Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1272 Chapter 23: Pathogens and Infection
yeast-form cells
in macrophage
(A)
MOLD FORM
in the environment
(B)
YEAST FORM
in the host
(C)
10 µm
Histoplasma capsulatum, for example, grows as a mold at low temperature in the
soil, but it switches to a yeast form when inhaled into the lung, where it can cause
the disease histoplasmosis (Figure 23–8).
Protozoan parasites are single-celled eukaryotes with more elaborate life
cycles than fungi, and they frequently require more than one host. Malaria is the
most devastating protozoal disease, infecting more than 200 million people every
year and killing upward of 500,000. It is caused by four species of Plasmodium,
which are transmitted to humans by the bite of the female Anopheles mosquito.
release of
sporozoites
and migration
to salivary gland
mosquito
sucks blood
and injects
sporozoites
MBoC6 m24.09/23.08 merozoites
replication
in liver
Figure 23–8 Dimorphism in the
pathogenic fungus Histoplasma
capsulatum. (A) At low temperature in the
soil, H. capsulatum grows as a multicellular
filamentous mold consisting of many
individual cells connected together. (B) After
it is inhaled into the lung of a mammal, the
increase in temperature causes a switch
to a yeast form consisting of small clumps
of round cells. (C) A stained histologic
section of a mouse lung infected with
H. capsulatum, showing a macrophage
containing yeast forms of the pathogen.
(A and B, courtesy of Sinem Beyhan and
Anita Sil; C, courtesy of Davina Hocking
Murray and Anita Sil.)
invasion of
gut and growth
infection of red
blood cells
schizont
ring
stage
ookinete
replication
zygote
MOSQUITO
HOST
HUMAN
HOST
fertilization
gametocytes
differentiate
into gametes
mosquito
sucks blood
and picks up
gametocytes
gametocytes
(A)
(B)
5 µm
(C)
(D)
10 µm 10 µm
Figure 23–9 The complex life cycle of
malaria parasites. (A) The sexual cycle of
Plasmodium falciparum requires passage
between a human host and an insect host
(Movie 23.3). (B)–(D) Blood smears from
people with malaria, showing three different
forms of the parasite that appear in red
blood cells: (B) ring stage; (C) schizont; and
(D) gametocyte. (B–D, courtesy of the
Centers for Disease Control, Division of
Parasitic Diseases, DPDx.)