L7- Bacteria External Cell Structure

What external features let bacteria respond to other cells and their environment

• glycocalyces (singular glycocalyx)

• flagella

• fimbriae

• pili

What is a glycocalyx

• a sticky gelatinous substance outside the cell

• specifically outside the cell wall

What is a glycocalyx made of?

• Mainly polysaccharides but can also be made of polypeptides or both, made inside the cell and pushed onto its surface

2 functions of glycocalyces

1. protects cell from drying out (desiccation bc sugar is polar and traps water)

2. helps pathogens survive and cause disease

3. ( helps bacteria adhere to surfaces)

What two forms can a glycocalyx take

• A capsule→ organized, repeating units of organic chemicals firmly attached to the cell surface

  • coats outside cell wall

• Slime layer→ loose, water soluble

Slime layer function

• It’s sticky so it helps bacteria attach to surfaces as biofilms

  • biofilms= aggregates of many bacteria living together on a surface by fimbriaea and glycocalyces (3D slimy mass of microbes stuck togeter to another another and surface)

Dental example of a biofilm

• dental plaque on teeth

• bacteria in it can make acid→ cavities

• another ex: Slime inside medical catheters

How does the capsule help pathogens survive and cause disease

• chemicals in bacterial capsules are similar to chemicals normally found in the body so the bacterium isn’t recognized or devoured by the hosts’ defensive cells

• The capsule is slippery (made of sugar) preventing immune cells from gripping it and engulfing the pathogen (phagocytosis)

Capsule example

• Streptococcus pneominiae + Klebsiella pnemoniaea

• avoids destruction by defensive cells in resp tract→ pneumonia

• not all bacteria have capsules

Why is cellular motility useful

• the cell can flee harmful environments or move toward favorable ones (food or light)

What are flagella

• long structures extending beyond the cell surface + glycocalyx that propel cell through its envt

Do all bacteria have flagella

• No but in those that do the flagella are similar in composition, structure, and development

• (can be found in euks too)

What 3 parts make up a bacterial flagellum?

• filament

• hook

• basal body

Describe the filament

• A long hollow shaft (~20 nm wide) extending into the envt

• No membrane covers it

What’s flagellin

• The protein that makes up the filament

• identical globular molecules

How does a flagellum grow

• the cell secretes flagellin through the hollow core

• It deposits at the tip of the filament in a clockwise helix which lengthens it

What inhibits flagella growth

• in dry habitats their growth is inhibited bc bacterial flagella react to external wetness

What is the hook

• a curved structure where the filament inserts at its base

• made of different protein than the filament (flagellin)

What is the basal body made of, what does it do?

• made of diff proteins

• It anchors the filament and hook to the cell wall + cell membrane using a rod and a series of 2/4 protein rings

What lets the filament rotate 360 degrees

• the hook, rod, and rings

What are serovars

• strains of a species classified by differences in flagellar proteins

Flagellar arrangements

• Peritrichous

• flagella cover the whole cell surface

Flagellar arrangements

• Monotrichous

• a single flagellum at one end of the cell (polar)

Flagellar arrangements

• lophotrichous

• Tuft of several flagella at the same spot

Flagellar arrangements

• Amphitrichous

• flagella at both ends (poles) of the cell

Which bacteria have endoflagella

• Spirochetes (spiral shaped bacteria)

How are endoflagella arranged

• flagella at both ends spiral tightly around the cell instead of sticking out

• the axial filament wraps around the cell bw the cell membrane and an outer membrane

How do spirochetes move

• rotating the endoflagella rotates the axial filament around the cell, so it corkscrews through its medium

  • Ex) Syphillis (treponema pallidum)

  • some scientists think this corkscrew motility helps it invade human tissues (cause disease)

How does a flagellum move and what powers it

• rotates 360 like a propeller not a whip

• Powered by ion flow through the cytoplasmic membrane near basal body

What is a “run”

• CCW rotation moves the cell in one direction for a while

• if there are multiple flagella they align + rotate together as a bundle

What’s a “tumble”

• A brief abrupt random change in direction that interrupts a run

• Its caused by CW rotation with each flagellum rotating independently

What is taxis

• movement in response to a stimulus

• phototaxis→ toward light

• chemotaxis→ toward a chemical

How does taxis work?

• receptors for light or chemicals on the cell surface signal the flagella to adjust their speed and direction of rotation

Positive taxis

• Movement toward a favorable stimulus (ex-nutrient)

• ‘Runs’ towards an attract get longer so cell drifts toward it

Negative taxis

• Movement away from an unfavorable stimulus

• ‘runs’ away from a repellent get longer

What are fimbriaea

• sticky bristlelike projections that adhere to each other and to substances in the envt (drives pathogenesis)

• May be hundreds/cell and usually shorter than flagella

Fimbriaea example in disease

• Neisseria gonorrhoeae uses fimbriaea to colonize mucous membrane of reproductive tract

• cells lacking fimbriae are nonpathogenic (bc need for attachement)

4 functions of fimbriae

1. metabolic activity: some carry enzymes that turn soluble toxic metal ions→ insoluble nontoxic forms

2. Movement: extend, attach by the tip then retract to pull cell forward (like pulling on rope)

3. Electrical signaling: some act as wires conducting signals among cells in a biofilm

4. Play imp role in biofilms (allowing adhesion to surfaces and other cells)

Biofilms in industry

How does biofilm arichtechture protect bacteria

• industry→ clog drains and corrode industrial pipes

Architecture of biofilms protecting bacteria

• gives protection that free-floating bacteria lack

• Lower O2 in interior reduces effectiveness of some antibiotics

What is a pilus (plural pili)

• Cells use pili to transfer DNA from one cell to another by conjugation

• also called a conjugation pilus

How do pili compare to fimbriae and flagella, and how many are there

• Longer than fimbriae but shorter than flagella

• thicker than fimbriae

• only 1 to a few per cell