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.
Diversity of Eubacteria
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).
A unique group of eubacteria that bears mentioning is the mycoplasmas. Classified as Gram-positive based on their relatedness to other Gram-positives, because mycoplasmas lack a cell wall they are functionally gram-negative. Mycoplasmas are both the smallest eubacteria and the smallest organisms capable of independent reproduction. They are barely larger than some viruses. Mycoplasmas have an extremely simple cell structure, a small genome, and are therefore of special evolutionary interest.
As we just saw, eubacteria are extremely diverse and specialized to their environments. Surprisingly, the structure of most eubacterial cells is relatively simple.
Many eubacteria have cell walls that lie outside of their plasma membranes. These are similar to the cell walls found in plants and fungi, but are composed of peptidoglycan rather than cellulose or chitin. In some eubacteria, this cell wall is covered by another layer called the outer membrane. Many eubacteria have yet another coating layer called a capsule. It is composed mostly of complex sugars and serves to protect the cell against environmental dangers, such as attack by host immune defenses or dehydration.
Many eubacteria are motile. In most cases, rotating structures called flagella enable them to move. The term flagella is also used to refer to similar motility structures in protists and other eukaryotic cells, but the two are not the same and should not be confused. Prokaryotic flagella are composed of protein subunits called flagellin, while eukaryotic flagella are made of arrays of microtubules made of tubulin. Prokaryotic flagella are anchored in the plasma membrane and move in a spiral motion. Eukaryotic flagella are enclosed by the plasma membrane and can only move by beating back and forth. Exceptions to this structure of prokaryotic flagella are found in some species of spirochetes, whose flagella resemble those of eukaryotes. It is believed that eukaryotes may have developed flagella through symbiotic relationships with these spirochetes.
Eubacteria are often classified by their shape. They fall into three main shape categories. Spherical eubacteria are called cocci; rod-shaped eubacteria are known as bacilli; spiral or helically-shaped eubacteria are spirilla.
Unlike eukaryotic cells, which divide by mitosis or meiosis, eubacteria reproduce by binary fission. In this process, the genetic material is replicated, and the two copies move to separate nucleoid regions. Next, the plasma membrane pinches inward, producing two equal daughter cells. While these daughter cells are completely independent of each other, in some species they remain together, forming colonies and filaments. Binary fission can take place very rapidly, on the order of about one split every 20 minutes, accounting for the amazing replicative ability of eubacteria.