Figure 6.17 (a) Flasks like this may be used to culture human or animal cells for viral culturing. (b) These plates
contain bacteriophage T4 grown on anEscherichia colilawn. Clear plaques are visible where host bacterial cells
have been lysed. Viral titers increase on the plates to the left. (credit a: modification of work by National Institutes of
Health; credit b: modification of work by American Society for Microbiology)
Animal viruses require cells within a host animal or tissue-culture cells derived from an animal. Animal virus
cultivation is important for 1) identification and diagnosis of pathogenic viruses in clinical specimens, 2) production
of vaccines, and 3) basic research studies. In vivo host sources can be a developing embryo in an embryonated bird’s
egg (e.g., chicken, turkey) or a whole animal. For example, most of the influenza vaccine manufactured for annual flu
vaccination programs is cultured in hens’ eggs.
The embryo or host animal serves as an incubator for viral replication (seeFigure 6.18). Location within the embryo
or host animal is important. Many viruses have a tissue tropism, and must therefore be introduced into a specific
site for growth. Within an embryo, target sites include the amniotic cavity, the chorioallantoic membrane, or the yolk
sac. Viral infection may damage tissue membranes, producing lesions called pox; disrupt embryonic development; or
cause the death of the embryo.
Figure 6.18 (a) The cells within chicken eggs are used to culture different types of viruses. (b) Viruses can be
replicated in various locations within the egg, including the chorioallantoic membrane, the amniotic cavity, and the
yolk sac. (credit a: modification of work by Hoang Chung)
For in vitro studies, various types of cells can be used to support the growth of viruses. A primary cell culture is
freshly prepared from animal organs or tissues. Cells are extracted from tissues by mechanical scraping or mincing to
release cells or by an enzymatic method using trypsin or collagenase to break up tissue and release single cells into
suspension. Because of anchorage-dependence requirements, primary cell cultures require a liquid culture medium
in a Petri dish or tissue-culture flask so cells have a solid surface such as glass or plastic for attachment and growth.
Primary cultures usually have a limited life span. When cells in a primary culture undergo mitosis and a sufficient
density of cells is produced, cells come in contact with other cells. When this cell-to-cell-contact occurs, mitosis is
252 Chapter 6 | Acellular Pathogens
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