Bacteria Differences

Sizes and Shapes

• Bacteria come in different sizes and shapes. The smallest bacteria are not visible to the naked eye and have diameters of about 100 nanometers whereas some of the largest can be seen without any magnification. Bacteria have three approximate shapes: The spherical coccus, the rod-like bacillus and the curved spirals. The cocci and bacilli exist singly or in colonies linked together in pairs, chains or clusters. The spirals tend to be the largest bacteria because they can reach extensive lengths. A few bacteria have unusual shapes such as squares, stars or blobs.

Wall Composition

• Bacteria have cell walls made of peptidoglycan, a protective substance that gives bacteria their shape. Some bacteria have more peptidoglycan in their cell walls than others, absorbing more of an identifying dye called the gram stain; these are the gram-positive bacteria. Gram-negative bacteria, which tinge pink because their cell walls take up less of the stain, are usually the organisms that are more pathogenic, or cause disease. Antibiotics such as penicillin stop bacteria from making peptidoglycan, so gram-positive bacteria die because they cannot grow or divide without their protective cell walls. Antibiotics rarely affect gram-negative bacteria.

  In addition to the peptidoglycan layer, the cell walls of gram-negative bacteria contain another layer made of lipopolysaccharides, complexes of fatty substances and sugar molecules. These bacteria shed the toxic LPS within human bodies, initiating fevers, damaging blood vessels and dangerously lowering blood pressure.

Motility

• Only certain bacteria can move, and this motility confers a distinct advantage for defense or nutrient procurement. Bacteria can move in response to chemicals detected -- chemotaxis -- or to earth's magnetic field -- magnetotaxis. The flagellum, a whip-like structure, spins in a counterclockwise manner to propel the bacterium forward in a straight line; when the flagellum rotates in a clockwise direction, the organism flips around awkwardly. The bacterium resumes its straight motion once the flagellum turns counterclockwise again. A bacterium can have a single flagellum, multiple flagella at both ends of the cell or flagella distributed all over its surface. The energy molecule adenosine triphosphate, or ATP, fuels the movement of flagella.

Source of Nutrition

• Like plants, some bacteria use energy from the sun, in combination with carbon dioxide and water, to make sugar for energy needs. Other bacteria can make their own food as well, but using chemicals such as ammonia or sulfur compounds instead. The last category of bacteria, the heterotrophs, must consume other organisms for nutrition, and includes the saprophytes, symbionts and many pathogenic varieties. Saprophytes feed on and decompose dead organisms. Symbiotic bacteria, such as the bacteria that change nitrogen into a usable form for plants, form mutually beneficial, intimate relationships with their hosts. Pathogenic heterotrophs are parasites that seize nutrients from their hosts while harming them.

Formation of Endospores

• A number of the more pathogenic bacteria such as anthrax and the organisms that cause food poisoning produce endospores in response to adverse environmental conditions. The bacteria wrap a nearly impenetrable coat made of proteins around themselves. Within the spore coat, the once-liquid cytoplasm -- the jelly-like substance distributed throughout the cell -- loses much of its water, and a protective structure forms around the bacterial DNA. These hardy structures have no observable activity; however, once conditions become favorable again, the spore coats break down, and the bacteria emerge to resume their usual activities. Endospores can survive unfavorable conditions such as high temperatures, lack of water, radiation and treatment with acids. Some bacteria have survived for millions of years in this inactive state.