Microbiology (Lippincott Illustrated Reviews Series)
Microorganisms can be found in every ecosystem and in close association with every type of multicellular organism. They populate the healthy human body by the billions as benign passengers (normal flora, see p. 7) and even as participants in bodily functions. For example, bacteria play a role in the degradation of intestinal contents. In this volume, we primarily consider the role of microorganisms (that is, bacteria, fungi, protozoa, helminths, and viruses) in the initiation and spread of human diseases. Those relatively few species of microorganisms that are harmful to humans, either by production of toxic compounds or by direct infection, are characterized as pathogens.
Most infectious disease is initiated by colonization (the establishment of proliferating microorganisms on the skin or mucous membranes) as shown in Figure 1.1. The major exceptions are diseases caused by introduction of organisms directly into the bloodstream or internal organs. Microbial colonization may result in: 1) elimination of the microorganism without affecting the host; 2) infection in which the organisms multiply and cause the host to react by making an immune or other type of response or 3) a transient or prolonged carrier state. Infectious disease occurs when the organism causes tissue damage and impairment of body function.
All prokaryotic organisms are classified as bacteria, whereas eukaryotic organisms include fungi, protozoa, and helminths as well as humans. Prokaryotic organisms areÂ divided into two major groups: the eubac – teria, which include all bacteria of medical importance, and the archaebacteria, a collection of evolutionarily distinctÂ organisms. Cells of prokaryotic and eukaryotic organisms differ in several significant structural features as illustrated in Figure 1.2.
Fungi are nonphotosynthetic, generally saprophytic, eukaryotic organisms. Some fungi are filamentous and are commonly called molds, whereas others (that is, the yeasts) are unicellular (see p. 203). Fungal reproduction may be asexual, sexual, or both, and all fungi produce spores. Pathogenic fungi can cause diseases, ranging from skin infections (superficial mycoses) to serious, systemic infections (deep mycoses).
Protozoa are single-celled, nonphotosynthetic, eukaryotic organisms that come in various shapes and sizes. Many protozoa are free living, but oth- 2 1. Introduction To Microbiology
A. Typical bacteria
Most bacteria have shapes that can be described as a rod, sphere, or corkscrew. Prokarytoic cells are smaller than eukaryotic cells (Figure 1.3). Nearly all bacteria, with the exception of the mycoplasma, have a rigid cell wall surrounding the cell membrane that determines the shape of the organism. The cell wall also determines whether the bacterium is classified as gram positive or gram negative (see p. 21). External to the cell wall may be flagella, pili, and/or a capsule. Bacterial cells divide by binary fission. However, many bacteria exchange genetic information carried on plasmids (small, specialized genetic elements capable of self-replication) including the information necessary for establishment of antibiotic resistance. Bacterial structure, metabolism, and genetics as well as the wide variety of human diseases caused by bacteria are described in detail in Unit II, beginning on p. 49.
B. Atypical bacteria
Atypical bacteria include groups of organisms such as Mycoplasma, Chlamydia, and Rickettsia that, although prokaryotic, lack significant characteristic structural components or metabolic capabilities that separate them from the larger group of typical bacteria
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