Bacteria (singular: Bacterium) is one of the two important members of the prokaryotes (i.e., cells in which the chromosomes are not separated from the cytoplasmic membrane). The second is Archaea. Bacteria are prokaryotic organisms with very simple cell structures. They are single-celled organisms with complex cell walls. Bacterial cells are the simplest possible forms of microorganisms, and they lack mitochondria, Golgi bodies and nuclear membranes. Bacteria are ubiquitous microorganisms with the majority of them occurring in soil, plants, the human body, and as free-living organisms in the environment. They are very small and can only be seen with the aid of a microscope. Bacteria are capable of self replication by binary fission (a type of asexual reproduction in which the parent cell splits to produce the daughter cells or offspring) because they contain all the enzymes and biologically active materials required for their growth and self replication.


The two basic forms of bacteria based on their cell wall are: (1) Gram-positive bacteria and (2) Gram-negative bacteria.Bacterial cells that lack a cell wall include Mycoplasma and L-form bacteria. L-form bacteria are forms of bacteria with different shapes. They are sensitive to osmotic shock. L-form bacteria are formed when bacteria including Gram-positive and Gram-negative bacteria go through a period of nutrient depletion or mutational changes. Bacterial cells without a cell wall (which is very critical to the survival of a bacterium) usually exist or live inside other cells under controlled osmotic conditions. Bacterial organisms have been implicated in a variety of infectious diseases that occur in humans. The diseases they cause have a plethora of negative economic consequences which affect humans, livestock, and even animals. Some of these diseases are mild while others are debilitating and can result in the death of the sufferer.

Notably, the bubonic plague (popularly known as the Black Death) caused by the bacterium Yersinia pestis is one of the bacterial diseases that spread from China through Europe in the 14th century killing millions of people at the time, and the disease was believed to drastically reduce the world’s population during the period. Though, the Black Death has been contained and eradicated by mankind; tuberculosis caused by Mycobacterium tuberculosis, bacterial vaginosis caused by Bacteriodes species, malaria caused by Plasmodium species), typhoid fever (caused by Salmonella), diphtheria and cholera caused by Vibrio cholerae are amongst the long list of bacterial related infections that are still causing morbidity and mortality in the human race; and these diseases have taken a huge toll on humanity in terms of their economic devastation. Some bacterial species have also developed resistance to some already available antibiotics and this has further worsened the plights of sick people as most of these drugs are no longer as effective in the treatment of infections. Despite the growing cases of some non-infectious diseases such as cancer, hypertension, diabetes and stroke, microbial infections and diseases caused by pathogenic bacteria still account for a significant amount of death in human population, as well as hospital visits across the globe.


All bacterial cells are extremely infinitesimal (i.e., microscopic), and are never visible to the naked eyes. They can only be seen with the aid of a microscope. Bacteria exist in different sizes and shapes which may range from 0.1 µm to 0.3 µm wide and 1 µm to 10 µm in length depending on whether they are rod or spherical in shape (Figure 1). Bacteria occur in various shapes. However, most bacteria are spherical in shape (cocci)while others are rod-like in shape (bacillus). The bacteria that assume a spherical shape are generally known as cocci (singular: coccus) while those that assume a rod-like shape are known as bacilli (singular: bacillus). However, bacterial cells generally exist in three forms which are spheres, rods and spirals. Figure 1 shows the various shapes in which a bacterial cell can occur. Generally, bacteria are classified into five different groups according to their basic shapes: (1) spherical (cocci), (2) rod (bacilli), (3) spiral (spirilla), (4) comma (vibrios) or (5) corkscrew (spirochaetes). Bacteria can also exist as single cells, in pairs, in chains or in clusters (Figure 1).


Bacilli (Rod bacteria) may be long or short, thick or slender in form (Figure 1). These features of rod bacteria are seen when they are viewed microscopically. Rod-like bacteria are usually cylindrical in shape, and they may appear as single cells known as bacillus (plural: bacilli) or bacterium. They can also appear as a double cell called diplococci or diplobacterium, and can also appear in chains called streptobacillus or streptobacterium. Coccobacilli are very short rods that can sometimes be mistaken to be cocci (i.e., spherical bacteria). There is also another group of rod-like bacteria known as fusiform bacteria because they have tapered or pointed ends. Rod-like bacteria can also be curved or bent. Some examples of rod-shaped bacteria include: Corynebacterium, Clostridium, Bacillus, Shigella, Escherichia, Salmonella and Listeria.


Bacteria that are spherical in shape are generally called cocci (singular: coccus). Spherical bacteria (which can also be known as oval bacteria because they are not always perfect sphere) may be large or small (Figure 1). They can also group themselves in various ways. Cocci can appear as a single cell (coccus), double cell (diplococcus), or combined as four cells (tetracoccus or tetrad). They can also be in chains (streptococcus) or in bulky form or clusters (staphylococcus). Examples of spherical bacteria include: Streptococcus, Staphylococcus, and Neisseria.


Spiral-shaped bacteria can also be called Vibrio, and they are generally known to have shape that looks like a screw or a cork (Figure 1). They are known as spirilla (singular: Spirillum) if the cells are rigid, and spirochaetes if the cells are undulating and more flexible. Spirilla contain one or more loosed turns while spirochaetes contain many coils. Examples of spiral shaped bacteria include: Vibrio cholerae that causes cholera disease. Others are Treponema, Borrelia and Leptospira – which are generally known as spirochaetes.   

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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