300
U N I T 4
Infection and Immunity
the Epstein-Barr virus, hepatitis B virus, and human
papillomavirus (see Chapter 7).
Bacteria
Bacteria are autonomously replicating unicellular
organisms known as
prokaryotes
because they lack an
organized nucleus. Compared with nucleated eukary-
otic cells (see Chapter 1), the bacterial cell is small and
structurally primitive (Fig. 14-3). Similar to eukaryotic
cells but unlike viruses, bacteria contain both DNA and
RNA, although their genome is considerably smaller
than eukaryotes and typically is encoded on a single
chromosome. Many bacteria transiently harbor smaller
extrachromosomal pieces of circular DNA called
plas-
mids.
Occasionally, plasmids contain genetic informa-
tion that increases the virulence or antibiotic resistance
of the organism.
The prokaryotic cell is organized into an internal
compartment called the
cytoplasm
, which contains
the reproductive and metabolic machinery of the cell.
The cytoplasm is surrounded by a flexible lipid mem-
brane, called the
cytoplasmic membrane.
This in turn
is enclosed within a rigid cell wall. The structure and
synthesis of the cell wall determine the microscopic
shape of the bacterium (e.g., spherical [cocci], helical
[spirilla], or elongate [bacilli], Fig. 14-4). Most bacteria
produce a cell wall composed of a distinctive polymer
known as
peptidoglycan.
This polymer is produced only
by prokaryotes and is therefore an attractive target for
antibacterial therapy. For example, the penicillin antibi-
otics target the peptidoglycan cell wall. Several bacteria
synthesize an extracellular capsule composed of protein
or carbohydrate. The capsule protects the organism
from environmental hazards such as the immunologic
defenses of the host.
Certain bacteria are motile as the result of external
whip-like appendages called
flagella.
The flagella rotate
like a propeller, transporting the organism through a
liquid environment. Bacteria can also produce hairlike
structures projecting from the cell surface called
pili
or
fimbriae
, which enable the organism to adhere to sur-
faces such as mucous membranes or other bacteria.
Reproduction.
Most prokaryotes reproduce asexu-
ally by simple cellular division. The manner in which
an organism divides can influence the microscopic mor-
phology. For instance, when the cocci divide in chains,
they are called
streptococci;
in pairs,
diplococci
; and in
clusters,
staphylococci
(Fig. 14-5). The growth rate of
bacteria varies significantly among different species and
depends greatly on physical growth conditions and the
availability of nutrients. In the laboratory, a single bac-
terium placed in a suitable growth environment, such as
an agar plate, reproduces to the extent that it forms a
Malignancy
Absorption
Animal cell
Penetration
Nucleus
Cytoplasm
Uncoating
Viral replication
Viral maturation
Protein coat
synthesis
Host cell lysis
Latency
Oncogenesis
Budding
release of
enveloped
viruses
FIGURE 14-2.
Schematic representation of the
many possible consequences of viral infection of
host cells, including cell lysis (poliovirus), continuous
release of budding viral particles, or latency
(herpesviruses) and oncogenesis (papovaviruses).
FIGURE 14-3.
Electron micrograph of the rod-shaped, gram-
positive bacterium Listeria monocytogenes showing the
simple prokaryotic cell structure including the cytoplasm,
the cytoplasmic membrane, and the rigid cell wall. (From the
Centers for Disease Control and Prevention Public Health Images
Library. No. 10828. Courtesy of Balasubr Swaminathan, Peggy
Hayes.)
