The Bacterial cell

Bacteria(sing. Bacterium) are microscopic, single-celled organisms that found in diverse environments. That means these organisms can live in soil, the ocean and inside the human gut.

Humans’ relationship with bacteria is complex. Sometimes bacteria serve as a helping hand, such as by curdling milk into yogurt or helping with our digestion. In other cases, bacteria are destructive, because they cause many diseases like pneumonia and others.


Bacteria (singular: bacterium) are classified as prokaryotes, which are single-celled organisms with a simple internal structure that lacks a nucleus, and contains DNA that either floats freely in a twisted, thread-like mass called the nucleoid, or in separate, circular pieces called plasmids. Ribosomes are the spherical units in the bacterial cell where proteins are assembled from individual amino acids using the information encoded in ribosomal RNA.

Bacterial cells are generally surrounded by two protective coverings: an outer cell wall and an inner cell membrane. Certain bacteria, like the mycoplasmas, do not have a cell wall at all. Some bacteria may even have a third, outermost protective layer called the capsule. Whip-like extensions often cover the surfaces of bacteria — long ones called flagella or short ones called pili — that help bacteria to move around and attach to a host.


A few different criteria are used to classify bacteria. The organisms can be distinguished by the nature of their cell walls, by their shape, or by differences in their genetic makeup.

The Gram stain is a test used to identify bacteria by the composition of their cell walls, named for Hans Christian Gram, who developed the technique in 1884. The test stains Gram-positive bacteria, or bacteria that do not have an outer membrane. Gram-negative bacteria don’t pick up the stain. For example, Streptococcus pneumoniae (S. pneumoniae), which vauses pneumonia is a Gram-positive bacterium, but Escherichia coli and Vibrio cholerae, which causes cholera, are Gram-negative bacteria.

There are three basic bacterial shapes: Round bacteria called cocci (singular: coccus), cylindrical, capsule-shaped ones known as bacilli (singular: bacillus); and spiral bacteria, aptly called spirilla (singular: spirillum). The shapes and configurations of bacteria are often reflected in their names. For example, the milk-curdling Lactobacillus acidophilus are bacilli, and pneumonia-causing S. pneumoniae are a chain of cocci. Some bacteria take other shapes, such as stalked, square or star

Most bacteria multiply by a process called binary fission, in this process, a single bacterial cell, called the “parent,” makes a copy of its DNA and grows larger by doubling its cellular content. The cell then splits apart, pushing the duplicated material out and creating two identical “daughter” cells.

The DNA found in parents and offspring after binary fission or budding is exactly the same. 

There are three ways  by which bacterial reproduction takes place that means horizontal gene transfer occurs by .. transformation, transduction and conjugation

Transformation is the most common process of horizontal gene transfer and occurs when short DNA fragments are exchanged between donors and recipients. Transduction, which typically only occurs between closely related bacteria, requires the donor and recipient to transfer DNA by sharing cell surface receptors. Conjugation requires physical contact between the cell walls of bacteria.Conjugation also refers as Sexual reproduction of bacteria where the DNA transfers from the donor cell to the recipient. Through conjugation, a bacterial cell can transfer DNA to eukaryotic cells (multi-celled organisms). 

Bacteria in human health and disease

Bacteria can be beneficial as well as harmful to human health. Commensal, or “friendly” bacteria, share space and resources within our bodies and tend to be helpful. There are about 10 times more microbial cells than human cells in our bodies; the highest numbers of microbial species are found in the gut, 


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