Figure 1. Amoeba proteus. This photomicrograph shows the various structures that can be seen with the aid of Nomarski or differential interference contrast. This amoeba can be quite difficult to find in the wild and prefers well oxygenated and clean water to live in. The nucleus is about 35 µm in diameter.

Figure 2. This photomicrograph shows a food vacuole and the granulated nucleus of the Amoeba proteus. The nucleus is about 35 µm in diameter while the food vacuole measures about 20 µm across. Many old vacuoles can also be seen with most of the prey digested. The small blebs seen protruding from the main cell body are pseudopodia in the process of forming. The average size range of this organism is 750 µm.

Figures 1 and 2 are images of the common protozoan Amoeba proteus, belonging to the class Sarcodina. A. proteus is typically found in freshwater ponds where there is an abundance of decaying organic material (Lee et al., p. 161). A. proteus varies between 250 and 750 µm depending on the availability of food. It is found on the surface of materials as it prefers a solid surface on which to move. A. proteus is a predator that captures and ingests a variety of other microbes, including bacteria, protozoa, and algae.

Amoebas are a successful group of microbes that inhabit a wide variety of niches including freshwater lakes and ponds, sewage, moist soil, salt water, hot springs, chemically polluted water, animal intestines, and even the human mouth. A number of amoeba are pathogenic to humans and other animals (Lee et al., p. 167; Madigan et al., p. 727). Amoeba are notable for their ability to alter their shape rapidly; the term amoeboid means to readily change shape. Amoebas consist of an inner fluid, called the endoplasm, and an outer region, the ectoplasm, which is more rigid than the endoplasm. Many amoeba, but not all, both move and capture prey by sending out pseudopodia (false feet) in the desired direction (Fig. 1 & 2). Once the prey is detected, pseudopodia extend and engulf the prey into a food vacuole (Fig. 1 & 2). Human phagocytic immune cells resemble amoeba both morphologically and physiologically. The bacterial agent of Legionnaires' disease (Legionella pnuemophila) is able to survive and grow within certain species of freshwater amoebas because it is able to avoid being digested by the amoeba's digestive system. The fact that a comparable mechanism is also used by the L. pneumophila to avoid destruction by human phagocytic cells suggests a similarity in the two evolutionarily diverse hosts (Gao et al., 1997).

For a more complete description of the habitat where this organism was found, please see Lost Pond, Epping Forest, England.

References