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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
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