Maintaining the reservoir
Reservoir: the habitat in which the agent normally lives, grows and multiplies, can include animals (cows: E.coli), humans (Typhoid Mary for salmonella Typhi), environment(soil: clostridium tetani)
may or may not be the source from which an agent is transferred to a host and show the effects of illness
Typhoid Mary
Mary Mallon
Healthy carrier of Salmonella Typhi
Worked as a cook
Known to have infected 53 ppl
Refused to stop working as a cook
was eventually quarantined
Transport of bacterial pathogens to the host
two methods : indirect and direct
direct :
- skin-skin, kissing, sexual contact
- droplet spreads : sneezing, coughing, talking
indirect:
- airborne
- vehicule borne (food, water)
- vector borne (mosquitoes, fleas, ticks…)
Adherence / colonization / invasion
adherence :
- mediated by special moelcules or strcutures called adhesin
colonization :
- establishment of a site of microbial reproduction on or within host
4 types of Adhesin
non-fimbrial adhesin (embedded in the bacterial membrane)
Fimbriae ( hairy surface, thin filaments protruding from the cell surface)
Type IV pili (rope-like structure made of many threads interwined)
curli (coiled surface structure intertwined, not rope-like, curved/curled)
Adhesin
they attach to specific structures on host cells
usually proteins, glycoproteins, or glycolipids
presence or absence of the receptor can determine the host susceptibility
Invasion of the bacterial pathogen
active penetration into host cells
active penetration between host cells
passive (not related to the pathogen itself) : skinlesions, insect bites…
can use existing host pathways of internalization
Multiply or Complete life cycles on or in host
appropriate environment is found, access to nutrients, pH, temperature, redox potential, protection from host attacks, can evolve to survive in different environment
Intracellular/extracellular pathogens
Evasion of host defenses by bacteria
must have mechanisms to evade/ resist the host defences such as a complement systems, phagocytosis, and specific immune responses.
Bacterial Capsules
Chemicals not recognised as foreign
slippery and difficult to phagocytose
Leaving the Host
Occurs in microbe is to be perpetuated
leaves by passive mechanisms(feces, urine, droplets)
symptoms of disease in the process (sneezing, coughing, runny nose, diarrhea)
E. coli
can get you sick, (killer germ)
is part of the microbiome,
can be used for research
there are different strains, different properties
Virulence factors confer an increased ability to cause disease
improves :
1- colonization of invasion of the host
2- ability to multiply and complete life cycles
3- ability to evade host defenses
4- ability to leave host and enter new one
examples of types of virulence factors
adhesins
nutrient acquisitions systems
capsule
virulence-associated secretions system s
toxins (endo/exotoxins)
factor 1 : adhesin
receptor decoys used as anti-virulence drugs.
factor 2: Nutrient Acquisition systems
Bacterial siderophores scavenge iron, which is in limiting quantities in the host
Some are specifically associated with pathogenes
factor 3: capsule
outisde cell wall
pollysaccharides, can be other
can protect from phagocytosis by host cells
e.g. Streptococcus pneumoniae
- capsules -> kills mouse
- no capsule -> no death
Factors 4: Virulence-associated secretion
systems
several different types
T3SS: a tiny ‘’ molecular syringe’’
encoded by 20 genes
present in Gram-negative pathogens
‘‘syringe’’ genes: similar between different species
injected proteins: vary between the different bacteria
cellular effect : varies between the different bacteria
Factor 5: Toxins
Endotoxins : part of bacterial cell
Exotoxins: secreted
LPS of gram-negative bacteria
endotoxin
structural component
released when bacteria lyses, and multiplication
Lipid A is toxic at high dose
heat stable
fever, shock, etc..
Increased production of host cell proteins
which produce severe host-mediated
inflammatory response
Botulinum Toxin
exotoxin
most poisonus naturally occuring substance in the world
threats for biological warfare
Cleaves a protein involved in neurotransmission from motor neurons to muscle
characteritics of exotoxins
- Usually easily inactivated (eg by heat)
- inactivated toxin can be used to elicit immune response
- Toxin-specific antiserum can be used to treat disease
- Antitoxin= antibody used to neutralize a specific toxin
Intoxications
diseases that result from entry of a specific
pre-formed toxin into host
Does not require entry of bacteria into host
or bacterial replication in host
Usually very fast onset: e.g. 30 min- 6 h
Bacillus cereus
- Spore forming bacteria found in the soil and several foods
- Produces a toxin that causes vomiting
- Contamination and improper storage of food-> growth
- Ingestion of toxin pre-formed in food-> vomiting
- Onset within 1–5 hours of ingestion
- Recovery usually within 6–24 hours
