Domain Bacteria

Most known prokaryotes are bacteria. They occur in many shapes and sizes and have distinct biochemical and genetic characteristics. Most bacteria have one of three basic shapes, as shown in Figure 23-3. Rod-shaped bacteria are called bacilli (buh-SIL-lE). Sphere-shaped bacteria are called cocci (KAHK-SIE), and spiral-shaped bacteria are called spirilla (spie-RIL-uh). When cocci occur in chains, they are called streptococci (STREP-tuh-KAHK-siE). Grapelike clusters of cocci are called staphylococci (STAF-uh-loh-KAHK-siE).

Gram Stain

Biologists group most species of bacteria into two categories based on the structure of their cell walls as shown by a laboratory technique called the Gram stain. Figure 23-4a shows that Gramnegative bacteria have cell walls that are complex and have relatively small amounts of peptidoglycan. Gram-negative bacteria take up the second, red dye of the Gram stain process which makes the cells appear reddish pink under a microscope. Figure 23-4b shows that the walls of Gram-positive bacteria are simpler and have more peptidoglycan. Gram-positive bacteria retain the purple dye in their cell walls and appear purple.

figure 23-3

The most common bacterial shapes are (a) bacillus, (b) coccus, and (c) spirillum.

figure 23-4

After the Gram-staining procedure, a Gram-negative bacterium (a) appears pink because the cells become counterstained by the safranin red stain after being decolorized by an alcohol/acetone wash. Gram-positive cells (b) retain the crystal violet stain and appear purple.

(a) GRAM-NEGATIVE

(b) GRAM-POSITIVE

(a) GRAM-NEGATIVE

(b) GRAM-POSITIVE

Cell wall Cell membrane

Cell Cell wall membrane

Cell wall Cell membrane

Cell Cell wall membrane

Careers in BIOLOGY

Microbiologist

Job Description Microbiologists are scientists who study microorganisms—organisms that are too small to be seen by the naked eye. Microbiologists interact with health workers and agencies to reduce the frequency and spread of diseases caused by microorganisms. They also work in agriculture, education, research, wastewater management, pharmaceuticals, biotechnology, and other industries.

Focus on a Microbiologist

James Rogers, Ph.D., majored in microbiology. "Learning to look closely is key to any microbiologist's success. Keen observation may lead you to a new discovery," says Rogers. Rogers and his colleagues at the U.S. Army's Chemical

and Biological Command are helping the United States prepare against bioterrorism. They develop methods to test environmental samples for the presence of deadly viruses and bacteria such as the anthrax bacterium (Bacillus anthracis). After new methods are devised, Rogers conducts additional research to make the tests run faster and more efficiently. "We are helping the United States with what we are developing. We might be saving lives one day in the future."

Education and Skills

• High school—three years of science courses and four years of math courses

• College—bachelor of science in biology, including course work in chemistry, physics, microbiology, bacteriology, and virology; a master's (M.S.) or doctoral degree (Ph.D.) is needed for research • Skills—good communication skills, good observation skills, curiosity, ability to use fine tools, and patience

For more about careers, visit ■bm go.hrw.com and type in the ^^ keyword HM6 Careers.

figure 23-5

Bacteria provide essential nutrients to many organisms. Members of the genus Rhizobium live symbiotically within these soybean root nodules. They fix nitrogen from the air into ammonia, a form of nitrogen that both bacteria and plants can use to make molecules such as amino acids, proteins, and nucleic acids.

figure 23-5

Bacteria provide essential nutrients to many organisms. Members of the genus Rhizobium live symbiotically within these soybean root nodules. They fix nitrogen from the air into ammonia, a form of nitrogen that both bacteria and plants can use to make molecules such as amino acids, proteins, and nucleic acids.

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