I table 2-1

FIG. 2-5 A, Micrograph of cells infected with measles viruses. B, A child with a typical measles rash.

(TMV) infects the cells of that plant. We easily recog­nize the red rash of measles when the measles virus replicates within certain human cells (FIG. 2-5).

Viruses, viroids, and prions are nonliving acellular microorganisms that can only replicate within living cells.

Viruses

A virus is made up of two essential parts: a central genetic nucleic acid molecule and a protein coat called a capsid (see FIG. 1-5). The capsid surrounds and protects the viral nucleic acid. The capsid gives the virus a characteristic shape. It also helps establish the specificity of the virus for a particular host cell. Some viruses also have an external viral envelope made up of plasma membrane that they acquire from host cells within which they replicate. Unlike prokaryotic and eukaryotic cells, viruses contain only one type of nucleic acid—RNA or DNA—as their hereditary molecule. Some viruses are RNA viruses and others are DNA viruses.

Viruses are composed of a central genetic core— DNA or RNA—surrounded by a protein coat called a capsid.

Viruses are not capable of carrying out metabolism independent of a host cell. They replicate only in spe­cific host cells. For example, viruses that replicate within a bacterial cell do not replicate within the cells of other organisms. This specificity, as well as other characteristics of viruses, is the basis for believing that they evolved from host cells rather than begin­ning as primitive or independent entities.

Interactions between the viral capsid and the outer layer of the host cell determine whether the viral nu­cleic acid will be able to enter the host cell. Viral repli­cation within host cells causes numerous plant and animal diseases, including, in humans, the common cold, influenza, AIDS, herpes, measles, mumps, chickenpox, yellow fever, hepatitis, and many more.

There are three groups of viruses. Animal viruses infect and replicate within animal cells, plant viruses within plant cells, and bacteriophage or phage with­in bacterial cells. Viruses that infect bacterial cells cannot infect and replicate within cells of other or­ganisms. Often the specificity is even greater than this. A virus that infects a tomato plant generally can­not infect other plants. This results in greater speci­ficity of viral diseases. A phage that infects the bac­terium Escherichia coli often cannot infect other bacte­ria or even other subtypes of E. coli. It is possible to utilize this specificity to identify bacteria, a proce­dure called phage typing.

Viroids

Unlike viruses, viroids are composed exclusively of RNA that contains their genetic information. They have no structures surrounding their genetic mole­cules. Inside a suitable host cell the RNA of a vi-roid is capable of initiating its own replication. This sometimes manifests as disease symptoms in the Host organism. It was discovered in the early 1970s that viroids cause potato spindle tuber disease, chry­santhemum stunt, citrus exocortis and a few other plant diseases. Thus far they appear to affect only plants.

Viroids are infectious RNA molecules found in plants.

Prions

The most recently discovered and least understood microorganisms are the prions (for proteinaceous in­fectious particles). What is so unusual about prions is that they seem to be composed only of protein. Like viroids, these "organisms" have no structures. They are only individual protein molecules that contain the information that codes for their replication when they infect a suitable host cell. They are properly called infectious proteins.

SURVEY OF MICROORGANISMS 33

Unlike cellular and even other acellular organisms, prions do not store their genetic (hereditary) material in nucleic acid molecules. This presents a problem in understanding how prions replicate. Prions do not fit into our current understanding of how genetic infor­mation in nucleic acid molecules is replicated and how it can determine the specific structural and func­tional characteristics of each organism. Scientists do not know how a protein can direct its replication, and thus they do not understand how prions replicate.

Prions are infectious proteins.

Prions can cause diseases of the nervous system. They were discovered during the search for the cause of scrapie, an infectious and usually fatal disease of sheep. This disease was known to be caused by an agent that could pass through a filter that could trap bacteria and larger organisms. Therefore it was be­lieved to be caused by a virus. No virus could be found, and eventually scrapie was shown to be caused by an infectious protein, that is, a prion. Pri­ons have been found to cause some exotic human diseases such as kuru, a disease restricted to certain tribes of New Guinea that practice cannibalism. Some scientists have also hypothesized that prions may cause various degenerative nervous disorders, including Alzheimer's disease, that up until now have had no known cause.

Prokaryotes

Microorganisms with prokaryotic cells include the archaebacteria and the eubacteria (Table 2-2). The prokaryotic cell is structurally not as complex as the eukaryotic cell. However, don't be misled by this

statement. All living systems are extremely complex. Eubacteria and archaebacteria, like all other living or­ganisms, must meet their energy and material needs through their metabolic activities, replicate their hereditary (genetic) information, and reproduce. Failure to accomplish any or all of these tasks results in death.

The distinction between archaebacteria and eubac­teria is relatively new. Before 1980 all organisms with prokaryotic cells were called bacteria. However, as we discussed earlier, beginning in the late 1970s, sci­entists discovered that there were two distinct lines of evolutionary descent among the prokaryotes. The distinctions between these two groups will be con­sidered later. One group was given the name archae­bacteria, implying that these were early evolutionary forms. The archaebacteria have subsequently been called by some the Archaea or archaeobacteria. The sec­ond group was given the name eubacteria. Most of­ten these prokaryotes are simply called bacteria.

Naming Bacteria

Like all other living organisms, each bacterial species is given a formal name that distinguishes it from all other organisms (Table 2-3). The formal names of bacteria and all other organisms are given in Latin. Latin is used because it was the classical language of science at the time when formal names were first given to organisms on a systematic basis. When typed or handwritten, species names are underlined. In print, species names are italicized, for example, the name Streptococcus pneumoniae. (This bacterial species sometimes causes pneumonia in humans.)

The formal name of a species is a binomial name, indicating that it has two parts. The use of binomial