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Chapter 1: Bacteria Compared With Other Microorganisms

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 helminths (worms) are classified in the Eukarya domain (Table 1–1). Protists and fungi are distinguished from animals and plants by being either unicellular 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 have a 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.

Table 1–1Biologic Relationships of Pathogenic Microorganisms
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Table 1–1 Biologic Relationships of Pathogenic Microorganisms
Domain (Superkingdom) Kingdom Pathogenic Microorganisms Type of Cells
Eukarya Animal Helminths (Worms) Eukaryotic
Eukarya Protist Protozoa Eukaryotic
Eukarya Fungi Fungi (Yeasts and Molds) Eukaryotic
Bacteria Prokaryote Bacteria Prokaryotic
Viruses Noncellular

IMPORTANT FEATURES OF MICROBES

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. This distinction is based primarily on three criteria:

  1. Structure. Cells have a nucleus or nucleoid (see below), which contains DNA; this is surrounded by cytoplasm, where proteins are synthesized and energy is generated. 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 their nucleic acid and protein, and then reassemble into multiple progeny viruses. Furthermore, 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 acid. Cells contain both DNA and RNA, whereas viruses contain either DNA or RNA, but not both.

Table 1–2Comparison of Medically Important Organisms
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Table 1–2 Comparison of Medically Important Organisms
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Characteristic Viruses Bacteria Fungi Parasites
Cell wall No Yes Yes +/–1
Approximate diameter (µm)2 0.02–0.2 1–5 3–10 (yeasts) 15–25 (trophozoites)
Nucleic acid Either DNA or RNA Both DNA and RNA Both DNA and RNA Both DNA and RNA
Type of nucleus None No distinct nuclear compartment Membrane-bound nucleus Membrane-bound nucleus
Ribosomes Absent 70S 80S 80S
Mitochondria Absent Absent

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