Chapter 4

Resolution
Resolution: The ability to distinguish two adjacent objects as distinct and separate.
Light microscopes optimize image resolution by using lenses with high light-gathering characteristics (numerical aperture).

Contrast
Differences in intensity between two objects, or between an object and background
Total Magnification
Product of the magnification of the objective and ocular lenses.
Upper limit=1500

Lenses
Ocular: magnify 10-15X and objective of 10-100X

At 1000 x objects 0.2 um in diamater can just be resolved.

Immersion oil increase the light gathering ability of a lens by allowing rays emerging from the specimen at angles that would otherwise be lost to the objective lens to be collected and viewed.
Simple Stains

Gram Stain and Cell Wall
The structural differences between the cell walls of gram-positive and gram-negative Bacteria are responsible for differences in the Gram stain reaction.

Alcohol can readily penetrate the lipid-rich outer membrane of gram-negative Bacteria and extract the insoluble crystal violet-iodine complex from the cell.

The Gram Stain
Electron microscopy
Two major types of electron microscopy are performed: transmission electron microscopy, for observing internal cell structure down to the molecular level, and scanning electron microscopy, for three-dimensional imaging and examining surfaces.

Cytoplasmic : Part 4.5
A highly selective permeability barrier
Constructed of lipids and proteins
Forms a bilayer with hydrophilic exteriors and a hydrophobic interior.
Permeability Layer
The attraction of the nonpolar fatty acid portions of one phospholipid layer for the other layer helps to account for the selective permeability of the cell membrane.

Transport: Simple, Group translocation and the ABC system.


Translocases
Proteins are exported out of prokaryotic cells through the actions of proteins called translocases, which are specific in the types of proteins exported.

Bacterial Cell Wall: Part 4.8
There are two types of cell walls: Gram-positive and Gram-negative
The wall consists of the following:
Alternating repeats of N-acetylglucosamine and N-acetylmuramic acid
N-acetylmuramic acid is cross-linked between strands by short peptides.


Cross-Linkage
Each peptidoglycan repeating subunit is composed of four amino acids:
L-alanine,
D-alanine,
D-glutamic acid
Either lysine or diaminopimelic acid)
Two N-acetyl-glucose-like sugars.

Gram stain reaction.
The structural differences between the cell walls of gram-positive and gram-negative Bacteria are thought to be responsible for differences in the Gram stain reaction.
Alcohol can readily penetrate the lipid-rich outer membrane of gram-negative Bacteria and extract the insoluble crystal violet-iodine complex from the cell.

Protoplasts
Some prokaryotes are free-living without cell walls
They have unusually tough membranes
Or live in osmotically protected habitats, such as the animal body.


Surface Structures- Part 4.10
Prokaryotic cells often contain various surface structures, including fimbriae and pili, S-layers, capsules, and slime layers. A key function of these structures is in attaching cells to a solid surface.

Capsule: Part 4.10
Composed of repeating units of molecules
Attached to cell surface
Protects from drying
Hides bacteria from host

Slime Layer: Part 4.10
Loosely attached to cell surface
Water soluble
Protects cells from drying
Sticky layer that allows attachment

Fimbriae Versus Flagella

Pilus Versus Fimbriae
Cell Inclusions: Part 4.11
Prokaryotic cells often contain internal granules that function as storage materials or in magnetotaxis.
Some gram-negative prokaryotes can store elemental sulfur in globules in the periplasm

Gas Vesicles: Part 4.12
Gas vesicles are small gas-filled structures made of protein that confer buoyancy on cells.
Gas vesicles contain two different proteins arranged to form a gas-permeable, but watertight, structure .
Cyanobacteria.

Gas vesicles
decrease the density of cells
allow a means of motility
allows organisms in water to position themselves for optimum light harvesting.
they are common in many species of cyanobacteria.

Endospores: Part 4.13
The endospore is a highly resistant differentiated type of bacterial cell produced by certain gram-positive Bacteria.

Endospore formation leads to a highly dehydrated structure that contains essential macromolecules and a variety of substances.

Endospores can remain dormant indefinitely but germinate quickly when the appropriate trigger is applied.

Endospores differ significantly from the vegetative, or normally functioning, cells

Endospore Stain

Motility : Part 4.14
Bacterial Flagella
Motility in most microorganisms is accomplished by flagella.
In prokaryotes, the flagellum is a complex structure made of several proteins, most of which are anchored in the cell wall and cytoplasmic membrane.

Flagella move the cell by rotation. A speed of about 60 cell lengths/second achievable.

Gliding Motility: Part 4.15
Prokaryotes that move by gliding motility do not employ rotating flagella They creep along a solid surface by gliding using the slime layer
or by a ratchet-protein mechanism that moves the outer membrane of the cell.

TAXIS: Part 4.16Chemotaxis
Motile bacteria can respond to chemical and physical gradients in their environment.
In chemotaxis and phototaxis, random movement of a cell is biased either toward or away from a stimulus by controlling the degree to which runs or tumbles occur

Taxis: Part 4.16
The latter are controlled by the direction of rotation of the flagellum, which in turn is controlled by a network of sensory and response proteins.
Counterclockwise rotation moves the cell in a direction called a run. Clockwise rotation causes the tuft of flagella to spread, resulting in tumbling of the cell