Bacteria are the oldest, most abundant life form on Earth. Their simplicity has allowed these microscopic, single-celled organisms to adapt and thrive in virtually all environments. The body of a healthy adult hosts roughly 100 trillion bacteria. Despite their bad reputation, most bacteria do not cause disease.
Structure
Bacteria are the simplest stand-alone life form on Earth. A bacterium's goals are simply to stay alive and reproduce. They contain only the essential structures needed to accomplish these goals.
Plasma Membrane
A bacterium is a single cell surrounded by a plasma membrane. Think of this structure as the skin of the bacterium. The plasma membrane is made up of protein with some fat and a bit of carbohydrate. Channels in the membrane allow nutrients to enter and waste to be expelled.
Cytoplasm
Within the plasma membrane is the liquid cytoplasm of the bacterium. It is a mixture of water, enzymes, genetic material and other substances needed for survival and reproduction.
Cell Wall
Most bacteria associated with humans have a cell wall surrounding the plasma membrane. This is the bacterial cell's armor. It is a semi-rigid structure comprised of interwoven strands of peptidoglycan. The cell wall protects the bacterium from the outside environment.
Appendages
Some bacterial species have appendages on their outer surface. Flagella and axial filaments propel bacterial motion. Fimbriae are short projections that help bacteria adhere to surfaces.
Nucleoid
The genetic material of bacteria is DNA (deoxyribonucleic acid). However, a distinguishing characteristic of bacteria--compared to other types of cells--is that they do not have nuclei. A nucleus is a walled-off compartment in a cell that holds its DNA. Without a nucleus, bacterial DNA floats in the cytoplasm as an unprotected circular clump called the nucleoid.
Ribosomes
Ribosomes are abundant within the bacterial cytoplasm. These tiny structures perform a remarkable task on which all life is dependent--reading the cell's genetic instructions and making the necessary protein ingredients to sustain life and reproduce.
Shape and Staining Characteristics
Each bacterial species has a specific shape and staining characteristics.
Shape
Bacteria have three basic shapes: round, rod-like and spiral. Spherical bacteria are called cocci. Rod-like bacteria are termed bacilli. Spiral shaped bacteria are known as spirochetes.
Gram Positive and Negative
Bacteria are stained with dyes to be seen under the microscope. The Gram stain is most commonly used. Gram-positive bacteria appear purple and Gram-negative bacteria are pink. Bacteria stain differently due to compositional differences in their surfaces.
Reproduction and Mutation
Binary Fission
Sexual reproduction is complicated--and bacteria are simple. Therefore, bacteria typically reproduce asexually by a process called binary fission. The contents of the bacterium are replicated and the cell then splits into two identical daughter cells, clones of the lone parent cell. Depending on the conditions, most common bacterial species can replicate every 15 to 60 minutes.
Mutation and Adaptation
The speed and simplicity of bacterial cell replication make them prone to genetic mutation. This enables bacteria to quickly adapt to their environment and thrive. The downside of this survival feature is that disease-causing species can develop resistance to antibiotics, fueling an ongoing battle between scientists' struggle to develop new antibiotics and bacteria's capacity to become resistant to them.
References
- "Bacteriology of Humans: An Ecological Perspective"; Michael Wilson, Ph.D.; 2008
- "Molecular Biology of the Cell, Third Edition"; Bruce Alberts, Ph.D., Dennis Bray, Ph.D., Julian Lewis, D.Phil., Martin Ruff, M.D., Keith Roberts, Ph.D., James D. Watson, Ph.D.; 1994
- "Zinsser Microbiology, Eighteenth Edition"; Wolfgang K. Joklik, D.Phil., Hilda P. Willett, Ph.D., D. Bernard Amos, M.D., Editors; 1984
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