INTERACTIONS BETWEEN MICROORGANISMS and HOST Chapter 19, pp
459-484
Terminology: Table 19-1, p460
Pathogenicity = ability to cause a disease
Virulence = level or degree of pathogenicity (e.g. LD50 or ID50 =
infectious dose)
Attenuation = reduction or loss of virulence
Virulence factors = factors that contribute to virulence or
pathogenicity
Infection = invasion or colonization by pathogens
Disease = when infection results in a change of health - abnormal
state
Bacteremia = presence of bacteria in blood
Septicemia = infectious agents or their products in the blood
Toxemia = toxins in blood
Viremia = viruses in blood
(review Koch’s postulates p 464; also normal flora Fig 19-1,
p461)
Symbiosis
1) mutualistic = both benefit
2) commensalistic = one benefits (part of normal flora ?)
3) parasites = one benefits, the other is harmed - pathogens
opportunistic pathogens = mutualistic or commensalistic organisms
that become
pathogens under certain conditions
I. Virulence - virulence factors
A) Adherence - adhesins
1) Streptococcus mutans adhere to teeth with via its
glycocalyx (dextran)
2) E. coli - pathogenic strains have adhesins on pili to
attach to small intestine
3) Neisseria gonorrhoeae also has pili containing
adhesins to attach to urogenital tract
4) Streptococcus pyogenes adheres to epithelial cells of
throat by “M” protein =
component of cell wall
B) escaping internal defenses
1) capsules resist phagocytosis e.g. Streptococcus
pneumoniae, Klebsiella pneumoniae,
Hemophilus influenzae, Yersinia pestis (Fig 19-10, p 471)
2) M protein also resists phagocytosis (Streptococcus
pyogenes )
3) lipids in cell wall of Mycobacterium resist destruction by
phagocytes = Mycobacterium
can multiply in phagocytes
4) leukocidins destroy neutrophils (leukocytes)
5) hemolysins lyse RBC e.g. beta hemolytic streptococci
6) coagulases coagulate or clot blood - used by Staphylococcus
to isolate it from host defenses
7) streptokinase produced by Streptococcus breaks down
clots (fibrin) (we make use of it to dissolve blood clots in
heart attacks)
C) toxins = exotoxins and endotoxins (comparison Table 19-3, p
477)
1) exotoxins = heat labile proteins secreted by cell Table 19-2,
p 473; inactivated for
vaccines = toxoids (exotoxins include enterotoxins = act on small
intestine = diarrhea)
a) diphtheria toxin (Corynebacterium diphtheriae ) =
encoded for by prophage;
inhibits protein synthesis in eucaryotic cells
b) erythrogenic toxins (Streptococcus pyogenes ) =
cytotoxins that damage capillaries under skin = rash (scarlet
fever)
c) botulinum toxins (Clostridium botulinum ) released
when clostridium cell dies;
a very potent neurotoxin producing paralysis with lack of muscle
tone
(flaccid paralysis)
d) tetanus toxin (Clostridium tetani ) = neurotoxin
resulting in contraction of
skeletal muscles (i.e. lockjaw)
e) Vibrio enterotoxin (Vibrio cholerae ) = severe
diarrhea, fluid/electrolyte loss
f) Staphylococcal enterotoxin (Staphylococcus aureus ) =
severe diarrhea
2) endotoxins = lipid A portion of LPS, released upon lysis of
bacterial cell; heat stable,
no toxoids; produces fever (Fig 19-14, p 476)
II. Outcome of exposure to infectious agents:
(depends on amount (numbers) exposed to; your resistance;
virulence of the pathogens)
A) Amount exposed to - the more exposed to, the worse the outcome
B) Resistance - the better your resistance and health = less
problems
1) genetic factors
2) overall health (sleep, diet, exercise, stress)
3) presence of other diseases/conditions
4) age
C) Virulence of pathogens - the more virulent the pathogen = more
likely to result in disease or death (ID50 vs. LD50)