Post on 27-Jan-2017
Bacterial Cell Structure
Dr Shireen RafiqMBBS, M.Phil, Ph.D
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Microbiology
• Agents of human infection:– Bacteria Prokaryotes– Helminths Eukaryotes– Protozoa Eukaryotes– Fungi Eukaryotes– Viruses Noncellular
• Difference between Eukaryotes and Prokaryotes
• Structure
• Replication
• Nucleic acid
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Earliest Prokaryotes
• Most numerous organisms on Earth
• Include all bacteria• Earliest fossils date 2.5
billion years old
BACTERIA
• Bacteria are large group of single celled prokaryotic microorganisms
• 10 times as many bacteria cells in the human flora as there are human cell in the body
Shape of Bacteria
• Three basic shapes• Cocci: streptococci, Staphylococci,
Diplococci• Bacilli: E.coli, Klebsiella, Bacillus.• Spirochetes: Treponema, Borrelia
Bacterial Size • Bacteria range in size from about 0.2 to 5 µm.
• The smallest bacteria (Mycoplasma) are about the same size as the largest viruses (poxviruses) and are the smallest organisms capable of existing outside the host.
• The longest bacteria rods approach the size of some yeasts and human R.B.Cs
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According to staining
• Gram positive Thick peptidoglycan layer and teichoic acid
• Gram negative Thin peptidoglycan layer and
lipopolysaccharide- endotoxin
• Acid fast bacilli Mycolic acid (lipids)
• Some bacteria are variable in shape
• PLEOMORPHIC----many shaped
• Shape of the bacteria is determined by its rigid cell wall
• The microscopic appearance of bacterium is most important criteria for its identification
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pH requirements• Most grow best at pH of 6.5 to 7.0• Many act as decomposers
recycling nutrients• Some cause disease (Pathogenic)
The Prokaryote
• Structural Components MACROMOLECULE SUBUNIT POSITION IN CELL
PROTEIN Amino Acid Flagella, pili, cell wall, cytoplasmic membrane, ribosomes, cytoplasm
POLYSACCHARIDE Sugar/Carbohydrate Capsule, Inclusions, Cell wall
PHOPHOLIPID Fatty Acid Membranes
NUCLEIC ACID(DNA/RNA)
Nucleotide DNA, Nucleoid, Plasmids, Ribosomes,
Structural Components
Prokaryotes have 5 essential components• Nucleoid (DNA)• Ribosomes• Cell membrane• Cell wall• Surface layer (Capsule)• Appendages
Structural Components
sugars (carbohydrates)
BACTERIAL STRUCTURECOVERING LAYERS
• Cell wall• Peptidoglycan
Sugar back bone with peptide side chains,which are cross linked
Rigidity osmotic protection , site of action of antibiotic, lysozyme degrade.
Outer membrane Gram Negative bacteria
Lipid A
Polysaccharide
Toxic component of endotoxin.Surface antigen.
Surface fiber on Gram Positive bacteria
Teichoic acid Surface antigen
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Protection
• Cell Wall made of Peptidoglycan
• May have a sticky coating called the Capsule for attachment to host or other bacteria
FUNCTION OF CELL WALL• Maintaining the cell's characteristic shape
• Countering the effects of osmotic pressure
• Providing attachment sites for bacteriophages-teichoic acids
• Providing a rigid platform for surface appendages- flagella, fimbriae
Peptidoglycan
COMPARISONProperty Gram Positive Gram Negative
Thickness of wall 20-80 nm 10 nm
Number of layers in wall 1 2
Peptidoglycan content >50% 10-20%
Teichoic acid in wall + -
Lipid and lipoprotein content 0-3% 58%
Protein content 0% 9%Lipopolysaccharide 0 13%Sensitive to penicliiin + - (not as much)
Digested by lysozyme + - (not as much)
Properties of cell wall
• Gram negative bacteria contains endotoxin---lipopolysaccharide
• Polysaccharides and proteins are antigens• Porin proteins helps entry of hydrophilic
molecules• Teichoic acid are fibers on outer surface of
gram positive ---ability to induces septic shock
Cell Membrane
• Composed of phospholipid bilayer
• FUNCTIONS• Active transport• Energy generation---oxidative phosphorylation• Synthesis of precursors of cell wall• Secretion of enzymes and toxins
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• Infoldings of cell membrane carry on photosynthesis & cellular respiration
• Infoldings called Mesosomes
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MesosomesMESOSOME
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Sticky Bacterial Capsule
Plasmids
• Molecules of DNA that are found in bacteria separate from the bacterial chromosome.
• A circular molecule only much SMALLER than the genomic DNA
• REPLICATE AUTONOMOUSLY from the genomic chromosome. Often there are MANY PLASMID COPIES present in one cell. Further, a cell may contain SEVERAL DIFFERENT PLASMIDS or it may contain NO PLASMIDS at all. Plasmids generally carry genes that are NOT ESSENTIAL for a cell's survival
• May carry genes for ANTIBIOTIC RESISTANCE
Transposons
• Transposons are pieces of DNA move from one site to another ---- within or between the DNAs of bacteria plasmid or bacteriophage.
• Nick name as Jumping genes• Genes for one or more (usually more) proteins
imparting resistance to antibiotics. When such a transposon is incorporated in plasmid, it can leave the host cell and move to another. This is the way that the alarming phenomenon of multidrug antibiotic resistance spreads so rapidly.
Appendages
Flagella: FlagellinFunction: Motility/chemotaxis
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Flagella• Bacteria that are
motile have appendages called flagella
• Attached by Basal Body
• A bacteria can have one or many flagella
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Flagella• Made of Flagellin• Used for Classification• Monotrichous: 1 flagella• Lophotrichous: tuft at
one end• Amphitrichous: tuft at
both ends• Peritrichous: all around
bacteria
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Pili• Short protein appendages PILIN• Smaller than flagella• Adhere bacteria to surfaces• Used in conjugation for Exchange of
genetic information• Aid Flotation by increasing buoyancy
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Pili in Conjugation
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Bacterial Shapes
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Shapes Are Used to Classify• Bacillus: Rod shaped• Coccus: Spherical (round)• Vibrio: Comma shaped with flagella• Spirillum: Spiral shape• Spirochete: wormlike spiral shape
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Grouping of Bacteria
• Diplo- Groups of two• Strepto- chains• Staphylo- Grapelike clusters
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Bacillus and E. coli
Spirochetes
ACCORDING TO STAINING
GRAM STAINING
Crystal violet
Gram's iodine
Decolorise with acetone
Counterstain withe.g. methyl red
Gram-positives appear purple
Gram-negatives appear pink
The Gram Stain
Gram-positive rods
Gram-negative rods
Gram-positive cocci
Gram-negative cocci
FORMATION OF BACTERIAL SPORE
• Found in Gram positive bacteria
• Tough, heat resistant
• Peptidoglycan > Picolinic acid