Entry, Exit and Transmission Flashcards
In order to cause an infection, what is the very first thing a pathogen needs to do?
Pathogens must attach to, or penetrate, the host’s body surfaces
What are the host’s body surfaces?
The skin
The mucosal linings
The cleansing mechanisms (e.g., tears)
How do pathogens attach to the host’s body?
Receptor molecules
Example: CD4 and CCR5 (HIV); C3d (EBV)
What is the last thing a pathogen will do (after causing infection)?
The pathogen must exit the body via either shedding in secretions and excretions or by blood uptake
What are different sites of entry?
The skin The conjunctiva The respiratory tract The GI tract The urogenital tract The oropharynx
Describe the skin’s defences
It has a pH of 5.5
It secretes fatty acids
There are sebaceous gland secretions
There is the presence and the secreted compounds of the natural flora
The skin peels (acts as a barrier and protects particularly from viral infection)
What can lower the skin’s defences?
Wounds, abrasions, burns create a portal of entry
What are the conjunctiva’s defences?
Continuous flushing of tears (which contain lysozyme, IgA, lactoferrin) and the wiper action of the eyelids
What happens if there is damage to the conjunctiva or the eyelids?
Non-specialized microbes can cause an infection. The source if often contaminated fingers, towels, etc.
What are the defences of the respiratory tract?
Inhales dust, particles, smoke and microbes are entrapped in mucous, which is carried up to the throat by a ciliary escalator and swallowed
Also, alveolar macrophages are strategically placed where there might be attachment
How do some organisms avoid cleansing?
They attach via adhesins to specific cell-surface receptors on epithelial cells
They inhibit ciliary action (e.g., Bordetella pertussis produces a toxin paralysis the cilia, which allows it to attach)
How do some organisms avoid destruction by alveolar macrophages?
Mycobacterium tuberculosis can survive inside macrophages
What are the defences of the oropharynx?
Flushing action of saliva (1 L/day)
Chewing and other activities of the tongue, lips and cheeks
Secretory IgA
Lysozyme
Antimicrobial activities of leukocytes in saliva and at the mucosal surfaces
Microflora
How do microbes invade the oropharynx?
Attachment to mucosa or tooth surfaces
Decreased resistance to infection (e.g., patients with vitamin C deficiencies are more likely to develop gum infections; patients taking antibiotics are more likely to develop trush from Candida)
What are the defences of the GI tract?
Peristalsis There are no particular cleansing mechanisms, except vomiting and diarrhea Mucous (mechanical barrier, contains IgA Acid Proteolytic enzymes Bile
How do bacteria cause infection in the GI tract?
They must attach to the intestinal epithelium
How does Vibrio cholerae and rotaviruses cause infection in the GI tract?
They establish specific binding to receptors.
How do bacteria that cause infection in the GI tract counteract mucous, acids, enzymes and bile?
Motile organisms can propel through mucous (V.cholerae, E.coli)
Production of mucinase (V.cholerae)
H. pylori produces a urease
How does H.pylori cause infections?
It produces an enzyme called urease, which produces ammonia. It uses urease to create this neutral pH bubble around itself and then it degrades the mucous layer of the stomach. The epithelial cells of the stomach are exposed to the stomach acid and they are degraded, creating an ulcer
What happens if you eradicate H.pylori from the body?
There are different strains of H.pylori (virulent and avirulent). If you eradicate H.pylori (particularly in children), you are more susceptible to developing asthma. So it’s important to only kill the virulent strain
