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Microbial Growth In NaturalEnvironments; Biofilm and Quorum

Sensing

LECTURE 20:

Microbiology and Virology; 3 Credit hours

Atta-ur-Rahman School of Applied Biosciences (ASAB)National University of Sciences and Technology (NUST)

Biofilm Formation

Planktonic

Loosely Bound EPS secreting

Biofilm Definition

• Biofilms are communities of microorganisms in a matrix that joins them together onto living or inert substrates

• Biofilms are surface attached communities of bacteria, encased in an extracellular matrix of secreted – proteins, – carbohydrates, – and/or DNA,

Biofilm Formation

Biofilm Formation

Biofilm Formation

Biofilm Formation

Biofilm Formation Steps

• Biofilms can form on virtually any surface, once it has been conditioned by proteins and other molecules present in the environment

• Microbes reversibly attach to the conditioned surface and eventually begin releasing polysaccharides, proteins, and DNA

• As the biofilm thickens and matures, the microbes reproduce and secrete additional polymers

• The end result is a complex, dynamic community of microorganisms.

Biofilm Formation

Microbial Interaction in Biofilm

• The microbes interact in a variety of ways. • For instance, the waste products of one microbe

may be the energy source for another microbe. • Finally, the presence of DNA in the extracellular

slime can be taken up by members of the biofilm community.

• Thus genes can be transferred from one cell (or species) to another.

Antimicrobial compound and Biofilm

• In the biofilm, microbes are protected from numerous harmful agents such as UV light, antibiotics, and other antimicrobial agents.

• This is due in part to the extracellular matrix in which they are embedded.

• The resistance of biofilm cells to antimicrobial agents has serious consequences.

• When biofilms form on a medical device such as a hip implant, they are difficult to kill and can cause serious illness.

• Often the only way to treat patients in this situation is by removing the implant.

• Another problem with biofilms is that cells are regularly sloughed off.

Continous shedding of bacteria from Biofilm

Quorum Sensing

Cell-Cell Communication Within Microbial Populations

• Previously microbiologists used to think of bacterial populations as collections of individual cells growing and behaving independently.

• But about 30 years ago, it was discovered that the marine luminescent bacterium Vibrio fischeri controls its ability to glow by producing a small, diffusible substance called auto-inducer (acyl homoserine lactone).

Euprymna scolopes (Hawaiian bobtail squid)

Euprymna scolopes and Vibrio fischeri

• The bacteria are housed within epithelial cavities, of this light organ, encouraged to produce light through bacteria regulated quorum sensing and oxygen provided by the squid.

• The squid uses this light organ in counter illumination, a technique that camouflages the squid within the environment.

• Since the squid is active at night, the light from the moon and stars can penetrate the shallow water and illuminate the squid’s presence to its prey or predators.

• Through sensors on the squid’s back, the squid is able to match the amount of ambient light by manipulating the amount of light given off by the light organ with its ink sac.

• Thus, the squid does not have a shadow. Every morning, the squid expels about 90% of the bacteria from the light organ, allowing a fresh culture for the remaining bacteria to flourish in as the squid buries itself in the sand for the day.

G (-ve) Bacteria and SQ

• Pseudomonas aeruginosa use AHLs to regulate the expression of virulence factors

• The plant pathogens Agrobacterium tumefaciens will not infect a host plant and Erwinia carotorvora will not produce antibiotics without AHL signaling

G (+ve) Bacteria and SQ• Gram positive bacteria usually exchange short peptides

called oligopeptides instead of autoinducer like molecules.• Enterococcus faecalis, whose oligopeptide signal is used to

determine the best time to conjugate (transfer genes).• Oligopeptide communication by Staphylococcus aureus

and Bacillus subtilis is used to trigger the uptake of DNA from the environment.

• The soil microbe Streptomyces griseus produces a gamma-butyrolactone known as A-factor.

• This small molecule regulates both morphological differentiation and the production of the antibiotic streptomycin.

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