Bacterial Form and Function

• Microbiology- Ch. 4 pp 87-101

Structure of a Prokaryotic Cell

Prokaryote Structures:

1. Appendages- flagella, pili, fimbrae

2. Cell envelope- glycocalyx, cell wall , cell membrane

3. Cytoplasm- ribosomes, granules, nucleoid/chromosome.

Bacterial Appendages:• Pili (pl), pilus (s)

– Only found in gram negative bacteria– hollow, hairlike structures of protein larger and more sparse than

fimbriae. – allow bacteria to attach to other cells. – sex pilus, - transfer from one bacterial cell to another- conjugation.

• fimbriae (pl) fimbria (s)– Adhesion to cells and surfaces– Responsible for biofilms.– Pathogenesis of gonococcus and E.coli

• Flagella (pl), flagellum(s)– Motility- – long appendages which rotate by means of a "motor" located just under

the cytoplasmic membrane. – Bacteria may have one, a few, or many flagella in different positions on

the cell.– All spirilla, half of bacilli, rare cocci– Advantages- chemotaxis-positive and negative.

Motility-

• Flagella vary in number and arrangement.• Polar arrangment-

– Monotrichious- 1 flagellum at one end• Fastest; Pseudomonas -example

– Lophotrichious- tuft at one end– Amphitrichious- bipolar

• Peritrichious-– Multiple flagella; randomly dispersed around the

bacterial cell– E.coli -example

Structure of flagellaallows for 360 degree filament rotation

Flagellar arrangements

Detection of Motility

1. Stab line in semisolid motility agargrowth out from the streak line indicates motility.

A= motile; B=nonmotile

2. Motility plate

3. Hanging drop- from actively

growing culture 18-24 hrs old.

directional movement

vs. “brownian movement

Bacterial Surface Structure- cell envelope

Bacteria have some or all of the following structures:

1. Glycocalyx- capsule or slime layer–

layer of polysaccharide (sometimes proteins) – Different composition in certain bacteria-

• Streptococcus pneumoniae- capsule- tighter • Slime layer- looser, washes off

– protects the bacterial cell from phagocytosis– associated with pathogenic bacteria -Staphylococcus aureus.– Glycocalyx- colonize nonliving materials- plastics, catheters,

medical devices.

1. Cell wall –• peptidoglycan (polysaccharides + protein),• Support and shape of a bacterial cell.

The three primary shapes in bacteria are: » coccus (spherical),» bacillus (rod-shaped) » spirillum (spiral).» Mycoplasma are bacteria that have no cell wall and therefore

have no definite shape.

2. Cell wall – peptidoglycan (polysaccharides + protein)

Repeating glycan chains (N acetyl glucosamine and N acetyl muramic acid) with crosslinked peptides.

Support and shape of a bacterial cell. The three primary shapes in bacteria are:

» coccus (spherical),» bacillus (rod-shaped) » spirillum (spiral).» Mycoplasma are bacteria that have no cell wall

and therefore have no definite shape.

Differences in Cell Wall Structure

• Basis of Gram Stain Reaction– Hans Christian Gram- 1884

• Differential Stain• Gram Positive vs Gram Negative Cells• Gram Positive Cells-

– Thick peptidoglycan layer with embedded teichoic acids

• Gram Negative Cells-– Thin peptidoglycan layer, outer membrane of

lipopolysaccharide.

Gram Stain Reaction

• Hans Christian Gram- 1880s• Divides bacteria into 2 main groups-

– Gram positive – Gram negative

• Also- gram variable• Gram nonreactive

• Gram positive bacteria– many layers of peptidoglycan and teichoic acids.– Form a crystal violet-iodine-teichoic acid complex

• Large complex,difficult to decolorize

• Gram negative cells– Very thin peptidoglycan– No teichoic acids– Alcohol decolorizer readily removes the crystal violet.– Alcohol also dissolves the lipopolysaccharide of the cell wall.

• Gram variable cells– Some cells retain crystal violet; some decolorize and take up the

safranin– 4 factors-

• Genetics- variable amount of teichoic acid.• Age of culture- older cultures have variable amount of teichoic acid• Growth medium- necessary nutrients not available• Technique-

– smear not thin or evenly made.– Staining procedure not done correctly- decolorizer left on too long.

• Gram nonreactive cells– Have peptidoglycan but have very waxy- thick

lipids –waterproof, dyes cannot enter either.– Examples- Mycobacterium- tuberculosis and

leprosy. • Alternative staining- acid fast stain-

Cell wall deficient formsFigure 4.17

• L- forms ( Lister Institute where discovered)– Bacteria loses cell wall during the life cycle

• Result of a mutation in cell wall forming genes• Induced by treating with lysozyme or penicillin which disrupts

the cell wall

– Protoplast- • G + bacterium with no c. wall, only a c. membrane• Fragile, easily lysed

– Spheroplast-• G – bacterium loses peptidoglycan, but has outer membrane• Less fragile but weakened.

Surface structures continued:

• Outer membrane – This lipid bilayer is found in Gram negative

bacteria and is the source of lipopolysaccharide (LPS) in these bacteria

– LPS is toxic and turns on the immune system.

– Not found in Gram positive bacteria.

Cell membrane• Located just beneath cell wall• Very thin• Lipid bilayer, similar to the plasma membrane of

other cells. Transport of ions, nutrients and waste across the membrane

• Typical – 30-40% phospholipids– 60-70% proteins

• Exceptions- – Mycoplasma- sterols– Archaea- unique branched hydrocarbons

MesosomeExtension of cell membrane

– Folding into cytoplasm – internal pouch– Increases surface area.

• Gram-positive bacteria-prominent• Gram negative bacteria- smaller,harder to see.• Functions-

– Cell wall synthesis– Guides duplicated chromosomes into the daughter

cells in cell division.

Photosynthetic Prokaryotes

• Cyannobacterium- dense stacks of internal membranes with photosynthetic pigments.

Functions of Cell Membrane

• Carries out functions normally carried out by eukaryote organelles.

• Site for energy functions• Nutrient processing• Synthesis• Transport of nutrients and waste• Selectively permeable• Most enzymes of respiration and ATP synthesis• Enzyme synthesis of structural macromolecules

– Cell envelope and appendages

• Secretion of toxins and enzymes into environment.

Cell cytoplasm

• Encased by cell membrane

• Dense, gelatinous

• Prominent site for biochemical and synthetic activities

• 70-80% water- solvent

• Mixture of nutrients- sugar, amino acids, salts– Building blacks for cell synthesis and energy

Bacterial chromosome• Singular circular strand of DNA• Aggregated in a dense area- nucleiod• Long molecule of DNA tightly coiled around

protein molecules.

• Plasmids-– Nonessential pieces of DNA

• Often confer protection- resistance to drugs– Tiny, circular– Free or integrated– Duplicate and are passed on to offspring– Used in genetic engineering

Ribosomes

• Site of protein synthesis

• Thousands – Occurs in chains –polysomes

• 70S – 2 smaller subunits– 30S and 50S

Inclusions

• If nutrients abundant- stored intracellularly

• Granules- – Crystals of inorganic compounds not enclosed

by membranes• Sulfur granules- photosynthetic• Polyphosphate- corynebacterium• Metachromatic- Mycobacterium

Bacterial Internal Structures• Endospores

– inert, resting, cells produced by some G+ genera: Clostridium, Bacillus and Sporosarcina

• have a 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» Features of spores- size, shape, location=identification

– sporulation -formation of endospores • hardiest of all life forms• Forms inside a cell- functions in survival • not a means of reproduction• withstands extremes in heat, drying, freezing, radiation and

chemicals

– germination- return to vegetative growth

Endospores• Resistance linked to high levels of calcium and

dipicolinic acid• Dehydrated, metabolically inactive• thick coat• Longevity verges on immortality - 25,250

million years.• Resistant to ordinary cleaning methods and

boiling• Pressurized steam at 120oC for 20-30 minutes

will destroy

Bacterial Shapes, Arrangements, and Sizes

• Variety in shape, size, and arrangement but typically described by one of three basic shapes:– coccus - spherical– bacillus – rod

• coccobacillus – very short and plump• vibrio – gently curved

– spirillum - helical, comma, twisted rod, • spirochete – spring-like

Bacterial Shapes, Arrangements, and Sizes

• Arrangement of cells is dependent on pattern of division and how cells remain attached after division:– cocci:

• singles• diplococci – in pairs• tetrads – groups of four• irregular clusters • chains• cubical packets

– bacilli:• chains• palisades