Bacteria Notes

Characteristics of Cells

 All living things (single and multicellular) are made of cells that share some common characteristics:

–      Basic shape – spherical, cubical, cylindrical

–      Internal content – cytoplasm, surrounded by a membrane

–      Genetic material (DNA), ribosomes (protein synthesis), metabolic capabilities

Two basic cell types: eukaryotic and prokaryotic

Eukaryotic cells: animals, plants, fungi, and protists

–      contain double-membrane bound nucleus with DNA chromosomes

–      contain membrane-bound organelles

Prokaryotic cells: bacteria and archaea

–      no nucleus or other membrane-bound organelles

 

Bacterial Structure

•       All have the following:

–      Cell membrane

–      Bacterial chromosome or nucleoid

–      Ribosome

–      cytoskeleton

–      Cytoplasm

Majority have cell wall and surface coating (glycocalyx)

             

EXTERNAL STRUCTURE

 

     Appendages

–      Two major groups of appendages:

•       Motility – flagella and axial filaments

•       Attachment or channels – fimbriae, nanotubes/wires and pili

•       Glycocalyx – surface coating

•       Flagella:

              - long protein structure used in motility of cell through environment

              - spins like a propeller to move bacterium through liquid

•       3 parts:

–      Filament – long, thin, helical structure

–      Hook- curved sheath

–      Basal body – stack of rings firmly anchored in cell wall

–      Rotates 360o

 

 

Types of flagella

 

    Monotrichous – single flagellum at one end

Lophotrichous – small bunches arising from one end of cell

Amphitrichous – flagella at both ends of cell

Peritrichous – flagella dispersed over surface of cell; slowest

     Periplasmic flagellum: internal flagellum located between the cell wall and cell membrane

•       Found in Spirochetes: corkscrew shaped bacterium

•       Worm or serpentine motion

•       Example: Borrelia burgdorferi

•       Lyme disease

 

Function of Flagell

Guide bacteria in a direction in response to an external stimulus:

Chemical stimuli – chemotaxis:

                            positive = movement of cell to favorable chemicals (nutrients)

                            negative=movement away from harmful chemical

Light stimuli – phototaxis

Signal sets flagella into rotary motion clockwise or counterclockwise:

Counterclockwise – results in smooth linear direction – run

Clockwise – tumbles

•       Some Pathogenic bacteria use flagella to invade mucous membranes

 

Identification of Flagella

 

•       Flagella too small to be seen with microscope

•       Use semisolid medium—growth spreading rapidly through entire media indicates motility

•       Use the hang drop slide method

 

Fimbriae

•       Fine, hairlike protein bristles on the cell surface

•       Function in adhesion to other cells and surfaces

–      Lead to biofilm formation/colonization on inanimate surfaces

–      Example: colonization of E.coli on intestine and then invades tissue

–      Allow tight adhesion between fimbriae and epithelial cells, allowing bacteria to colonize and infect host tissues

•       Structure play a role in invasion of human tissue-pathogenicity

Pili:

•       Rigid tubular structure made of pilin protein

•       Function to join bacterial cells for partial DNA transfer called conjugation

–      The pilus connects to another cell and DNA is transferred

–      Gram neg bacteria only

–       

Glycocalyx: Outermost layer

 •       Used as protection & helps cells adhere to Surfaces

        Two types:

  1. Slime layer - loosely organized and attached

                                           protects against water loss

2.      Capsule - highly organized, tightly attached to bacterium—sticky mucoid

                                                                        called “encapsulated bacteria”

                                   Have greater disease-causing abilities

–      Protect against host white blood cells called phagocytes

–      Biofilms: living layers

–      Form readily moist thin layer of organic material

–      Bacteria attach, multiply and lay down sticky matrix

–      More cells join and biofilm forms

–      Cells can break away to other habitats

–      Plaque on teeth protects bacteria from becoming dislodged

–      Responsible for persistent colonization of plastic catheters, IUDs, metal pacemakers, and other implanted medical devices

•       Treatment is difficult

–      Protected by thick outer layer

–      Antibiotic resistant cells-protects biofilm

 

Cell Envelope: External covering (beneath glycocalyx)

•       Composed of two basic layers: Cell wall and Cell (plasma) membrane

 

   1. Cell Membrane

•       Beneath the cell wall

•       Lipid bi-layer, selectively permeable

•       Very similar to eukaryote cell, except eukaryote cell has cholesterol

•       Thin, fluid structure (phospholipid bilayer) that surrounds the cytoplasm and defines the boundary of the cell

•       Functions in:

–      Providing site for energy reactions, nutrient processing, and synthesis

–      Passage of nutrients into the cell and discharge of wastes

–      Selective permeability

 

 

 

 

 

 2.Cell Wall

•       Helps determine the shape of a bacterium

•       Provides strong structural support to keep the bacterium from bursting or collapsing because of changes in osmotic pressure:

–      Certain drugs target the cell wall, disrupting its integrity and causing cell lysis (disintegration or rupture) of the cell

•       Peptidoglycan: the chemical component of the cell wall that makes it very rigid (found only in bacteria)

Unique macromolecule composed of a repeating framework of long chains (NAG & NAM) cross-linked by short peptide fragments the composition vary between bacteria

Typically a rigid structure that determines the shape of the organism

 

         2 types of Bacteria based on external structure

–      Gram-positive bacteria: thick cell wall composed primarily of peptidoglycan and cell membrane

–      Contain negatively charged molecules that are attached to the peptidoglycan layers (teichoic acids)

–      Toxic properties to human cells

 

–      Gram-negative bacteria: outer cell membrane, thin peptidoglycan layer, and cell membrane

–      Far more complex than Gram- Positive organisms:

–      Contains an outer membrane: a unique lipid bilayer embedded with proteins, serves as a barrier

–      The outer membrane contains lipopolysaccharides (LPS) rather than phospholipids

                     - May be toxic when released during infections (endotoxin)

              cause fever, inflammation, hemorrhage, diarrhea--fatal

              - Also functions to block immune response

              - resistant to disinfectants

        Medical Importance

Compounds that specifically target peptidoglycan synthesis or integrity are used to destroy bacteria

•       These substances have no effect on eukaryotic cells because peptidoglycan is unique to bacteria

•       Examples include: penicillin and lysozyme

•       Enzyme that breaks the bonds between molecules of peptidoglycan, destroying the structural integrity of the crosslinking.

•       Found naturally in tears and saliva

 

 

 

 

GRAM STAIN

•       The Gram Stain is a differential stain used to classify Gram positive and negative bacteria.

•       Developed by Hans Christian Gram in 1884 when he was studying bacteria from different respiratory diseases.

The Gram Stain is the single most important technique in microbiology

•       Distinguishes cells with a Gram-positive cell wall from those with a Gram-negative cell wall:

–      Gram-positive - retain crystal violet and stain purple

–      Gram-negative - lose crystal violet and stain red from safranin counterstain

–      Important basis of bacterial classification and identification

•       Important in diagnosing infection and guiding drug treatment

 

Acid Fast Bacteria: Bacteria with different structure than Gram + and Gram –

–      Some peptidoglycan, but more lipid (mycolic acid (fatty acid)

•       Lipids are harmful to human cells

•       Highly resistant to certain chemicals

–      Example, mycobacterium & Nocardia

•       Responsible for tuberculosis and leprosy

•       M. leprae & M.tuberculosis

 

Bacteria without a cell wall

            Mycoplasmas contain sterols in membrane that prevents them from lysis

•       Variety of shapes

•       Mycoplasma pneumoniae

•       Walking pneumonia

•       L forms:

–      Some bacteria that naturally have a cell wall but lose it during part of their life cycle

–      Role in persistent infections

–      Resistant to antibiotics

 

INTERNAL STRUCTURES

 

The Chromosome (DNA):

                            - Makes up the nucleoid which contains the genetic material in center of cell

- Tightly coiled (supercoiled) so not to take up much room in the already small cell (10% of the total volume of the cell)

DNA carries information required for maintenance and growth

Plasmids:

              - Extra pieces of genetic material that carry 5-100 genes

              - Makes up 0.1-10% of the size of the chromosome

Not essential for survival

- Can be advantageous: may contain genetic info for enzymes responsible for degrading antibiotics and toxin production

-can be passed onto to offspring

 

Ribosomes:

              -RNA and Protein

              - involved in protein synthesis

              - facilitate the joining of amino acids to make a protein

Inclusions and granules

–      Intracellular storage bodies that perform variety of functions

–      Vary in size, number and content

–      Bacterial cell can use them when environmental sources are depleted

•       Store nutrients

–      Example: gas vesicles for floating-found in aquatic bacterium

Endospores

•        Dormant structures produced by certain Gram-positive bacteria

–      Clostridium and Bacillus for example

•       Dehydrated, metabolically inactive with a thick coat

•       Typically found in soil and water where they may survive for a very long time

•       Longevity verges on immortality - 250 million years!!

•       Resistant to ordinary cleaning methods and boiling

•       Pressurized steam at 120oC for 20-30 minutes will destroy

 

2-phase life cycle:

 

–      Vegetative cell – metabolically active and growing

–      Endospore – when exposed to adverse environmental conditions; capable of high resistance and very long-term survival (think hibernation)

•       Sporulation – process of forming endospores

–      Endospores are the hardiest of all life forms

–      Withstands extremes in heat, drying, freezing, radiation, and chemicals

–      Not a means of reproduction

•       Germination – process of returning to vegetative growth

 

Bacterial Shapes

•       Vary in shape, size, and arrangement but typically described by one of three basic shapes:

–      Coccus – spherical

–      Bacillus – rod

–      Spiral - helical, comma, twisted rod

vibrio-gentle curve

                          coccobacillus-short rod and Plump

•       A bacterium having a slightly curled or spiral-shaped body is called a spirillum

•       Another spiral cell (which contains periplasmic flagella) is the spirochete

•       Pleomorphism: different shapes of the same bacteria caused by nutritional or hereditary differences

Bacterial Arrangement

•       Influenced by pattern of division

–      diplococci=pairs

–      Tetrads=groups of 4

–      Staphylococci or micrococci: irregular clusters

–      Streptococci: chains

Archaea: The Other Prokaryotes

•       Constitute third Domain: Archaea

•       More closely related to Eukarya than to Bacteria

•       Contain unique genetic sequences in their RNA

•       Have unique membrane lipids and cell walls

•       Live in the most extreme habitats in nature, called extremophiles

•       Adapted to heat, salt, acid pH, pressure, and atmosphere

•       Very few (if any) are medically relevant so we will not discuss Archaea further