What Are the Types of Bacteria?
by Paul Dohrman
What Are the Types of Bacteria?
The major motivation for classifying bacteria is to make identification easier. This aids doctors in making diagnoses of bacterial infections.
There are seven main groups of bacteria, distinguished by their shape and cell wall type. The four types that constitute the majority of bacteria are distinguished by whether they are Gram-positive or Gram-negative, and whether they are cocci (round) or bacilli (rod-shaped).
The remaining three are spirochaetes (corkscrew-shaped; includes the causes of syphilis and Lyme disease), rickettsia (includes the cause of typhus), and mycoplasma (lacks a cell wall; includes the cause of pneumonia).
A broader division of bacteria is between those that produce their own food (through photosynthesis) and those that do not. Cyanobacteria is the name of the former. The latter are often just called bacteria.
Bacteria predated cyanobacteria, which showed up no later than 2.15 billion years ago. The early Earth atmosphere held little oxygen but a great deal of carbon dioxide. Cyanobacteria reversed the concentrations. Eukaryotes (cells with nuclei) that symbiotically absorbed cyanobacteria a billion years ago became green plants. Today these plants perform photosynthesis in their chloroplasts, the descendants of those ancient cyanobacteria. That they are bacterial descendants is the reason chloroplasts have genetic material.
Cyanobacteria are Gram-negative.
Reaction to the Gram Stain
The Gram stain test begins with staining a culture with a purple dye called crystal violet. In Gram-negative bacteria, the stain can be washed away with alcohol or acetone.
This criterion may seem arbitrary, but it distinguishes two fundamentally different kinds of cell wall. Gram-positive bacteria have a thicker wall and lack an outer membrane that gram-negative bacteria have.
Hans Gram's original use was clinical, to distinguish bacteria with similar symptoms: Streptococcus pneumoniae (Gram-positive) and Klebsiella pneumoniae (Gram-negative).
Gram-positive and Gram-negative Membranes (http://users.rcn.com/jkimball.ma.ultranet)
Ability to Form Spores
Many bacteria dry up to form a single spore when food runs out. Metabolism ceases. Spores are so resistant to adverse conditions of temperature that they may be revivable after 50 years of dormancy.
A study at the German Aerospace Center in 2000 indicates that bacterial spores are resistant enough for 0.01% to survive interplanetary travel on a meteor and the resulting impact.
Method of Energy Production
Bacteria can be distinguished by method of energy production. Anaerobes engage in glycolysis while aerobes can also engage in cellular respiration. "Anaerobic" means not requiring oxygen. Many anaerobes find oxygen poisonous. This is one reason why drinking stagnant water can be dangerous. Running water is aerated, killing such anaerobes.
Glycolysis occurs with or without oxygen. Without oxygen, small amounts of the high-energy compound ATP are produced. With oxygen, glycolysis becomes the first step in cellular respiration, a more efficient way of producing ATP.
Aerobes were absorbed by eukaryotes (cells with nuclei) over a billion years ago for the benefit of ATP production. Their descendants are the organelles called mitochondria. Like chloroplasts, they also have genetic material.
Harmful vs. Friendly
Of course, the differentiation in bacteria that most people are concerned about is whether a bacterium is harmful or not. Toxicity to humans traverses multiple types. For example, as Gram's original intent showed, toxicity can be found in both Gram-positive and Gram-negative species.
The same bacterium can be benign or deadly, depending on where it is. Staphylococcus aureus lives on our hands benignly, but is deadly if it enters the body surgically -- thus the importance of surgeons using an antiseptic to clean their hands. E. coli is harmful if ingested, but safe in our intestines, below the acidic barrier of the stomach.
This is not to say that all bacteria are benign or toxic. Some are helpful, by aiding digestion in the intestines, producing nutrients, and taking up space in different parts of the body to crowd out harmful bacteria. In fact, there are ten times more bacterial cells in the human body than there are human cells.
Various Bacterial Diseases (Wikipedia/Mikael Häggström)