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Micro - Pathogenesis of bacterial infection (i) (Pathogenesis…
Micro - Pathogenesis of bacterial infection (i)
Pathogenesis
pathogenicity = ability to produce disease in a host
mechanisms involved in disease production
portal of entry
faecal-oral (ingestion)
gastroenteritis
campylobacter
salmonella
salmonella typhi: typhoid fever
vibrio cholera: cholera
viruses: hep A+E
inhalation
upper: meningococcus
lower: TB, pneumonia
penetration/inoculation
break in skin (C tetani, S aureus)
needle stick infection = rarely bacterial, usually blood-borne viruses (HepB, HIV)
insect/tick bite
borrelia burgdorferi: lyme disease
plasmodium: malaria
sexual
bacteria
chlamydia trachomatis
N gonnorrhoeae
treponema pallidum
look for chancre
painless ulcer
people rarely present with it
on males' outer genitals, on females' cervix/upper vagina
goes away as disease goes dormant
then patients presents with secondary/tertiary syphilis
symptom of primary syphilis
viruses
HPV
herpes
hep B
HIV
increased risk by having a genital lesion
transplacental/vertical
congenital syphilis
listeria monocytogenes
toxoplasmosis (parasitic)
rubella
VZV
sometimes perinatally (during birth)
gonorrhoea/chlamydia in birth canal - ophthalmia neonatorum (neonatal conjunctivitis)
attach to host cells
defeat/evade immune system
damage host cells
transmission (leave + infect other cells)
i.e. dissemination (spread)
through tissue/tissue planes
haematogenous
lymphatic
within macrophages
ascending/descending in a tract
factors affecting infection occurance
virulence
size of inoculum
portal of entry
host state (immunocompetancy, underlying conditions)
pathogen survival in host
virulence
ability to invade + cause disease
weak virulence - rarely causes disease
avirulent - no ability to invade/cause disease
virulence factors
genetic/biochem/structural features of a pathogen that enable it/enhance its ability to cause disease
help invasion, cause pathology, evade host responses
capsule
evades immune system - antiphagocytic
around pneumococcus, N meningitides, H influenza B
aids attachment
flagellae
pili
attach to complementary structures on host cells (adherence)
P pilus on E coli present in strains causing pyelonephritis
fimbrae
short pili
endo/exo toxins
other extracellular products (e.g. enzymes)
invasins
proteins that bind to host cell Rs + facilitate invasion
adhesions
on fimbriae/pili of N gonorrhoeae + B pertussis
bind to specific host cell epithelium Rs
colonisation of high-virulence organisms can occur within normal gut flora - doesn't always cause disease - asymptomatic carrier
MRSA
high virulence
prevalent hospital pathogen
colonises in nose
spreads to hands + hence is easily transmissible
extremely virulent stains (e.g. community-acquired MRSA in US) can cause primary skin + lung infection (uncommon in Ire)
host-pathogen interaction
dynamic
outcome depends on...
pathogen virulence
host susceptibility (effectiveness of its defences)
Normal flora
skin
upper resp tract
intestine (e.g. E Coli in colon)
GUT
rarely cause infection, unless...
they move from their usual location (e.g. E coli causes UTI if it gets into GUT)
host is immunocompromised (e.g. on high dose steroids)
a barrier is breached (S epidermidis infection via IV line breaching skin)
Opportunistic bacteria
low intrinsic virulence but can cause infections when immunocompromised/foreign body/anatomic lesion is present
e.g. endogenous normal flora