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Pili-fimbriae-endospore
Bsc nursing (blaw 213)
Kerala University of Health Sciences
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FIMBRIAE & PILI IN BACTERIA
FIMBRIAE
⇒ are very short, fine, hair like appendages projecting from the surface as straight filaments.
⇒ Fimbriae are also called “short attachment pili”
⇒They are shorter and thinner than flagella, i. 0 – 1 long and less than 10nm thick.
⇒ Fimbriae are found in some gram-negative bacteria, but not in many Gram- positive bacteria.
⇒ they are slender tubes composed of helically arranged protein subunitsThere are 3 types of fimbriae have been identified – common, ‘sex’ Pili and col I (colicin) pili.
⇒They attach to the host surface and help bacteria colonise and cause infection
⇒They are present on the overall surface or concentrated towards the poles.
⇒Many aerobic bacteria get attached to the surface of the culture medium by fimbriae.
⇒This allows them to colonise near the air and also take nutrients from the medium.
⇒The attachment helps them to avoid flushing.
⇒The thin film at the surface of broth culture is known as a pellicle.
PILI
⇒Pili are generally referred to as the appendages, which are involved in the conjugation. They are also known as long conjugative pili.
⇒pili are typically longer and fewer in number than fimbriae and involved in the cell to cell attachment during conjugation for DNA transfer.
⇒The fimbriae and pili have a shaft composed of a protein called pilin. At the end of the shaft is the adhesive tip structure having a shape corresponding to that of specific glycoprotein or glycolipid receptors on a host cell.
⇒Because both the bacteria and the host cells have a negative charge, pili may enable the bacteria to bind to host cells without initially having to get close enough to be pushed away by electrostatic repulsion.
⇒ Once attached to the host cell, the pili can depolymerize and enable adhesions in the bacterial cell wall to make more intimate contact.
Adhesive Tip of Bacterial Pili Binding to Host Cell Receptors
⇒There are two basic types of pili: short attachment pili and l ong conjugation pili.
⇒ Short attachment pili , also known as fimbriae or common pili are usually short and quite numerous and enable bacteria to colonize environmental surfaces or cells and resist flushing.
⇒They contribute to the pathogenicity of certain bacteria.
Scanning electron micrograph of E bacteria exchanging genes.
⇒The donor bacterium or ‘male bacterium’ with sex pilus takes in charge and establishes direct contact with the recipient cell by forming a mating bridge. It then transfers its DNA to the recipient cell.
⇒The DNA, which gets transferred contains genes for the formation and transfer of pili. Other genes such as antibiotic-resistant genes also get transferred along with it.
⇒Pili are classified based on their susceptibility to bacteriophages, e. F-pili, I- pili, etc. Some bacteriophages attach to sex pili before multiplication. They are encoded by a conjugative plasmid. F-pili of E is encoded by F-plasmid.
TYPE IV PILI
⇒They are responsible for the twitching motility. They adhere to the surface and bring about the movement by contraction.
⇒ The external ends of the pili adhere to a solid substrate, either the surface to which the bacterium is attached or to other bacteria. Then, when the pili contract, they pull the bacterium forward like a grappling hook. Movement produced by type IV pili is typically jerky, so it is called twitching motility.
⇒ Many archaea contain this type of pili, which help them to adhere to various surfaces.
⇒The disease-causing strains of bacteria possess fimbriae or pili. Pili increases the bacterial ability to adhere to tissues and colonise by multiplying rapidly.
⇒Bacteria without fimbriae or pili are generally non-pathogenic.
⇒ Fimbriae or pili are responsible for virulence through the attachment and also provide resistance to the phagocytosis by white blood cells.
⇒Bacteria with type IV pili include Pseudomonas aeruginosa, Neisseria gonorrhoeae, Neisseria meningitidis , and Vibrio cholerae.
SPORE FORMATION IN BACTERIA
⇒ Some bacteria have the ability to form the highly resistant resting stage during unfavorable environmental conditions called as Spores.
⇒ Bacterial spores are formed inside the bacterial cell, are called as endospores.
⇒ where as exospore s is produced outside the cell. Bacteria producing exospore: Methylosinus
⇒ Endospores are defined as round or ovoid, thick-walled, highly refractile, metabolically inert resistant structures
⇒ Sporulation is the process by which bacterial cell forms spore under unfavorable conditions.
⇒ Spore develops from a part of protoplasm is called as forespore near one end of the cell and the remaining part of the cell is called sporangium.
⇒Spores are not obligatory part of the bacterial life cycle. ⇒Endospore formation is a mechanism of survival rather than a mechanism of reproduction ⇒The endospores can survive possibly hundreds of years until a variety of environmental stimuli trigger germination; they are probably the most durable cells produced in nature.
⇒Spore formation is usually triggered by detrimental environmental growth conditions, specially limitation of C-supply. ⇒Sporulation occurs in the late log phase / early stationary phase of bacterial growth cycle. ⇒The endospores are produced at the rate of one per cell inside bacteria.
Electron microscope shows that endospore structure is complex
The spore is surrounded by a thin delicate covering called the exosporium
A spore coat lies beneath the exosporium, is composed of several protein
layers and may be fairly thick
It is impermeable to many toxic molecules and is responsible for the spores
resistance to chemicals.
The coat also contain enzymes that are involved in germination.
The cortex lies beneath the spore coat; it is made of a peptidoglycan that is
less cross-linked than that in vegetative cells.
The spore cell wall (core wall) is inside the cortex & surrounds the
protoplast or spore core.
The core has normal cell structures such as ribosomes & a nucleoid but is
metabolically inactive.
15 % of the spores dry weight consists of dipicolinic acid complexed with
calcium ions which is located in the core.
It has been thought that dipicolinic acid was directly involved in heat
resistance, but heat resistant mutants lacking dipicolinic acid have been isolated.
Calcium provides resistance to wet heat, oxidizing agents & some times dry
heat.
The calcium- dipicolinate stabilizes the spores nucleic acids.
In addition specialized small, acid soluble DNA -binding proteins
(SASPs)are found in the endospore; which saturate spore DNA & protect it from heat, radiation, dessication and chemicals. SPORULATION
It is a complex process & may be divided into seven stages.
An axial filament of nuclear material forms (stage 1)
Followed by an inward folding of the cell membrane to enclose part of the
DNA & produce the fore spore septum(stage2)
The membrane continuous to grow and engulfs the immature endospore in a
second membrane (stage3).
Next cortex is laid down in the space between the two membranes, and both
calcium & dipicolinic acid are accumulated (stage4).
Protein coats are then formed around the cortex (stage5).
And then maturation of endospores occur (stage6).
Finally the lytic enzymes destroy the sporangium releasing the
spore(stage7).
Sporulation requires about 10 hrs in Bacillus megaterium.
The transformation of dormant spores into active vegetative cells involves
three stages 1. Activation 2. Germination 3. Outgrowth
Pili-fimbriae-endospore
Course: Bsc nursing (blaw 213)
University: Kerala University of Health Sciences
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