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(a) Coccus

(d) Vibrio
Figure 3.20 Typical Shapes of Common Bacteria (a) Coccus; (b) rod; (c) coccobacillus; (d) vibrio; (e) spirillum; (f) spirochete. Electron micrographs.

often called a coccobacillus. A short curved rod is called a vibrio (plural: vibrios), whereas a curved rod long enough to form spirals is called a spirillum (plural: spirilla). A long helical cell with a flexible cell wall and a unique mechanism of motility is a spirochete. Bacteria that characteristically vary in their shape are called pleomorphic (pleo meaning many and morphic referring to shape).

Perhaps the greatest diversity in cell shapes is found in aquatic environments, where maximizing their surface area helps microbes absorb dilute nutrients (figure 3.21). Some aquatic bacteria have extensions on their surface, prosthecae. In some cases these give the organisms a starlike appearance. Square, tilelike cells have been found in the salty pools of the Sinai Peninsula in Egypt. ■ prosthecate bacteria, p. 286

Groupings

Most prokaryotes divide by binary fission, a process in which one cell divides into two cells. Cells adhering to one another following division form a characteristic arrangement that depends on the planes in which the organisms divide. This is seen especially in the cocci because they may divide in more than one plane (figure 3.22). Cells that divide in one plane may form chains of varying length. Cocci that typically occur in pairs are routinely called diplococci. An important clue in the identification of Neisseria gonorrhoeae is its characteristic diplococcus arrangement. Some cocci form long chains; this

Figure 3.21 Diverse Shapes of Aquatic Bacteria (a) Square, tilelike bacterial cell. (b) Ancalomicrobium, an example of a prosthecate bacterium. Note that the cytoplasmic membrane and the cytoplasm are a part of each arm.

Figure 3.21 Diverse Shapes of Aquatic Bacteria (a) Square, tilelike bacterial cell. (b) Ancalomicrobium, an example of a prosthecate bacterium. Note that the cytoplasmic membrane and the cytoplasm are a part of each arm.

Nester-Anderson-Roberts: I. Life and Death of Microbiology, A Human Microorganisms Perspective, Fourth Edition

3. Microscopy and Cell Structure

© The McGraw-H Companies, 2003

characteristic is typical of some, but not all, members of the genus Streptococcus.

Cocci that divide in two or three planes perpendicular to one another form cubical packets. Members of the genus Sarcina form such packets. Cocci that divide in several planes at random may form clusters. Species of Staphylococcus typically form characteristic grapelike clusters.

The various groupings are sometimes described with a Latin word. For example, chains of cocci may be called streptococci, cubical packets may be called sarcinae, and clusters may be called staphylococci. Unfortunately, these same words have also been assigned as genus names. Many microbiologists intentionally avoid using these words as descriptive terms because they are so easily confused with the genus names Streptococcus, Sarcina, and Staphylococcus.

Multicellular Associations

Some types of bacteria typically live as multicellular associations. For example, members of a group of bacteria called myxobac-teria glide over moist surfaces together, forming a swarm of cells that moves as a pack. These cells release extracellular enzymes, which enables the pack to degrade organic material,

3.3 Morphology of Prokaryotic Cells 51

including other bacterial cells. When water or nutrients are depleted, cells aggregate to form a structure called a fruiting body, which is visible to the naked eye (see figure 11.18). Within this structure, cells of myxobacteria form a dormant resting stage. ■ myxobacteria, p. 284

Many types of bacteria live on surfaces in associations called biofilms. Cells within these aggregates alter their activities when a critical number of cells are present. They sense the density of cells within their population by a mechanism called quorum sensing. ■ biofilms, p. 104 ■ quorum sensing, p. 186

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