Prescott’s Microbiology,10
thEdition
4
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McGraw-Hill Education.
GUIDELINES FOR ANSWERING THE COMPARE, HYPOTHESIZE,
INVENT QUESTIONS
1.Many classification schemes are used to identify bacteria. These start with Gram staining, progress to
morphology and arrangement characteristics, and include a battery of metabolic tests. Build an
analogous scheme that could be used to identifyviruses. You might start by considering the host, or you
might start with viruses found in a particular environment, such as a marine filtrate.
Students determine which of the characteristics of viruses are general enough to split them into two or
three large groups, and then further distinguish them. Students might first determine if the nucleic acid
content were DNA or RNA. Then they could consider the geometry of the capsid, enveloped or not, host
cell, natural environment, etc.Note they could not propose biochemical tests since viruses are
metabolically inactive.Nor could they propose aerobic versus anaerobic, or motility.
2.The origin and evolution of viruses is controversial. Discuss whether you think viruses evolved before
the first cell or whether they have coevolved and are perhaps still coevolving with their hosts.
While there is no doubt that virusescontinue tocoevolve with their hosts, the question of viral origin is an
open question, so students are free to propose ideas.The simplicity ofnucleic acids surrounded by a self-
assembling protein capsid sounds primitive enough to have come together independent of cellular
organization, arguing viruses originated before cells. On the other hand, since viral replication requires a
cellular host for propagation, evolution of viruses independent of host cells seems improbable. This
question encourages students to consider the concept that “right and wrong” in science are not as important
as models that are “more or less” consistent with observationsand data. They may propose that viruses are
some ‘escaped genes’ from cells, or some descendant of a primitive, ancestral life form.
3.Consider the separate stages of an animal virus life cycle. Assemble a short list of structures and
processes that areunique to the virus and would make good drug targets for an antiviral agent. Explain
your rationale for each choice.
Refer to figure 6.10.Attachment-drug could block ligands on virus surface that interact with host
receptors. Penetration-for viruses whoseentire nucleocapsid enter the cell, drugs can be targeted to the
nucleocapsid to block release of nucleic acid (process called uncoating). Synthesis-for RNA viruses, block
unique enzymes used for RNA replication or production of mRNA (such as drugs targeting reverse
transcriptase in HIV). Release-block viral proteins involved in the exiting of the virus from the cell (as
some anti-influenza drugs do).The general concept students should grasp is that an critical step in the life
cycle can potentially be interfered with by a small molecule, thus making it an antiviral drug (as long as it
does not also block an essential function in the cell).
4.Chronic wasting disease (CWD) is a neurodegenerative disease of cervid animals (e.g., deer, elk,
moose) caused by prions. Unlike other prion diseases, CWD is readily transmitted from one animal to
another in a herd. How this occurs in not understood. However, evidence suggests the mode of
transmission is by animal excreta: saliva, urine, and feces. In this mechanism, a healthy animal would be
exposed to the prion present in excreta from a diseased animal. Since many cervids congregate, the prion
would easily spread from one animal to another. One problem facing scientists studying CWD is
measuring the amount of infectious prions in excreta and determining their source (i.e., whether they are
produced solely in nervous system tissue or in organs of excretion). Why is CWD of concern to fish and
wildlife biologists and game managers in states where CWD is prevalent (e.g., Colorado, Wyoming,
Wisconsin)? Why would the presence of CWD prions in tissues other than those of the central nervous
system be important to know? (Hint: Learn about the variant Creutzfeldt-Jakob epidemic that occurred in
the 1990s.)