MICROBES THAT CAUSE INFECTIOUS DISEASES

The agents of human infectious diseases belong to five major

groups of organisms: bacteria, fungi, protozoa, helminths, and

viruses. Bacteria belong to the Bacteria domain) whereas fungi

(yeasts and molds), protozoa, and helmiñnths (worms) are clas

sified in the Eukarya domanh. (Table 1-1). Protists and fungi are

distinguished from animals and plants by being either unicel-

lular or relatively simple multicellular organisms. In contrast,

helminths are complex multicellular organisms.Viruses, are

quite distinct from other organisms. They are noncellular; that

is, they do not havea nucleus and cytoplasm, cannot make their

own energy, and are unable to synthesize proteins. They are

completely reliant upon host cells for replication and are thus

considered obligate intracellular pathogens.

Many of the essential characteristics of these organisms are

described in Table 1-2. One salient feature is that bacteria,

fungi, protozoa, and helminths are cellular, whereas viruses are

not.

(1)Structure Cells have a nucleus or nucleoid (see below), which contains DNA; this is surrounded by cyto-

plasm, where proteins are synthesized and energy is gener-

ated. Viruses have an inner core of genetic material (either

DNA or RNA) but no cytoplasm, and so they depend on

host cells to provide the machinery for protein synthesis and

energy generation.

(2) Method of replication Cells replicate either by binary

fission or by mitosis, during which one parent cell divides to

make two progeny cells while retaining its cellular structure.

Prokaryotic cells/(e.g., bacteria) replicate by binary fission,

whereas eukaryotic cells replicate by mitosis. In contrast, viruses

disassemble, produce many copies of theír nucleic acid and pro-

tein, and then reassemble into multiple progeny viruses. Fur-

thermore, viruses must replicate within host cells because, as

mentioned previously, they lack protein-synthesizing and

energy-generating systems. With the exception of rickettsiae

and chlamydiae, which also require living host cells for growth,

bacteria can replicate extracellularly.

(3) Nature of the nucleic actd.Cells contain both DNA and

RNA, whereas viruses contain either DNA or RNA, but not

both.