The simplest wholly contained life systems are bacteria, or prokary-otes, which are the most diverse group of microorganisms. As mentioned, they are among the most common microorganisms in water. They are primitive, unicellular (single-celled) organisms that possess no well-defined nucleus and present a variety of shapes and nutritional needs. Bacteria contain about 85% water and 15% ash or mineral matter. The ash is largely composed of sulfur, potassium, sodium, calcium, and chlorides, with small amounts of iron, silicon, and magnesium. Bacteria reproduce by binary fission.

Note: Binary fission occurs when one organism splits or divides into two or more new organisms.

Bacteria were once considered to be the smallest living organisms, but today it is known that smaller forms of matter exhibit many of the characteristics of life. They range in size from 0.5 to 2 pm in diameter and about 1 to 10 pm long.

Note: A micron (pm) is a metric unit of measurement equal to one-thousandth of a millimeter. To visualize the size of bacteria, consider that about 1000 bacteria lying side-by-side would reach across the head of a straight pin.

Bacteria are categorized into three general groups based on their physical form or shape (although almost every variation has been found; see Table 8.2). The simplest form is the sphere. Spherical-shaped bacteria are called cocci ("berries"). They are not necessarily perfectly round but may be somewhat elongated, flattened on one side, or oval. Rod-shaped bacteria are called bacilli. Spiral-shaped bacteria (spirilla) have one or more twists and are never straight (see Figure 8.4). Such formations are usually characteristic of a particular genus or species. Within these three groups are many different arrangements. Some exist as single cells; others as pairs, as packets of four or eight, as chains, or as clumps.


Technical Name












Bacillus typhosis

Curved or spiral



Spirillum cholera

Cocci (spherical shapes)

Singles (random arrangement)

00 Pairs (Diplococcus)


Cubical packets of 8 (Sarcina)





Grape-like clusters (Staphylococcus)

Grape-like clusters (Staphylococcus)

Bacilli (cylindrical or rod-shaped)


Side-by side (palisading)

One-half spiral turn


Loosely wound spiral

Loosely wound spiral

Side-by side (palisading)

Tightly wound spiral

Tightly wound spiral

Figure 8.4 Bacterial shapes and arrangements.

Most bacteria require organic food to survive and multiply. Plant and animal material that gets into the water provides the food source for bacteria. Bacteria convert the food to energy and use the energy to make new cells. Some bacteria can use inorganics (e.g., minerals such as iron) as an energy source and exist and multiply even when organics (pollution) are not available.

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