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

INTRODUCTION TO PATHOGENS AND THE HUMAN MICROBIOTA
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capsid containing
viral chromosome
(nucleocapsid)
capsid
protein
viral
genome
(DNA or RNA)
transmembrane
viral envelope
proteins
nucleocapsid induces
assembly of
envelope proteins
BUDDING
Figure 23–12 Acquisition of a viral
envelope. (A) Electron micrograph of
an animal cell from which six copies of
an enveloped virus (Semliki forest virus)
are budding. (B) Schematic drawing of
the envelope assembly and budding
processes. The lipid bilayer that surrounds
the viral capsid is derived directly from
the plasma membrane of the host cell. In
contrast, the proteins in this lipid bilayer
(shown in green) are encoded by the viral
genome. (A, courtesy of M. Olsen and
G. Griffith.)
lipid bilayer
progeny
virus
(A)
100 nm
(B)
eliminated smallpox infection from the planet, and the eradication of poliomyelitis
is approaching completion (Figure 23–13).
Summary
Infectious diseases are caused by pathogens, which include viruses, bacteria, and
fungi, as well as protozoan and metazoan parasites. All pathogens must have
mechanisms for entering their host and for evading immediate destruction by the
host. The great majority of bacteria are not pathogenic to humans. Those that are
pathogenic produce specific virulence MBoC6 m24.15/23.12
factors that mediate the bacteria’s interactions
with the host; these proteins change the behavior of host cells in ways that
promote the replication and spread of the bacteria. Eukaryotic pathogens such as
fungi and protozoan parasites typically pass through several different forms during
the course of infection; the ability to switch among these forms is usually required
for these pathogens to survive in a host and cause disease. In some cases, such as
malaria, parasites must pass sequentially through several host species to complete
their life cycles. Unlike bacteria and eukaryotic parasites, viruses have no metabolism
of their own and no intrinsic ability to produce the proteins encoded by their
DNA or RNA genomes; they rely on subverting the machinery of the host cell.
reported cases of polio per
100,000 population
40
30
20
10
inactivated
vaccine
oral
vaccine
0
1940 1950 1960 1970 1980 1990
Figure 23–13 Effective control of a viral disease through vaccination. The graph shows the
number of cases of poliomyelitis reported per year in the United States. The arrows indicate the
timing of the introduction of the Salk vaccine (inactivated virus given by injection) and the Sabin
vaccine (live attenuated virus given orally).