Lesson 1, Topic 1
In Progress

Characteristics of Bacteria

April 11, 2024

Learning Objective: Examine the characteristics of bacteria.
      As mentioned in a prior section, some bacteria can be helpful to the body, such as normal flora. Bacteria can also be harmful. Besides the helpful/harmful characteristics, bacteria have other characteristics. They are single-celled prokaryote organisms that reproduce asexually, by binary fission. This process of reproduction results in large numbers of bacteria being formed from a single cell. Because bacteria reproduce quickly, bacterial infections can swiftly overwhelm a person’s immune system. Some bacteria reproduce in as little as 14 minutes, whereas other bacteria may take days. For example, a single E. coli cell reproduces about every 30 minutes to produce about 3.5 trillion offspring in 24 hours.

FIGURE 35.1  (A) Gram stain red blood cells (RBCs) and gram-positive cocci. (B) RBCs with gram-negative bacilli. From De la Maza LM, Pezzlo MT, Baron EJ: Color atlas of diagnostic microbiology, St. Louis, 1997, Mosby.

      Bacteria often are classified according to their staining characteristics, shape, and the environmental conditions in which they thrive. Both shape and staining characteristics are direct results of their cell wall composition.

Bacterial Staining Properties
Learning Objective: Describe the various bacterial staining characteristics.
Three types of cell wall structures are found among pathogenic bacteria: gram-positive, gram-negative, and acid-fast structures. These labels are based on reactions to specialized stains used to see bacteria under a microscope. Bacterial cell walls are composed of peptidoglycan (PG), a molecule composed of carbohydrate and protein.
            • Gram-positive cells contain a thick layer of PG, which looks deep blue/violet when stained with Gram stain (FIGURE 35.1A).
            • Gram-negative cells contain a thin layer of PG, which looks pinkish red when stained with Gram stain (FIGURE 35.1B).
            • Acid-fast cells contain a thin layer of PG surrounded by a thick layer of waxlike lipids (fats). Acid-fast bacteria do not stain well with a Gram stain, but they stain pink with the acid-fast stain (FIGURE 35.2).

Bacterial Shapes
Learning Objective: Describe the various bacterial shapes.
Bacteria assume three different shapes:
            • Round bacteria are called cocci.
            • Rod-shaped bacteria are called bacilli.
            • Spiral-shaped bacteria are called spirilla. Tightly coiled spirilla are called spirochetes.
Certain arrangements are also seen with different bacteria. For example, when bacteria are in a chain formation, the prefix strepto- is used; when bacteria are found in pairs, the prefix diplo- is used; and when they are found in grapelike clusters, the prefix staphylo- is used. Cocci in packets of 4 are called tetrads, and in packets of 8 or 16, they are called sarcinae (FIGURE 35.3).


Bacterial Oxygen Requirements
Learning Objective: Describe the oxygen requirements of bacteria.
Bacteria are also classified according to oxygen requirements:
            • Aerobes: Bacteria that require oxygen to live. An aerobic example is Mycobacterium tuberculosis, which thrives in white blood cells in the lungs
            • Anaerobes: Bacteria that die in the presence of oxygen. An anaerobic example is Bacteroides fragilis, a gram-negative rod-shaped bacterium. B. fragilis is part of the normal flora in the colon.

FIGURE 35.2  Acid-fast stain. From De la Maza LM, Pezzlo MT, Baron EJ: Color atlas of diagnostic microbiology, St. Louis, 1997, Mosby.

            • Facultative anaerobes: Bacteria that are flexible regarding their oxygen needs; they can survive in the presence of oxygen but prefer to live without oxygen. E. coli, which is also found in the intestine, is an example of a facultative anaerobe.


Bacterial Physical Structures
Learning Objective: Describe the physical structures of bacteria.
Bacteria can be classified and identified according to additional physical structures. Some bacteria have long thin flagella that help them move. Some bacteria have a thick, jelly-like substance that surrounds the cell wall called a capsule, which can make the bacteria more pathogenic. Other bacteria form intracellular structures called endospore. An endospore allows the bacteria to remain viable when environmental conditions are poor. Clostridium tetani produces spores; if spores enter a wound and grow, they cause the disease known as tetanus.
See TABLE 35.1 for a list of some important infectious diseases caused by pathogenic bacilli, cocci, and spirilla.


35.1 Critical Thinking Application
In your own words, write a brief description of the term capsule. What advantage does a bacterium have if a capsule surrounds it? Share your thoughts with the class.

FIGURE 35.3  Typical bacterial arrangements.