By: Divya Chandramouli

Diagnostic Characteristics
There is a common misconception that all bacteria are harmful; this is not true at all. Most bacteria are either harmless or helpful.
Traditionally, bacteria were classified as prokaryotes (organisms that lack a cell nucleus), belonging to the kingdom Monera. But recently, prokaryotes have been divided into two domains, Bacteria and Archaea, the two major branches of prokaryote evolution. Most scientist agree that what separates bacteria from Archaea is peptidoglycan, a polymer of amino sugar units. Bacteria have this polymer in their cell wall, whereas Archaea lack it (5T2). Additionally, Bacteria have circular DNA and lack histones for their DNA (5T2). Most Bacteria are unicellular organisms. Some species of Bacteria stick together in groups of two or more cells and other species form colonies. Bacteria can be found in a variety of shapes, the three most common being spheres (cocci), rods (bacilli) and helices.


Bacteria are almost everywhere. Generally, they exist wherever there are other living organisms, and they also exist in habitats that are too extreme (too hot, too cold, too acidic, etc.) for eukaryotic organisms. Bacteria exist in marine ecosystems, freshwater ecosystems, terrestrial ecosystems, and they have even made their way into starring in Hollywood productions. :) In one breath of air, there are about 10,000 Bacteria. In each drop of human saliva, 150,000,000. They are literally almost everywhere, en masse. (JE)


Major Types

There are 5 major clades of Bacteria: Proteobacteria (sub-clades: Alpha proteobacteria, Beta proteobacteria, Gamma proteobacteria, Delta Proteobacteria, Epsilon proteobacteria), Chlamydias, Spirochetes, Gram-positive Bacteria, Cyanobacteria. Before DNA sequencing was used to determine the various types of bacteria, their grouping was determined based on shape alone. Bacteria were categorized by their three main shapes: rod-shaped, sphere-shaped, and spiral-shaped (CS 5).

Proteobacteria: group of gram-negative (outer membrane is present) bacteria including photoautotophs, chemoautotrophs, and heterotrophs. (Also include aerobic and anaerobic species).
1) Alpha Proteobacteria – The species of bacteria in this group are usually closely associated with eukaryotic hosts (mutualism or parasitism).
For example, the Rhizobium species live in nodules within the roots of legumes; the bacteria convert atmospheric N2 to compounds useful to the host plant.
Beta Proteobacteria – This group is a nutritionally diverse group that includes Nitrosomonas, soil bacteria that play an important role in Nitrogen recycling within ecosystems.
Gamma Proteobacteria – Among photosynthetic members, this group of bacteria includes Chromatium, a species which splits H2S as a source of electrons to make organic molecules. And example of an enteric (bacteria that live in animal intestines) Gamma is Salmonella, a species of bacteria that causes food poisoning.
Delta Proteobacteria – One group of Delta bacteria called the myxobacteria form the most elaborate colonies of all prokaryotes. This group contains both aerobic and anaerobic bacteria. (HL 22)
Epsilon Proteobacteria – This group is closely related to the Delta bacteria; one example helicobacter pylori, the bacteria that causes stomach ulcers.
*Vampirococcus is an aerobic gram-negative bacteria that is predatory. It feeds on Chromatium, a purple phototrophic bacteria, by degrading its cytoplasm. Predatory prokaryotes are rare and used to be thought of as parasites, but careful examination of predatory bacteria like Vampirococcus prove that the predator/prey relationship is not limited to eukaryotes (CH 8).

Chlamydias : This group of bacteria contains organisms that can only survive within the cells of animals (depend on their hosts for everything). The gram-negative walls of Chlamydias lack peptidoglycan (the polymer that forms mesh-like layer outside of the plasma membrane of Bacteria cell walls). An example is the species Chlamydia trachomatis, the most common cause of blindness in the world. Chlamydia cannot synthesize their own ATP, and were previously mistaken for viruses as a result. (AR 10)

Spirochetes: These helical heterotrophs are mostly free-living, but there are some pathogenic (disease-causing) species. They contain an arrangement of flagella-like axial filaments, which allow them to move by rotating in place (4 J. Stein). Borrelia burgdorferi is the pathogen that causes Lyme disease. Spirochetes include both aerobic and anaerobic species and can be very harmful to most organisms and animals. In addition to causing Lyme disease, they can also cause syphilis. (EK)

Gram-Positive Bacteria: This very diverse group of bacteria has a couple of subgroups: 1) Actinomycetes and 2) Solitary species. Actinomycetes form colonies with branched chains of cells. One species belonging to this subgroup helps decompose the organic litter in soil, whereas other species cause Tuberculosis and Leprosy. The solitary species are spore formers (ex. Bacillus and Clostridium). Anthrax is caused by Bacillus anthracis. The most structurally unusual members of gram-positive bacteria are mycoplpasmas, the only bacteria lacking cell walls; they are also the smallest of all known cells. Many mycoplasmas are soil bacteria, but some are pathogens. Gram-positive bacteria have certain characteristics that are generally present: cytoplasmic lipid membrane, thick peptidoglycan layer, capsule polysaccarides (in some species), and flagellum (in some species). (4-SC)

: This group of phototrophic bacteria contains the only prokaryotes with plant-like, oxygenic photosynthesis. Solitary and colonial cyanobacteria are abundant wherever there is water; they provide a huge amount of food for freshwater and marine ecosystems. Thought to be cyanobacteria’s greatest contribution is the origin of plants--The chloroplast in a plant is a type of plastid that is actually a cyanobacterium living within the plant’s cells. (5 AN)


Basic Anatomy

Almost all prokaryotes have cell walls; the cell wall retains the shape of the cell, provides physical protection, and prevents the cell from bursting. Most bacterial cell walls consist of a material called peptidoglycan. Gram-positive Bacteria have simpler walls with more peptidoglycan. Gram-negative bacteria have less peptidoglycan and have more complicated structures. Bacteria, being prokaryotes, do not have membrane-enclosed nuclei. Various prokaryotes do have specialized membranes that perform their metabolic functions. Bacteria also have smaller, simpler genomes (about 1/1000 as much DNA as a eukaryotic cell). The DNA is concentrated in the nucleoid region. Bacteria have one major chromosome; in addition to that, they have smaller rings of DNA called plasmids. Many prokaryotes secrete sticky substances that create a protective layer, called a capsule, over the cell wall.

Formation of a Bacterial Flagellum (KA)

The most common method of movement among bacteria is flagellar action. Flagella may be spread over the entire cell, or it may just be concentrated at one or both ends of the organism. Bacterial flagella are not covered by the plasma membrane. Helix-shaped bacteria called spirochetes use a second method of movement. Two or more helical filaments under the outer layer of the cell wall rotate, and the flexible cell moves like a corkscrew. In a third method of movement, prokaryotes secrete slimy threads that anchor to the surface; as the cell continues to secrete jets of slime, the organism glides along the growing end of the thread.


bacteria undergoing conjugation (18RM)
bacteria undergoing conjugation (18RM)

Bacteria produce asexually, through binary fission, a mode of cell division. Neither mitosis nor meiosis occurs in prokaryotes. Bacteria transfer genes between individuals in three ways.
1) Transformation: a prokaryotic cell absorbs genes from its surroundings, allowing considerable genetic transfer between prokaryotes (even across different species).
2) Conjugation: direct transfer of genes from one prokaryote to another.
3) Transduction: viruses transfer genes between prokaryotes.

Mutation is the major source of genetic variation in Bacteria.

The video above describes the simple process of cellular division known as binary fission that occurs in all prokaryotes and some eukaryotes. Bacteria do not practice mitosis or meiosis as methods of cell division, just binary fission (3 MB).

Environmental Adaptations

Ancestors of bacteria were the first forms of life to develop on Earth.
For Bacteria, generations can be created in hours and even minutes, so mutations can be spread to a very large number of offspring. This enables Bacteria to adapt to their environments rapidly; natural selection screens new mutations. Bacteria are surrounded by membranes. These membranes may be altered so that bacteria can be more resistant against its surroundings wich may include antibiotics or penicillin. This membrane alteration makes it more difficult for antibiotic cells to penetrate into bacteria (11 JSun). Some alternations are reversable, disappearing when the particular pressure has been lifted. (18 VK)Other alternations are maintained and can even be passed on to succeeding generations of bacteria. (18 VK)
Evidence of bacteria surviving in the moon's atmosphere has also been confirmed by NASA. With extremely low temperatures, the vacuum, and lack of liquid water, bacteria are said to use internal substances, such as glycerol, proteins, gums, and sugars, to protect themselves. (AS 23)

Nutrition and Metabolism

Bacteria are divided into four categories by where they obtain their energy from (mode of nutrition).
1) Phototrophs: photosynthetic organisms that obtain their energy from the sun and drive the synthesis of organic compounds from carbon dioxide. Phototrophic prokaryote: cyanobacteria.
2) Chemoautotrophs: organisms that need only CO2 as a carbon source; they obtain their energy by oxidizing inorganic substances. This mode of nutrition is unique to certain prokaryotes. Most chemoautotrophs are bacteria or archaea that live in hostile environments such as deep sea vents (24 SC).
3) Photoheterotrophs: organisms that use sunlight to generate ATP but have to acquire their carbon in organic form. This mode of nutrition is unique to certain prokaryotes.
4) Chemoheterotrophs: organisms that have to consume orgain molecules for both energy and carbon. This nutritional mode is found widely among prokaryotes and other kingdoms.

Review Questions
1. What diseases are caused by bacteria, and how do the bacteria affect the infected cells? (DPOD 8)
2. Explain flagellar action, the structures used, and how it works. (EG 6)
3. What type of bacteria was once mistaken for a virus and why? (AW)
4. How many different clades of bacteria are there, and what divides them? (EK18)
5. Bacteria may reproduce asexually, but they still exchange genes with each other. Explain the three mechanisms through which a bacterium acquires genes. (SW 16)
6.What characteristic of bacteria is responsible for its resistance to its surroundings
7. What are the four modes of nutrition in bacteria? How do these relate to the diversity of bacteria? (19 AL)

1. Campbell, Neil A., and Jane B. Reece. Biology, Sixth Edition. San Francisco: Benjamin Cummings, 2001. Print.
2. "Introduction to the Cyanobacteria." UCMP - University of California Museum of Paleontology. 24 Oct. 2009 <http://www.ucmp.berkeley.ed

3. "Bacterial Asexual Reproduction (binary fission)." 24 Oct. 2009
4. "Introduction to the Spirochetes." UCMP - University of California Museum of Paleontology. 25 Oct. 2009 <>.
5. "Bacteria - Bacterial Adaption." 25 Oct. 2009 <>.\
6. Klyce, Brig. "Bacteria: The Space Colonists." Cosmic Ancestry. Web. 1 Nov. 2009. <>.
7. "An Overview of Numbers and Sources of Bacteria." Watertalk - a forum to protect water quality. Web. 04 Nov. 2009. <>.
8. Irwin, Jesse. "Vampirococcus." Missouri S&T Biology. 4 Nov. 2009. <>