Bacteria are single-celled prokaryotes that lack a nucleus and other membrane-bound structures. They are found nearly everywhere on Earth, often existing in communities of millions in environments ranging from soil and water to the human body. While some bacteria can cause disease, many others are beneficial, playing roles in nutrient cycling, food production, and maintaining human health.
The Primary Method: Binary Fission
The primary method bacteria use to multiply is a form of asexual reproduction called binary fission, which produces two genetically identical daughter cells from a single parent cell. The process begins with the replication of the bacterium’s single, circular chromosome, starting at a specific point called the origin of replication. As the DNA copies, the two new origins of replication move toward opposite ends of the elongating cell, pulling the rest of the chromosome along.
Once the two identical chromosomes have been successfully segregated to opposite poles, a specialized ring of proteins, including the FtsZ protein, assembles at the cell’s midpoint. This protein ring directs the formation of a septum, which is a new dividing wall composed of cell membrane and cell wall material. The septum grows inward from the sides of the cell until it completely separates the cytoplasm and the two newly formed chromosomes. Because binary fission does not involve the mixing of genetic material from two different parents, the resulting offspring are clones of the original cell, barring any new mutations.
Exchanging DNA: Genetic Variation
While binary fission produces clones, bacteria possess sophisticated mechanisms to exchange genetic material, known as horizontal gene transfer, which introduces variation and allows for rapid adaptation. This process is distinct from reproduction and is a powerful mechanism for the spread of traits like antibiotic resistance.
One method is conjugation, where DNA is transferred through direct cell-to-cell contact. The donor cell uses a structure called a pilus to connect with a recipient cell, forming a temporary bridge. Through this bridge, a copy of a small, independently replicating piece of DNA, called a plasmid, is transferred.
Transformation occurs when a bacterium takes up “naked” DNA fragments that have been released into the environment by dead and degraded bacteria. The third mechanism, transduction, involves the transfer of bacterial DNA from one cell to another by a virus called a bacteriophage. During viral replication inside a host bacterium, the virus sometimes mistakenly packages a fragment of the host’s bacterial DNA into its new viral particles.
Understanding Rapid Growth
The simple, efficient nature of binary fission enables bacterial populations to grow at an exponential rate under favorable conditions. This rate of increase is often measured by the generation time, which is the time it takes for the population to double in number. For many common bacteria, such as E. coli, this doubling time can be as short as 20 minutes. Starting with just one cell, a population doubling every 20 minutes would yield millions of cells within a matter of hours.
However, this growth cannot continue indefinitely, as population expansion is always limited by environmental factors. As a colony grows, the available nutrients are quickly depleted, and waste products accumulate, which can create toxic or acidic conditions. Changes in temperature, pH, and the availability of water or oxygen will eventually slow the cell division rate and push the population into a stationary phase.