Biofilms: their role in Agriculture

By: Sunita Gaind

Biofilms are nature's gift to man kind that can be exploited for their diverse application in the area of food, agriculture, medical, biotechnology and environmental settings.

What is a biofilm:

A biofilm is an aggregate of microorganisms in which cells adhere to each other and/or to a living or nonliving surface. Close proximity of different microbial colonies can bring about complex and rapid microbial degradation requiring combined metabolic capabilities. A biofilm can be formed by a single bacterial spp or by group of microorganisms (bacteria, archaea, protozoa, fungi and algae), each group perform specialized metabolic functions.

Cells in the biofilm are held together by extracellular polymeric substances/ exopolysaccharide (EPS) that has DNA, proteins, and polysaccharides in various configurations. EPS provides an optimal environment for the exchange of genetic material between cells. Cells may also communicate via quorum sensing, which may in turn affect biofilm processes such as detachment. When a cell switches to the biofilm mode of growth, it undergoes a phenotypic shift in behavior in which large suites of genes are differentially regulated.

Advantages of biofilms

• Dense extra cellular matrix by outer layer of cells provides increased resistance to detergents and antibiotics as well as protection against antimicrobial agents.

• Biofilms improve the uptake of dissolved organic matter from suspending solution. Organisms can concentrate scarce nutrient extracellularly and utilize these compounds via activity of exoenzymes.

• Biofilms that contain water channels, help distribute nutrients and signalling molecules.

• Exchange of nutrients, metabolites or genetic material from close proximity to other micro organismsis is facilitated in biofilms

• Many sewage treatment plants have a treatment stage in which water passes over biofilms grown on filter that can extract or digest organic compounds. Bacteria can remove organic matter and protozoa can remove suspended solids including pathogens.

• Biofilms can also help eliminate petroleum oil from contaminated oceans or marine systems. The oil is eliminated by the hydrocarbon-degrading activities of microbial communities, in particular by hydrocarbonoclastic bacteria (HCB)

How do microbes reach plant tissue to form bioflm

Direct and indirect routes by which microbes encounter plant tissues and thereby initiate the interactions that lead to biofilm formation are

• Plant exudates generate nutrient rich conditions into the soil, provide a target for chemotaxis and allow soil-borne microbes to effectively colonize productive sites on the plant.

• Rainwater, splatter and aerosols can also transfer microbes onto leaves and flowers

• By wind microbes are deposited on aerial plant surfaces.

• Physical wounding of plants may introduce microbes from the external environment into internal tissues.

• Insect and nematode activity can potentially inoculate microbes at any site on the plant. Sap-feeding insects are a common mode of transmission for vascular pathogens

How do biofilm form?
Microbes form a biofilm in response to many factors, which may include
1. Cellular recognition of specific or non-specific attachment sites on a surface
2. Nutritional cues,
3. By exposure of planktonic cells to sub-inhibitory concentrations of antibiotics.

There are five stages of biofilm development
• Initial attachment
• Irreversible attachment
• Maturation
• Development
• Dispersion

1. Initially free-floating microorganisms attach themselves to a surface through weak, reversible adhesion via Vander Walls forces.

2. If the colonists are not immediately separated from the surface, they can anchor themselves more permanently using cell adhesion structures such as pili.

3. The first colonists facilitate the arrival of other cells by providing more diverse adhesion sites and begin to build the matrix that holds the biofilm together. Some species are not able to attach to a surface on their own but are often able to anchor themselves to the matrix or directly to earlier colonists.

4. Once colonization has begun, the biofilm grows through a combination of cell division and recruitment. The final stage of biofilm formation is known as development, and is the stage in which the biofilm is established and may only change in shape and size. The development of a biofilm may allow for an aggregate cell colony (or colonies) to be increasingly antibiotic resistant.

5. Dispersal enables biofilms to spread and colonize new surfaces. Enzymes that degrade the biofilm's extracellular matrix, such as dispersin B and deoxyribonuclease, may play a role in biofilm dispersal. Biofilm matrix degrading enzymes may be useful as anti-biofilm agents.

Role of biofilms in agriculture
The microbial cells growing in a biofilm are physiologically distinct from planktonic cells of the same organism, which, by contrast, are single-cells that may float or swim in a liquid medium.
Biofilms inocula can be used for successful establishment of introduced beneficial microorganisms. When biofilms are formed, high acidity suppresses the pathogens. High acidity reflects the production of Indole acetic acid like substances that can improve the plant growth.

Biofilms formed by nitrogen fixing bacteria and phosphate solubilizing fungi may prove beneficial for crop growth. Incorporation of a N2-fixing rhizobial strain to the fungal bacterial biofilm to form fungal-rhizobial biofilms (FRB) has potential applications in N-deficient settings and in the production of biofilmed inocula for biofertilizers and biocontrol in plants.

• The biofilmed inocula enabled Rhizobium culture survive at high salinity (400 mM NaCl) and tannin concentrations (0.4 mM tannic acid) by 105 and 1012 fold, respectively compared to the rhizobial monocultures.

• Pleurotus ostreatus and Pseudomonas fluroscence biofilm increased the endophyte colonization of tomato by 1000 % compared to inoculation with P. fluroscence alone.

• The inoculation of the edible mushroom (P. ostreatus) with a rhizobial strain showed that this association fixed N2 through biofilm formation and increased the protein content of the mushroom by 147 %.

• Penicillium frequentans and Bacillus mycoides formed a biofilmed inoculant which resulted in 14 % increase in the biodegradability of degradable polyethylene by P. frequentans.

• Application of fungal Rhizobium biofilm reported to increase N2 fixation in soybean by 30%, compared to a single inoculation with Rhizobium.

• Co-inoculation of plant growth-promoting rhizobacteria and arbuscular mycorrhizal fungi in rain-fed wheat fields produced the highest protein contents of grains compared to their monocultures.

• Mixed inoculation with arbuscular-mycorrhizal fungi and diazotrophic bacteria has been reported to generate synergistic interactions with the possible consequences of a significant increase in growth, in the phosphorus content of the plants, enhanced mycorrhizal infection, and an improvement in the uptake of phosphorus, nitrogen, zinc, copper and iron. These inocula stimulate plant growth through mechanisms that improve nutrient acquisition and inhibition of fungal plant pathogens.

• Biofilm inoula can also be used for rock phosphate solubilization.

Biotechnoogical applications of biofilm

• The microbes attached to particles of contaminated soils and aquatic sediments help degrade soil-bound contaminants occurring from chemical releases into the environment. Biofilm reactors have been designed to promote microbial growths that are effective for treating environmental wastes such as sewage, industrial waste streams or contaminated groundwater.

• Biofilms can also be used to produce a wide variety of biochemicals that on purification can be used in medicines, food additives or chemical additives for cleaning products.

• Cellulolytic enzyme activity and productivity of Aspergillus niger in biofilm was higher by 40 and 55 % than that attained by planktonic cultures.

• Beneficial biofilms have been found effective in decreasing the biological corrosion, through the inhibition of bacteria which corrode metals.

• Saccharomyces cerevisiae forms biofilms in packed bed continuous bioreactors, to produce ethanol from molasses.

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