Eubacteria, also known as the true bacteria, have a bad reputation. They are seen as disease causing agents. Every day new products come out adverstising their ability to destroy these microscopic but dangerous creatures. In reality, only a small percentage of these unicellular organisms cause disease. The rest fullfill many important roles in the natural world. Eubacteria can be photoautotrophs, saprophytes, or symbionts.
The Eubacteria are an ancient and diverse group. Different species have evolved to fit in every type of environment and lifestyle. They are often classified by their oxygen requirements and by the type of nutrition in which they engage.
A great many of the most familiar eubacteria are heterotrophs, meaning they must take food in from outside sources. Of the heterotrophs, the majority are saprophytes, which consume dead material, or parasites, which live on or within another organism at the host's expense.
In addition to the heterotrophs, there are many kinds of autotrophic bacteria, able to produce their own food. These autotrophs may be photosynthetic or chemosynthetic and may or may not use oxygen in their synthetic pathways. Cyanobacteria are the largest group of photosynthetic eubacteria. The cells of these bacteria are often much larger than other bacteria, which in the past led this group to be classified as algae rather than bacteria. In fact, cyanobacteria are still sometimes referred to as blue-green algae. These eubacteria possess pigment molecules, including chlorophyll a, the same type of chlorophyll found in higher plants. Unlike plants, in cyanobacteria the pigments are not contained within membrane-bound chloroplasts.
Respiration of eubacteria may be aerobic or anaerobic. The anaerobes undergo a form of respiration called fermentation. Among anaerobes, some can live in the presence or absence of oxygen. These are called facultative anaerobes. Some are indifferent to the presence of oxygen, but others have two respiratory pathways, one that uses oxygen and one that does not. The other group of anaerobes, the obligate anaerobes, are actually poisoned in the presence of oxygen.
In addition to respiratory and nutritional habits, one other important feature used to classify bacteria is Gram staining. Gram's stain will highlight peptidoglycan if it appears in a cell wall. Not all groups of eubacteria have peptidoglycan, so all eubacteria may be classified as either Gram-positive (able to bind Gram's stain) or Gram-negative (unable to bind Gram's stain).