BACTERIAL CELL: CHARACTERISTICS AND STRUCTURE (PARTS) - BIOLOGY - 2025

The bacterial cell is the simplest known organization of a living organism. Bacteria are unicellular organisms that do not have a nucleus or any organelle separated from the cytosolic content through a membrane (all bacteria are classified within the Prokaryotic domain).

Scientific studies have shown that, although bacterial cells lack organelles, they have a very controlled and precise organization, regulation and internal dynamics. They have all the necessary mechanisms to survive the hostile and changing conditions of the environment where they live.

General diagram of a bacterium and its parts (Source: This vector image is completely made by Ali Zifan / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons)

Such adaptability has meant for scientists an important tool and an ideal biological model for studying the basic principles of molecular biology; Basic knowledge about DNA replication, transcription, and translation was understood first in bacterial cells before eukaryotic cells.

All bacterial cells are microscopic, that is, they cannot be observed with the naked eye without using a microscope, which represents a great advantage for the study of these microorganisms, since they can be maintained and studied in a small space and with few nutritional resources. to millions of living cells.

Currently the bacterial cell is one of the most important biotechnological tools. Scientists manipulate bacteria's extra chromosomal DNA to synthetically produce almost any protein of human interest.

General characteristics of the bacterial cell

Morphologically, bacterial cells can be highly variable, but even so, they all share common characteristics. For example:

- Every bacterial cell has a cell wall that surrounds it and is composed of a combination of carbohydrates with peptides that is called "peptidoglycan".

- Bacterial cells are unicellular organisms, that is, each cell is a complete organism that can grow, feed, reproduce and die.

- The genetic material of bacteria is "scattered" or distended in a large tangle submerged in the cell cytosol, in a region known as the nucleoid region.

- Many bacteria have specialized structures for locomotion called "flagellum", which are in the outermost regions of their bodies.

- It is common to find bacterial cells forming colonies or maintaining a symbiotic relationship with other organisms and, in addition, many bacteria are pathogenic for humans.

- Most bacteria are almost 10 or 15 times shorter than the size of any animal cell (eukaryotic), as they do not exceed a unit of microns in length.

- They are found in all existing environments on the biosphere, since there are these microorganisms adapted to practically any environmental condition.

Structure of the bacterial cell (parts)

Many scientists subdivide the bacterial cell into three anatomical regions to facilitate study. These three regions that are common for any type of bacterial cell observed and are:

- The external region, made up of extracellular structures (flagella, hairs, cilia, among others)

- The cell coverage region, composed of the cell wall and the cytoplasmic membrane

- The internal region, formed by the cytosol and all the structures suspended in it.

Depending on the species of bacteria that is studied in each region, some structures and parts different from the "typical" of a bacterial cell are observed. However, the most common for any bacterial cell are explained and classified according to each region in which they are found.

Extracellular region

- Capsule: it is a polymer surface that covers the entire cell wall of bacteria. It is made up of slime and glycocalyx, which in turn are made up of abundant carbohydrate molecules bound to lipids and proteins. The capsule fulfills an important protective function for the cell.

- Film: it is a surface, a liquid or a viscous matrix in which the bacterial cells are immersed. They are formed of polysaccharides similar in composition to the polysaccharides of the capsule and generally serve functions in the protection and displacement of cells.

- Fimbriae: they are a kind of very numerous filamentous appendages that are found attached to the cell wall of bacteria. These serve for the mobility and adherence of bacterial cells to any surface. They are made up of a hydrophobic protein called pilin.

Fimbriae of E. coli bacteria. It has approximately 200. Source: (Image: Manu Forero) / CC BY (https://creativecommons.org/licenses/by/2.5)

- Sexual pili: some fimbriae (few) are modified to form a kind of "pipe", which is used by bacteria for conjugation (the transfer of genetic material between different bacteria), which is a kind of "sexual reproduction" primitive.

- Flagella: they are longer filaments than the fimbriae and are made up of proteins; they have a "tail" appearance. They fulfill the driving function for the movement of cells and are anchored to the cell membrane. From one to hundreds of flagella can be found in the same bacterial cell.

Vibrio vulnificus bacteria with flagella. CDC / Janice Haney Carr / Public domain

Diagram of a bacterial flagellum (Source: LadyofHats / Public domain, via Wikimedia Commons)

Region of coverage

The cell covering generally consists of a cytoplasmic membrane and a peptidoglycan layer that is called the "cell wall." The envelope is made up of complexes of lipids, carbohydrates and proteins. The chemical composition of the peptidoglycan envelope is used as a classification to distinguish between two types of bacteria.

Gram positive bacteria and Gram negative bacteria. Gram positive bacteria are characterized by having a thick layer of peptidoglycan, without an outer membrane covering it, whereas gram negative bacteria have only a thin layer of peptidoglycan with an outer membrane superimposed on it.

- Cytoplasmic membrane: it has a similar structure to the cell membrane of eukaryotic cells. It is a phospholipid bilayer with associated proteins (integral or peripheral). However, it differs from the membrane of eukaryotic cells in that it does not possess endogenously synthesized sterols.

The cytoplasmic membrane of bacterial cells is one of the most important structures, since it is there where cell fusion, electron transport, protein secretion, nutrient transport and lipid biosynthesis occur, etc.

Inner region

- Genome: unlike eukaryotic cells, the genome of bacterial cells is not contained in a membranous nucleus. Instead, it exists as a tangle of DNA that is compacted in a more or less circular shape and associates with some proteins and RNA. This genome is much smaller than the eukaryotic genome: it is approximately 3 to 5 MB in size and forms a single circular topology chromosome.

- Plasmids or extrachromosomal DNA molecules: they are small circular DNA molecules that are capable of replicating independently of cellular genomic DNA. Generally, plasmid DNA molecules are exchanged during conjugation, since the information necessary for resistance to antibiotics and / or toxins is encoded in these.

Bacterial DNA and plasmids. Original file: User: Spaully. Translation: Fibonacci. / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5)

- Ribosomes: ribosomes participate in the translation of RNA that was transcribed from the sequence of a gene that codes for a protein. Each bacterial cell has about 1500 active ribosomes inside. The ribosome subunits of the bacterial cell are 70s, 30s and 50s, while eukaryotic cells have 60s and 40s subunits.

It is common for antibiotics to attack the ribosomes of bacteria, blocking the translation of proteins and causing cell lysis or death.

- Endospores: bacteria have internal spores that are in a dormant state and are used for survival when environmental conditions are extreme. Endospores come out of their dormant state when different receptors on the surface detect that conditions are favorable again; This creates a new, fully functional bacterial cell.

Green endospores inside bacterial cells. CNX OpenStax / CC BY (https://creativecommons.org/licenses/by/4.0)

- Granules or inclusion bodies: these function as a kind of reserve for carbohydrates, phosphate compounds and other molecules. Their composition varies according to the species of bacteria and they are easily visible in the cytoplasm using optical microscopes.

References

1. Cabeen, MT, & Jacobs-Wagner, C. (2005). Bacterial cell shape. Nature Reviews Microbiology, 3 (8), 601-610.
2. Coleman, JP, & Smith, CJ (2007). Structure and Composition of Microbes.
3. Gitai, Z. (2005). The new bacterial cell biology: moving parts and subcellular architecture. Cell, 120 (5), 577-586.
4. Silhavy, TJ, Kahne, D., & Walker, S. (2010). The bacterial cell envelope. Cold Spring Harbor perspectives in biology, 2 (5), a000414.
5. Willey, JM, Sherwood, L., & Woolverton, CJ (2009). Prescott's principles of microbiology. Boston (MA): McGraw-Hill Higher Education.