the interior differentiate into planktonic and motile individuals while the cells in the periphery remain non-motile, surrounding the motile cells
Eventually the microcolony breaks from the local break out points and the motile cells enter the surrounding liquid. Usually only a few microcolonies or areas, and not the entire biofilm, is involved.
b. Induced dispersion: This happens during unfavourable external conditions such as nutrient depletion, stress, certain toxic agents. These conditions cause certain set of genes to be expressed and result into dispersion
Passive dispersal:
Detachment: it is the process wherein the biofilm is dispersed due to external forces. It’s more prominent when the shear is high or even when the biofilm has overgrown. The passive detachment can be of different types:
a. Abrasion: this occurs when the particles in the liquid collide with the biofilm.
b. Grazing: is when the biofilm is fed on by the eukaryotic organisms.
c. Erosion or shearing: is the process where small portions of biofilm is continuously removed due to shear effect of the liquid. The outermost cells are prone to shearing.
d. Sloughing: is rapid and heavy loss of biofilm sections. This could happen due to fluid frictional forces. This is also highly effected by the biofilm structure itself, like nutrient or oxygen depletion, overgrown biofilm structure, enzymatic degradation, etc.
Around 99.9% of the microbes have the ability to form biofilms. The biofilms have been seen in various bacterial species like P. aeriginosa, Staphylococcus epidermis, E. coli spp, S. aureus, Enterobacterlo, K. pneumoniae, Enterobacter cloacae and many more.
These biofilms can create lot of problems for public health, as the blanket of EPS confers resistance to antibiotics and other disinfecting agents. We will discuss about the role of biofilms in the human health in the next post.
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Next post: Biofilms: The Troublemaker
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