MICR 2123 Lecture 20: Microbial diseases (4.11.17)

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12 Apr 2017
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Microbial diseases
Classifying pathogens
Pathogens can be classified as food-borne, airborne, blood-borne, or sexually transmitted
Understanding infectious disease requires knowledge of the organ system, the portal of entry, and
the infectious organism
Patient histories are vital in diagnosing microbial diseases (farmers, travelers, campers)
Zoonotic disease are animal diseases accidentally transmitted to humans
Skin and soft tissue infections
Bacterial, vial, and fungal causative agents
oBacterial
Staphylococcus aureus
Boils: secrete coagulase that coats the organisms with fibrin to evade
immune response and wall off infection
Toxic shock: toxic shock syndrome toxin (TSST) a superantigen (non specific
T cell activation)
MRSA: methicillin-resistant S. aureus. Major cause of nosocomial infections
(in hospitals)
Scalded skin syndrome: some strains make exfoliate toxin which is also a
superantigen (non specific T cell activation)
Streptococcus pyogenes (group A streptococci)
Strep throat and immunological sequelae such as rheumatic fever
Necrotizing fasciitis
oViral
Several viruses can produce skin rashes, although their route of infection is usually
through the respiratory tract
Chicken pox, shingles dsDNA
Rubeola (measles) ssRNA+
Rubella (german measles) ssRNA-
oCan cause blindness or deafness in utero
Respiratory tract infections
Successful lung pathogens possess attachment mechanisms and countermeasures to avoid various
lung defenses
oBordatella pertussis
Cause of whooping cough
Adheres to tracheal epithelium and produces toxin that destroys lung cilia
oMycobacterium tuberculosis
Cause of tuberculosis
An acid-fast rod (actinomycete)
Forms calcified tubercles in lung
1/3 of world population infected
Top 10 cause of death
oPneumonia
Disease, not an infection
Caused by many different microbes
Streptococcus pneumoniae is primary pathogen
Has capsule that prevents phagocytosis
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Can invade the bloodstream (bacteremia) and the covering of the brain
(meningitis)
Gastrointestinal tract infections
Most gastroenteritis is caused by viruses
oRotavirus (dsRNA reovirus)
Highly infectious (#1 cause of infant diarrhea)
Can cause lethal dehydration (#1 infant mortality)
Antibiotics ineffective on viruses (if used,  secondary infections)
Now an effective vaccine against rotavirus
oBacterial infections
Enterobacterial toxin-producing strains
Inject virulence factors via Type III secretion
Bacteria that invade epithelial mucosal cells
oSalmonella, Shigella, enteroinvasive E. coli (EIEC)
Shigella can produce Shiga toxin  hemorrhagic colitis and
hemolytic uremic syndrome (HUS)
Blocks host protein synthesis, damages endothelia 
capillary damage, loss of blood, clots
Antibiotic treatment is indicated
Bacteria remaining outside epithelial cells
oE.coli: EHEC (O157:H7), ETEC
Enterohemorrhagic – toxigenic
Antibiotic treatment is contraindicaeed
Diarrhea does not occur until 12-24 hours after ingestion since the infectious
agent must first grow in the victim
Other bacterial agents of disease
Campylobacter jejuni
oMost frequent bacterial cause of diarrhea
Vibrio cholera
oCholera (cholera toxin)
Staphylococcus aureus
oFood poisoning
oSecreted toxin is the cause. After ingestion, the toxin travels to the
intestine where it enters the bloodstream
oSymptoms occur quickly after ingestion (w/I 2-6 hours)
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