The Web of Life (41-3) Social Amoebae

Social Amoebae

In good times, social amoeba live as single-celled organisms (myxamoebae), feasting on soil bacteria. When food runs short, tens of thousands band together to form a slug-like multicellular cluster, which then slithers away in search of a more bountiful patch of dirt.

During slug time, some of these social amoebae become specialized cells, roving around and vacuuming up invading toxins and unwelcome bacteria; in effect, an immune system.

The cells recognize foreign bacteria using a protein called toll/interleukin-1 receptor A (TirA). TirA is closely related to the protein that animals use in their immune systems to identify bacteria, indicating an evolutionary connection between amoeba and later, larger multicellular life.

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Dictyostelium discoideum (Dicty) is an ordinary soil amoeba that feeds on bacteria. Dicty form biofilms which enhance survival prospects for its members. A Dicty biofilm is commonly, and indelicately, called a slime mold.

Dicty life begins as a spore, released from a mature sorocarp (fruiting body). When conditions are favorable – warm and moist – individual cells (myxamoebae) hatch from their spores.

Myxamoebae are attracted to their prey by the smell of the folic acid which the bacteria secrete as a waste product. Once fat and happy, myxamoebae multiply by mitosis (cell division).