Microbial pathology

Disease

condition in which the normal structure or function of the body is impaired

infectious disease

damage due to infection by disease causing micro-organisms

infection

successful invasion of the body by a pathogenic microorganism, which leads to disease

pathogenicity

the ability of the pathogen to inflict damage on the host

horizontal transmission

the spread of communicable disease from one individual to another

contagiousness

the ease by which the disease is spread, dictated by the mode of transmission

respiratory transmission

spread via aerosols (droplets containing the virus expelled by infected individuals)

inhaled particles are carried to the pharynx and swallowed when trapped in mucus

may be destroyed by alveolar macrophages

-mycobacterium tuberculosis, streptococcus pneumonia

defences in the lungs

-mucus blanket

-alveolar macrophages

-ciliary cells lining nasal cavity and lower respiratory tract

conjunctival transmission

Ebola, chlamydia trachomatis, staphylococcus

oral-faecal transmission

infection via the GIT, can migrate to CNS liver and spleen

-shigella, salmonella, vibrio cholera

skin transmission

pathogens may enter through minor abrasions or burns

introduced via arthropod vector bite

-Yellow fever (mosquito) , Lyme disease

mechanical transmission

injection via contaminated medical equipment (Iatrogenic infections)

sexually transmitted disease examples

HIV-1/2, neisseria gonorrhoea

Urine transmission

larissa fever- spread via rat urine

zoonosis

Transfer of disease from animal to human

nosocomial infection

hospital acquired infection via contaminated equipment or poor hygiene

infect immunocompromised/ wounded patients

highly antibiotic resistant

vertical transmission

the transfer of a virus from mother to child

-measles, HIV-1, foetal rubella (congenital cataracts, born paralysed)

non-communicable disease

not spread from person to person

-tetanus

legionnaires disease caused by legionella pneumophilia

legionella pneumophilia

maintains symbiotic relationship with amoeba in water sources (eg in cooling systems)

acute infections

sudden, rapid onset of disease

may be killed by innate immune response or kill host

-influenza, scarlet fever

Sublinical Infection

no detectible sign of disease

-HIV-1; syphilis; typhoid

persistant/chronic infections

not cleared by the host but remains active in host immune system

HCV (Hep C) which can lead to liver cancer

leprosy; HIV

latent infection

dormant/inactive viral infection which may become productive under appropriate conditions

activated by stress response

shingles (from dormant chicken pox virus)

HSV1,2, HIV 1, syphilis

slowly progressive disease

irreversible damage to the host over time

-chronic

-HIV-1, syphilis, Lyme disease

primary pathogen

Cause disease regardless of the health status of the host

-vibrio cholera

opportunistic pathogen

cause disease in a compromised host- barriers, immune function, flora lacking

-pseudomonas aeruginosa

antibiotic effect on flora

antibiotics decrease the intrinsic flora of an individual.

therefore it is no longer able to compete with and overcome opportunistic bacteria

virulence

the degree of pathogenicity ie how effective an organism is at causing disease

LD50

No of pathogenic cells/virions / amount of toxin to kill 50% of infected animals

highly virulent cells have little difference in number needed to kill 100%

ID50

No of pathogen cells/virions required to cause active infection in 50% of inoculated animal

virulence attenuation

decrease or loss of virulence

-occurs when pathogens are kept in a lab culture, virulence decreases/ lost completely

-attenuated strains used for vaccine creation

attenuation in vaccine production

1 . isolate virus from diseased patient

2. multiple passages in rhesus macaques

3. multiple passages in mouse embryos

4. multiple passages in chicken embryos

5. final preparation of attenuated vaccine in fertilized chicken eggs

6. administration to elicit immune response w/out symptoms

4 steps pathogens cause disease

1. contact

2. adhesion

3. invasion

4. infection

virulence factors for attachment

-adhesins

-lipoteichoic acid (gram +)

-outer membrane proteins and vesicles

-exopolysaccharides; capsule/slime layer

virulence factors for invasion

-flagella

-enzymes (hyaluronidase, acid phosphatase )

virulence factors for survival

-capsule

-flagella

-heat shock proteins

-exotoxins

-metabolic end products

Microbial Adherence

-enhanced ability of microbes to attach to host tissues

-require dedicated receptors

-adhesions-glyo/lipoproteins enable binding to host cell

viral adhesion

attach to receptor molecules- protein, carbs, lipids

low affinity receptors

eg heparon sulfate

binding to receptor allows virus to overcome electrostatic forces

S.aureus adhesion factors

fibrinogen using N2/3 domains of sdrG adhesion protein

neisseria adhesion factors

Opa adhesion proteins that attach to CEACAM (carcinoembryonic antigen related molecules)

capsule

thick hydrated polysaccharide-based/peptide coating outside plasma membrane and cell wall

-sticky, containing receptors to facilitate attachment; phagocytosis by immune cells difficult

eg s. pneumoniae encapsulated strain

pili function

conjucation; genetic transfer of plasmids contributing to antibiotic resistance

flagella

facilitate adherence to host cells

Fimbriae

attachment to cells or ECM

Colonization

growth following access to host tissues; biofilm formation

Biofilm

hydrogel made of polysaccharides, proteins, lipids, DNA.

eg P. aeruginosa form biofilms on hospital equipment, damaged tissue surfaces

eg dental plaque

invasion

dissemination of a pathogen throughout local tissues or the body

-exoenzymes

H pylori invasion

invades lining of stomach; releases urease which neutralizes stomach acid. Mucin liquefies and bacterium invades epithelium

-leads to infection and rupture of stomach lining

hyaluronidase

breaks down host tissue by hyaluron hydrolysis (ECM constituent)

-inc tissue permeability

ECM degrading enzymes

Collagenase, Chondroitin sulfatase

Coagulase

forms clots

streptokinase

breaks down clots

Local infection

confined to a small area of the body, typically near the portal of entry. S. aureus

focal infection

localised pathogen/ toxins produced spread to secondary location

viral spread in host

can disseminate in lymphatic system

-or spread through blood and localise in organs

papilloma virus

replicate in epithilia and spread via neighbouring cells

HCV

Exotoxins

produced and excreted by living organisms

A-B toxins

cytolytic

superantigen

A-B toxin

A component- active component targeting cell

B component- binding component facilitating cell surface binding and entry of A component; responsible for cellular specificity to toxin and facilitates initial receptor binding.

A-B taken up in vacuole where dec in pH causes A-B to dissociate so A can interfere with cell

Diptheria toxin

A-B toxin of corynebacterium diptheriae.

A subunit catalyses ADP-ribosylation of EF-2

cessation of protein synthesis and death

-Toxin has enzymatic activity

Clostridum botulinum

Neurological exotoxin

A-B

blocks Ach release so no muscle contraction occurs

-flaccid paralysis, irreversible relaxation

- leads to respiratory failure

Clostridium tetani (tetanus)

A-B toxin

tetanospasmin blocks glycine

excess ach- muscles cannot relax

causing spastic paralysis

usually occurs in interneuron of spinal cord

-lock jaw, and paralysis spreading throughout body

cytolytic exotoxins

degrade cytoplasmic membrane integrity, causing cell lysis and death

-create pores in the membrane increase permeability

hemolysin

toxins that lyse red blood cells by targeting phospholipid membrane

staphylococcal alpha-toxins

cytolysins from acinetobacter species target immune cells, macrophages

staphylococcal alpha toxins

pore forming cytotoxin from s.aureus

-7 identical subunits oligomerise in cytoplasmic membrane to form a pore

Superantigen toxins

trace amounts may cause shock and death

overstimulate and dysregulate immune system

s.aureus/ s.pyogenes

powerful t-lymphocyte mitogen (stimulate cell proliferation)

How does superantigen work?

a conventional antigen presenting cell can only bind T cell which can bind to a specific receptor

a superantigen bypasses this process allowing for binding of the antigen presenting cell to any T cell- therefore a much larger portion of T cells are activated

- this releases a large amount of cytokines leading to shock and eventually death

Endotoxins

Lipopolysaccharide layer on gram -ve bacteria

endotoxin released when cell dies/ when cell wall disintegrates during binary fission

causes excessive inflammatory response, a drop in BP and organ failure

Lipid A- toxic properties

Limulus amebocyte lysate (LAL) assay

used to test for endotoxins; blood of horseshoe crabs contains amebocytes; amebocytes lyse in the presence of endotoxin, producing a clot

Siderophores

iron transporters, high affinity for iron

produced by gram -/+ ve bacteria

in host iron is complexed to storage proteins, or bound to heme, ferritin etc

siderophores scavenge for iron; direct toxicity to mitochondria

Siderophore trojan horse strategy

chemically engineered to contain linker, linking siderophore to antibiotic

enzymes cleave linker, releasing antibiotic into bacterial cell

prophage

A phage genome that has been inserted into a specific site on the bacterial chromosome.

eg diptheria and cholera

- necrotising faciitis (strep prophage)

integrated toxins and enzymes that are involved in tissue degradation

pathogenicity islands

refers to gene clusters responsible for virulence

acquired by horizontal gene transfer

biosafety level 1

low risk of causing disease in healthy individuals

biosafety level 2

moderate hazard but high disease risk, no respiratory transmission

eg s.aureus

biosafety hood

biosafety level 3

Indigenous or exotic agents with potential for aerosol transmission; disease may have serious or lethal consequences

mycobacterium tuberculosis

biosafety level 4

Dangerous or exotic agents that cause great risk for disease,

Lab completely isolated from external environment

enriched media

contains specific growth factors (NAD and haemin), to enhance the growth of fastidious pathogens

eg choc agar

selective media

isolates specific pathogens

-eg rogosa agar selects for acid tolerant bacteria

differential media

isolate bacteria based on growth characteristics

eg. mcconkey agar

an example of both differential and selective

eosin-methylene blue

selects for gram + bacteria

turns green/black when pH drops

immunoassays

independant of growth

used when pathogen can't be isolated/are not culturable eg viruses

monoclonal antibodies

a collection of identical antibodies that interact with a single antigen epitopes

-produced by isolating single clones of B cell fused with cancer cells to make immortalized cell lines

-very specific affinity, fingerprint for pathogen

-used to target cancer cells

-highly specific blood and tissue typing

fluorescent antibody method (indirect and direct)

direct method: antibody targeted against surface antigen bonded to dye

indirect method: non fluorescent antibody detected by secondary fluorescent antibody

ELISA test

enzyme-linked immunosorbent assay

Direct ELISA

Detects antigens

Sample containing antigens is mixed with antibody

Enzyme-linked antibodies react with the antigen

Detected by adding a substrate for the linked enzyme; a color is produced eg peroxidase

amount of colour proportional to amt antibody and amt pathogen

Competitive ELISA

Immobilized primary antibody, antigen competes with labeled antigen

viral concentration inversely proportional to the signal

sandwich ELISA steps

antigen in sample adheres to immobilized antibody and is detected by a 2nd antibody via the indirect/direct method

Lateral Flow Immunoassay

sample applied to absorbet support

move via capillary action over immobilized antibodies

the accumulation of antibody sandwiches at test line shows a visible positive line (colour change due to gold nanoparticles/latex beads)

control lines- antibody binding labelled antibody only

Western Immunoblot

-electrophoresis of proteins followed by transfer to membrane
-secondary antibody conjugated to enzyme added to bind to antigen-antibody complex

-enzyme substrate added to reveal only antibody labelled proteins

nucleic acid hybridization

Base pairing between a gene and a complementary sequence on another nucleic acid molecule.

uses unique nucleic acid probes detected by fluorescence imaging

respiratory pathogens

pathogens transmitted via aerosols

most survive poorly in air, so transmitted over short distances only

streptococcus pyogenes

Group A (GAS) found in low nos in upper respiratory tract of healthy individuals

gram +

mild infections - pharyngitis/impetigo

severe infections- scarlet fever, rheumatic fever, septicaemia, toxic shock syndrome

scarlet fever

erythrogenic toxin produced when phage T12 infects bacterium

-rash, high fever, strawberry tongue

septicaemia

blood stream infection which can lead to toxic shock

rheumatic fever

autoimmune response following poorly treated strep A infection

affects heart, joints, skin, brain

2-4 weeks after strep throat infection

mimicry of bacterium causes immune system to attack heart valves causing damage and scarring

strep A tissue infections

cellulitis, subcutaneous skin infections, necrotising faciitis

diptheria

severe upper respiratory tract infection, typically children

corynebacterium diptheriae

bacteriophage B produces exotoxin causing pseudomembrane in patients throat restricting airflow and swallowing

Bordetella pertussis

whooping cough

recurrent, violent cough may lead to pulmonary hypertension, encelophalopathy

TCT peptidoglycan fragment kills epithelial cells and prevemts mucus removal from URT

elevated white blood cells nos lead to pulmonary hypertension due to blockage of lung arteries

hypoxia + brain damage in infants

granuloma

infected macrophages aggregate with epitheloid , T and B cells and fibroblasts

this prevents dissemination, lung tissue access and protects them from immune response

cause cavities that can be viewed via x ray

bacteria can become dormant and may be reactivated

TB

vector borne pathogens

Pathogens that rely on a living host organism to be transmitted

eg mosquitos, ticks