Microbes That Cause Infectious Diseases
Important Features of Microbes
Eukaryotes & Prokaryotes
Practice Questions: USMLE & Course Examinations
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 prokaryote kingdom, fungi (yeasts and molds) belong to the kingdom of fungi, and protozoa are members of the kingdom of protists. Helminths (worms) are classified in the animal kingdom (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. Taken together, the helminths and the protozoa are commonly called parasites. Viruses are quite distinct from other organisms—they are not cells but can replicate only within cells.
TABLE 1–1 Biologic Relationships of Pathogenic Microorganisms
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:
TABLE 1–2 Comparison of Medically Important Organisms
(1) Structure. Cells have a nucleus or nucleoid (see below), which contains DNA; this is surrounded by cytoplasm, within which 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.
EUKARYOTES & PROKARYOTES
Cells have evolved into two fundamentally different types, eukaryotic and prokaryotic, which can be distinguished on the basis of their structure and the complexity of their organization. Fungi and protozoa are eukaryotic, whereas bacteria are prokaryotic.
(1) The eukaryotic cell has a true nucleus with multiple chromosomes surrounded by a nuclear membrane and uses a mitotic apparatus to ensure equal allocation of the chromosomes to progeny cells.
(2) The nucleoid of a prokaryotic cell consists of a single circular molecule of loosely organized DNA, lacking a nuclear membrane and mitotic apparatus (Table 1–3).
TABLE 1–3 Characteristics of Prokaryotic and Eukaryotic Cells
In addition to the different types of nuclei, the two classes of cells are distinguished by several other characteristics:
(1) Eukaryotic cells contain organelles, such as mitochondria and lysosomes, and larger (80S) ribosomes, whereas prokaryotes contain no organelles and smaller (70S) ribosomes.
(2) Most prokaryotes have a rigid external cell wall that contains peptidoglycan, a polymer of amino acids and sugars, as its unique structural component. Eukaryotes, on the other hand, do not contain peptidoglycan. Either they are bound by a flexible cell membrane, or, in the case of fungi, they have a rigid cell wall with chitin, a homopolymer of N-acetylglucosamine, typically forming the framework.
(3) The eukaryotic cell membrane contains sterols, whereas no prokaryote, except the wall-less Mycoplasma, has sterols in its membranes.
• The agents of human infectious diseases are bacteria, fungi (yeasts and molds), protozoa, helminths (worms), and viruses.
• Bacterial cells have a prokaryotic nucleus, whereas human, fungal, protozoan, and helminth cells have a eukaryotic nucleus. Viruses are not cells and do not have a nucleus.
• All cells contain both DNA and RNA, whereas viruses contain either DNA or RNA, but not both.
• Bacterial and fungal cells are surrounded by a rigid cell wall, whereas human, protozoan, and helminth cells have a flexible cell membrane.
• The bacterial cell wall contains peptidoglycan, whereas the fungal cell wall contains chitin.
Motility is another characteristic by which these organisms can be distinguished. Most protozoa and some bacteria are motile, whereas fungi and viruses are nonmotile. The protozoa are a heterogeneous group that possess three different organs of locomotion: flagella, cilia, and pseudo-pods. The motile bacteria move only by means of flagella.
Bacteria, fungi, protozoa, and helminths are named according to the binomial Linnean system, which uses genus and species, but viruses are not so named. For example, regarding the name of the well-known bacteria Escherichia coli, Escherichia is the genus and coli is the species name. Similarly, the name of the yeast Candida albicans consists of Candida as the genus and albicans as the species. But viruses typically have a single name, such as poliovirus, measles virus, or rabies virus. Some viruses have names with two words, such as herpes simplex virus, but those do not represent genus and species.
1. You’re watching a television program that is discussing viruses called bacteriophages that can kill bacteria. Your roommate says, “Wow, maybe viruses can be used to kill the bacteria that infect people! You’re taking the Microbiology course now; what’s the difference between viruses and bacteria?” Which one of the following would be the most accurate statement to make?
(A) Viruses do not have mitochondria, whereas bacteria do.
(B) Viruses do not have a nucleolus, whereas bacteria do.
(C) Viruses do not have ribosomes, whereas bacteria do.
(D) Viruses replicate by binary fission, whereas bacteria replicate by mitosis.
(E) Viruses are prokaryotic, whereas bacteria are eukaryotic.
2. Bacteria, fungi (yeasts and molds), viruses, and protozoa are important causes of human disease. Which one of the following microbes contains either DNA or RNA but not both?
3. Which one of the following contains DNA that is not surrounded by a nuclear membrane?
PRACTICE QUESTIONS: USMLE & COURSE EXAMINATIONS
Questions on the topics discussed in this chapter can be found in the Basic Bacteriology section of PART XIII: USMLE (National Board) Practice Questions starting on page 689. Also see PART XIV: USMLE (National Board) Practice Examination starting on page 731.