Reservoirs and vectorsReservoirs

Animal, soil, water etc - source of infection.

Vectors

Arthropods, especially fleas, ticks, and mosquitoes

Mechanical vectors

Insects carry pathogens on feet Biological vectors

Pathogen replicates in vector

Microbes: Portals of entry

1) Skin:Most microbes are not able to penetrate intact skinSome fungi infect skin

2) Mucous membranesRespiratory tract: eg. Influenza virusGastrointerstinal tract: eg. CholeraUrogenital tract: HIVEye (conjunctiva): eg. adenovirus

Microbes: Portals of entry

3) Parenteral (Blood-borne) route: Microbe is deposited below the skin into the tissue Eg. Blood transfusion, insect bite, tatooing (eg. HIV)

4) Vertical transmission: Transmission from mother to child (eg. HIV)

What are bacteria ?

Bacterial Structure

Chromosome: DNA material of the cell

Cell wall: Composed of peptidoglycan the cell wall maintains the overall shape of a bacterial cell

Cell membrane: Lining the inside of the cell wall it provides a boundary for the contents of the cell and a barrier to substances entering and leaving.

Cytoplasm: Describes the inside of the cell and the contents

Bacterial ShapesThere are 3 different shapes of bacteria

Spirals(Campylobacter)

Rods(Lactobacillus)

Balls or cocci(Staphylococcus)

Bacteria with flagella are motile

Gram stain

Diff. in cell wall

1884: Hans Christian Gram; he developed this method to better visualize bacteria

All bacteria may be classified as Gram positive and Gram negative

Classification of bacteria

• Shape

• Motility

• Gram stain

• Other property (eg.aerobic /ananerobic)

Mechanisms of bacterial pathogenicity

Portals of entryPenetration or evasion of host defenses

Damageto host cells

Portal of exit

Number of invading microbes

Adherence

Virulence factors

Microbial mechanism of pathogenicity

(1) Adherence

• Adherence (attachment) is often an essential step in bacterial pathogenesis or infection, required for colonizing a new host – Adhesion - Process by which microorganisms attach themselves to

cells.

• Requires the participation of two factors: a receptor and an ligand– Microbial adherence to a eukaryotic cell or tissue surface involves

complementary chemical interactions between the host cell or tissue surface and the bacterial surface.

(1) Adherence

• Adhesions/ligands bind to receptors on host cells– Fimbriae/ pili Escherichia

coli Neisseria gonorrhoeae– M protein

Streptococcuspyogenes– Glycocalyx

Streptococcus mutans

Adherence examples

M protein: Group A Streptococcus Pili /fimbriae: E.Coli / N. gonorrhoeae

Why is adherence important ?

• Ability to colonize and cause disease

• Ability to adhere determines the host specificity

• Potential drug target

(2)Formation of bacterial biofilms

Quorum sensing: ability to sense population density and alter gene expression

Examples of biofilms

• Dental plaques

Biofilms outside the body

Biofilm mats on stagnant water Biofilm mats on rocks – Yellowstone national park

What triggers biofilm formation ?

• Bacterial attachment to surfaces

• Nutritional depletion

• Sub-inhibitory concentrations of antibiotics

• A critical population density of bacteria

How do biofilms help bacteria ?

• Protection from immune repsonse

• Protection from antibiotics

• Help tide over periods of low nutrition

• Microbial chatter (communication)

Biofilms: Why do they matter ?

• Key mechanism in bacterial pathogenesis

• Important part of food chain

• Major cause of corrosion of metal pipes

(3) Bacterial capsule

• Some bacteria have a polysaccharide layer outside the cellwall called capsule

• Helps in attachment

• Helps evade host defences Streptococcus pneumoniae Haemophilus influenzae Bacillus anthracis

How does the capsule help bacteria evade host defences ?

Prevent complement bindingand phagocytosis

Escape TLR recognitionAllows survival inside phagocytosis

Capsulated bacteria

Capsule inhibits phagocytosis

Capsule allows survival inside phagocytes

Capsule may help resist digestion by lysosomal emzymes

Capsules allow escape of TLR recognition

(TLR)

No immune response

Capsules contribute to pathogenesis

(4) Bacterial cell wall

• Outer layer

• Peptidoglycan (sugars and aminoacids)

• The cell wall may carry some surface proteins

Mycolic acid in cellwall helps resists lysosomal digestion

Mycolic acid in the cell wall resists digestion by lysosomal enzymes

Mycolic acid

in the cell wall

Eg. Mycobacterium tuberculosis , Mycobacterium leprae

Proteins on the cell wall help evade phagocytosis

Phagocytosis Immune responseX Disease

X PhagocytosisX Immune response Disease

(5) Bacterial enzymes

(a) Coagulase and kinase(b) Hyaluronidase and collagenase(c) IgA protease

(a) Coagulase and kinase

Inhibits phagocytosis Immune response – no access Antibiotics not effective

Eg. Staphylococcus aureus – skin infections

Strains that do not produce coagulase are not pathogenic

(b) Hyaluronidase and collagenase

Digestion of connective tissues

and Invasion of tissues

Hyaluronidase: is present in Staphylococcus aureus (Skin infections)and Streptococcus pyogenes (Sore throat)

Collagenase: is present in Clostridium perfringens (gas gangrene)

(c) IgA protease

• An enzyme that can degrade IgA antibodies

• Eg. Haemophilus influenza – causes respiratory tract infection / meningitis

• Help negotiate mucosal defenses.

(6) Membrane ruffling

Invasins : proteins expressed on the cell surface of various pathogens that alter actin filaments of host cell cytoskeleton, allowing microbes to enter cells.