14 Entry, exit and transmission Flashcards
Pathogens have specific molecules that help bind to receptor molecules on host cells. Certain cells have certain receptors, which means that they are only susceptible to certain infections
CD4 molecule T cells - HIV
C3d receptor - EBV
Alpha-dystroglycan - M leprae
How does skin protect from infection?
Physical barrier
Dead layer
Fatty acids - pH 5.5
How do conjunctiva prevent infection?
What infections affect eye?
Lacrimal secretion - lysosyme
Blinking
Chlamydia - trachoma
Gonococci
HSV
Respiratory tract inhales 10 000 microbes per day, so needs good defenses.
What are the defenses?
How to microbes evade defenses?
Mucus + ciliary action
Cough
Macrophages
Attach firmly to surface of cell
Inhibit ciliary activity - Pertussis cytotoxin, pollution/ smoke
Ciliary destruction by virus - influenza, measles
Avoid phagocytosis - TB
How do following microbes attach to respiratory tract?
Microbial adhesion molecule/ receptor name
- Influenza A
- Rhinovirus
- Influenza A - haemagluttinin - sialyloigosaccharides
- Rhinovirus - capsid protein - ICAM-1
How do following microbes attach to respiratory tract?
Microbial adhesion molecule/ receptor name
- coxsackie A (common cold/ oropharyngeal vesicles)
- parainfluenza type 1
- RSV
- coxsackie A - capsid protein - ICAM-1 or integrin
- parainfluenza type 1 - envelope protein - sialoglycolipids
- RSV - envelope protein - sialoglycolipids
How do following microbes attach to respiratory tract?
Microbial adhesion molecule/ receptor name
- mycoplasma pneumoniae
- measles virus
- mycoplasma pneumoniae - fuses with cell - neuraminic acid
- measles virus - haemagluttinin - CD46
How do following microbes attach to respiratory tract?
Microbial adhesion molecule/ receptor name
- Haemophilus influenzae, strep pneumoniae, klebsiella pneumoniae
- surface molecule - carbohydrate sequence in glycolipid
Intestinal transit time means that any ingested bacteria, are excreted in faeces. Infecting bacteria must attach to epithelium to prevent this. Some have specific attachments for small or large bowel - which is shown by symptoms they cause
Where and how do these microbes attach to GI tract
- Poliovirus
- Rota virus
- Poliovirus - intestinal epithelium. Binds to Pvr (polio virus receptor)
- Rota virus - intestinal epithelium. Viral capsid VP4 attach to host cell glycans
Where and how do these microbes attach to GI tract
- Vibrio cholera
- Escherichia coli (EPEC and EHEC)
- Vibrio cholera - intestinal epithelium. Multivalent adhesion molecule 7 (MAM7)
- Escherichia coli - intestinal epithelium. Bacteria inject Tir an effector into host membrane, acting as receptor for bacterial surface protein intimin
Where and how do these microbes attach to GI tract
- Salmonella typhi (enteric fever)
- Shigella (dysentery)
- Salmonella typhi (enteric fever). Ileal epithelium. Binds using bacterial adhesins
- Shieglla (dysentery). Colonic epithelium. Surface protein IscA acts as adhesin and interacts with host cells after activating type III secretion system trigerring its uptake into epithelial cells
Where and how do these microbes attach to GI tract
- Giardia lamblia
- Entamoeba histolytica
- Ancylostoma duodenale (hookworm)
- Giardia lamblia. Duodenal/ jejunal epithelium. Binds to mannose-6-phosphate on host cell, in addition to mechanical succker called ventral disc help bind to microvilli
- Entamoeba histolytica. Colonic epihtelium. Lectin on surface of amoeba binds to host cell
- Ancylostoma duodenale. Intestinal epithelium. Binds using large mouth (buccal capsule)
Some GI pathogens do not just attach, but attach and penetrate
Can you name examples?
Trichinella
Trichuris
Echinococcus
Bacteria motility can help them travel through protective mucus, and reach susceptible epithelium
Which bacteria have motility properties?
Vibrio cholerae
E. coli
Mucinase is an enzyme which helps break down mucus barrier.
Which bacteria produce this?
V cholerae
Bacteria generally prefer alkaline environment. But need to evade gastric acid.
Which microbes are resistant to acid?
H Pylori
Mycobacterium tuberculosis - intestinal TB
Enteroviruses - poliovirus, coxsachieviruses, echoviruses
HAV - infects liver, then shed in biliary tract into faeces
What are examples of organisms which can resist bile?
Salmonella Shigella Enteroviruses Enterococcus faecalis E. coli Proteus Pseudomonas
Pathogens which are primarily anaerobic can thrive in GI tract.
What are examples of this?
Bacteroides fragilis
H pylori has special proteins to attach to gastric epithelium. It releases large amounts of urease, which creates a small surrounding area of ammonia in which they can survive. So treatment is antibiotics, as opposed to antacids
What are consequences of H pylori infection?
Gastritis
Gastric ulcers
MALT lymphoma
What lymphoid material is present in GI tract?
Peyer’s patches are localised collections of lymphoid tissues (MALT), lying just below epithelium surface. Contain “M cells” which take up foreign proteins and deliver them to underlying immune cells
Urogenital tract is continuum, so once infected, organism can quickly spread.
What defences does vagina have?
- Has glycogen on epithelium due to oestrogen. Lactobacilli colonise vagina, and metabolise glycogen to lactic acid - making pH about 5.0. Inhibits other bacterial growth except streptococci and diptherioids
- for STI to infect they need to enter via injury to epithelium (genital wars, syphilis) or impaired defences (tampons, oestogren imbalance)
Urinary tract has regular flushing as method of protection.
Exterior infection via urethra is almost always cause of infection.
Why are females at higher risk of UTIs?
14x more common in females
Shorter urethra 5cm (20cm men)
Urethra closer to anus
Sexual intercourse increases risk. Uncircumcised penis harbors bacteria
What are natural defences of oropharnyx
Flushing action of saliva - 1L per day
IgA antibodies
Lysosyme
Normal flora
What factors influence whether pathogen transmission will be successful?
Number of pathogens shed
Pathogens stability in environment - ability to resist drying, temperature etc. Some organisms produce spores (clostridium), or cysts (amoeba)
Number of pathogens required to infect fresh host - efficiency of infection
Ability to aid transmission - e.g causing coughing/ sneezing/ diarrhoea
Once in external environemtn, organisms become dried. Those which are more stable on drying, are able to survive longer, and spread
What are examples of organisms which are more stable on drying?
Spread in air via dust/ dry droplets -
- Tubercle bacilli
- Staphylococci
Spread from soil -
- Clostridial spores
- Anthrax spores
- Histoplasma spores
Spores can survive for years in soil
