Lecture 3 - Bacterial Anatomy And Physiology

Bacterial Anatomy and Physiology    

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Objectives: At the end of this session, students should be able to: Describe the properties bacteria Explain differences between Gram- positive and Gramnegative bacteria Discuss different bacterial structures and their functions

Bacterial Anatomy and Physiology 

Bacteria

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are unicellular organisms, small in size usually about 1µm in diameter. They are true living organisms and belong to the kingdom prokaryotes- lack nuclear membrane. Larger than viruses,

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Contains double stranded DNA of about 1mm in size which must be folded a thousand fold to fit the size of the bacteria.

Reproduce by binary fission Vary in size and arrangements: Single cells, or in regular groups of two or more cells Most bacteria can be grown on artificial culture media. Some bacteria can cause disease in humans, animals and plants (pathogens)

Assignment  

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Write an essay on the topic: Life in a world without microbes. Length: 2 pages

Prokaryote Vs Eukaryote

Prokaryote

Eukaryote

Differences between Procaryotes and Eucaryotes Procaryotes

Eucaryotes

Size:1-10µm

Size: 10-100 µm

No nuclear membrane

Nuclear membrane present

Single chromosome chromosomes

Multiple

No DNA-associated histones DNA

Histones associated

Binary fission

Mitotic division

Lack membranous compartments

Membranous compartments

Peptidoglycan cell wall

Chitin or cellulose cell wall

70S ribosomes

80S ribosomes

Steroids absent

Steroids present

Anaerobic respiration possible respiration

No anaerobic

Bacterial shapes & arrangements

The Gram stain The Gram stain divides bacteria into two broad groups; Gram- positive (purple) Gram- negative (red) The Gram staining technique consists of flooding a fixed smear of bacteria with crystal violet (purple dye) followed by Lugol’s iodine. The slide is washed with alcohol, (a decolorizer). The smear is then stained with a counter stain such as saffranin (red dye). Bacteria that retains the purple color is Gram positive. Those that stain with the color of the counter stain (red) are Gram – negative.

The Gram stain

Gram stain reactions of bacteria & yeast

Differences between Gram-positive and Gram-negative cell walls

Differences between Gram-positive and Gram-negative cell walls Properties Layers

Grampositive Two

Gramnegative Three

Peptidoglycan

Thick layer

Thin layer

Periplasmic space

Absent

Present

Teichoic acid

Present

Absent

Outer membrane LPS

Absent

Present

Absent

Present

Structure of bacterial cell 

Bacterial cell consists of a cell envelope and cytoplasm

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Cell envelope 

Consists of a cell wall and an underlying cell membrane

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Cell Wall:

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is a rigid structure that surrounds the bacterial cell. It provides shape, rigidity Is composed of peptidoglycan and autolysins. Autolysins:  Enzymes that dissolve the peptidoglycan layer.  Activity is essential for cell wall growth, cell septation and sporulation

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Functions of cell envelope and structures

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Essential for viability

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Contributes to ability to cause disease Ligands for adherence (attachment),  Resist phagocytosis  Antigenic (trigger innate immune response) sepsis  Form spores antigenic variation between bacteria 

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Protects against environmental stresses 

Bile salts, low pH, low osmotic pressure

Target for antibiotics 

Susceptible to cell wall active agents e.g betalactams and glycopeptides, etc

Bacterial Cell Structure Cell Wall- deficient organisms ( L-forms):  May emerge during antibiotic therapy cause persistent infections and resist the effect of antibiotics

Removal of cell walls- leads to bacterial lysis   

Gram positive bacteria--- protoplast, Gram- negative bacteria--- Spherolasts Are resistant to differences in osmotic pressure

Naturally occurring cell wall free organisms  Mycoplasma andUreaplasma

Cytoplasmic membrane (Cell or plasma membrane Cytoplasmic membrane (Cell or plasma membrane)

is the physical and metabolic barrier between the interior and exterior of the bacterial cell Functions: Selective permeability, electron transportation and oxidation Forms mesosomes (complex invaginations) in some bacteria.

The outer membrane Is composed of lipids, proteins and lipopolysaccharide (LPS)  The proteins include porin which form membrane channels for the transport of ions and hydrophilic compounds.  LPS is composed of :  a lipid portion, lipid A,  a polysaccharide rich core and  a polysaccharide side chain  The polysaccharide portion of LPS is antigenic and is designated the O- antigen  Lipid A is heat stable,  is responsible for the biologic effect of endotoxin.

Cytoplasmic compounds  

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Contains DNA, ribosome and storage granules. DNA: No nuclear membrane. DNA is concentrated in the cytoplasm as a nucleod (chromosomal DNA) The nucleod represents 2-3% of the cell dry weight or >10% of cell volume Plasmids: extrachromosomal DNA. Smaller than chromosomal DNA Code for variable numbers of of genes e.g. virulent property, antibiotic resistance. Ribosomes: Bacterial ribosome have coefficient of sedimentation of 70S Are composed of two subunits, 50S and 30S subunits, are associated with protein synthesis Are targets for antibiotics that inhibit protein synthesis Storage granules: Temporarily hold excess metabolites. Their presence vary with bacteria and metabolic activity. E.g.metachromatic or volutin granules, sulphur granules, poly-ß-hydroxybutyric acid, starch and glycogen.

External structures- Capsules 

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1. Capsule & slime layer: surround many bacterial cells including several pathogenic species. E.g. Haempphilus influenzae, Streptococcus pnemoniae, Klebsiella spp., Bacillus anthracis Composed of complex polysaccharides. are resistant to proteolytic enzymes, help capsulated strains evade phagocytosis and desiccation. Are useful for adherence

2. Flagella ( flagellum) 

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Are bacterial organs of motility enabling organisms to move towards nutrient, away from adverse conditions and penetrate host mucus. Are composed of flagellins (antigenic in nature) Flagella can be single (monotrichous) or multiple, located at one end (polar) or distributed over the surface of the bacterium (Peritrichous). Can exhibit phase variation. Flagellin is immunogenic and some bacteria have developed a system of switching between types of flagella (smooth S or rough, R), constant and variable regions.

Flagellar localization 

Polar or peritrichous location

E. coli

Bartonella

Vibrio

Spirillum

Bacterial spores

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Bacterial spores are metabolically inert forms triggered by adverse environmental conditions; Adapted for long term survival and it allows regrowth when conditions are favorable. Spores are resistant to heat, desiccation and many chemicals. Pathogenic bacteria that produce spores include Clostridium spp.(Cl tetani, Cl. perferenges) Bacillus species

Pili/fimbriae 

Fimbriae:

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thin projections that aid adhesion to host cells for colonisation.  E.g. Uropathogenic Escherichia coli have specialized fimbriae (P fimbriae) that bind to mannose receptors on ureteric epithelial cells  Fimbriae are also important in infections caused by Neisseria gonorrhoeae.  Not involved in motility Pili: long thin protein filaments that cover the entire surface of Gramnegative bacteria. Common Pili: play a role in bacterial adherence to mucosal surfaces. Sex (F) pili: are specifically involved in bacterial conjugation.

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Functions of Pili        

Conjugation (E. coli F pilus) DNA uptake (Neisseria) Phage receptors Adhesion Early steps in biofilm formation Twitching motility (Pseud aeruginosa) Resist phagocytosis Antigenic variation (Neisseria gonorrhea)

Transport mechanisms in bacteria 

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A microbial cell must take on nutrients from its surroundings by transporting them across the cell membrane. Three general types of transports are Passive transport and Active transport. Passive transport: Diffusion , Osmosis Atoms or molecules move in a gradient from area of high density or concentration to an area of lesser concentration.No energy is expended. Facilitated diffusion: large molecules are bound by membrane proteins and transported across without the expenditure of energy. The molecules are altered in configuration. Active transport: Substances are taken into the cell by a process that consumes energy. Substances are transported against concentration gradient or along a concentration gradient at a greater speed. Phagocytosis and pinocytosis are forms of active transport in which bulk quantities of solid and fluid are taken into the cell

Medically important groups of bacteria 

Gram-positive cocci: 

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Gram- negative cocci 

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Haemophilus, Brucella, Pasteurrella

Gram-positive bacilli 

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Neisseria meningitidis (meningitis), Neisseria gonorrhoea (gonorrhoea).

Cocco-bacilli 

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Staphylococci (catalase positive), Streptococci (catalase negative

Aerobic: Bacillus e.g bacillus anthracis Anaerobic: Clostridium spp

Gram-negative bacilli: Enterobacteria e.g. E.coli, Salmonella. Klebsiella sp. Spiral bacteria: Helicobacter, Campylobacter spp. Leptospira,Treponema Rickettsia,

Bacterial classification 

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Bacteria are classified or identified by using a series of observable characteristics. Staining: Gram reaction: Gram positive or Gram-negative Special stains e.g Ziehl Neelsen (Zn) stain for Mycobacterium tuberculosis Cell shape : cocc (spheres), bacilli(rods) or spira (spirilla, vibrios, and spirochetes) Presence, shape and position of endospores e.g. terminal, subterminal or central Atmospheric preference: aerobic – require oxygen Anaerobic organisms require little or no oxygen. Microaerophiles prefer a reduced oxygen tension; capnophiles prefer increased carbon dioxide Biochemical reactions Serology- antigen and antibody reaction Genetic relatedness- Molecular biology tests.