Microbial World

Organisms Infectious agents

I Domain B^S^Ta Rua® I Viruses Viroids Prions

Prokaryotes Eukaryotes (unicellular)

Algae Protozoa Fungi Helminths

(unicellular or (unicellular) (unicellular or (multicellular multicellular) multicellular) parasites)

multicellular) parasites)


Figure 1.12 The Microbial World Although adult helminths are generally not microscopic, some stages in the life cycles of many disease-causing helminths are.

The distinguishing features of the nonliving members of the microbial world are given in table 1.4. The relationships of the major groups of the microbial world to one another are presented in figure 1.12.

1.5 Size in the Microbial World 13

Table 1.4 Distinguishing Characteristics of Viruses, Viroids, and Prions




Obligate intracellular agents

Consist of either DNA or RNA, surrounded by a protein coat

Obligate intracellular agents Consist only of RNA; no protein coat

Obligate intracellular agents Consist only of protein; no DNA or RNA

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