3 Common Shapes of Bacteria - Rounded, Rod-like and Spiral

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Bacteria Morphology

Bacteria are single celled microorganisms which move with a flagellum, a tail-like appendage which whips back and forth to cause movement. Bacteria are found everywhere in nature and are particularly important in aiding the decomposition of dead materials, such as plants and animals. They are an integral part of the natural recycling that occurs in nature. There are 3 shapes of bacteria - cocci, which are round; bacilli, which are rod shaped; and spirillum, which are spiral shaped or can appear coiled. Bacteria will often exist singularly, however, some types join together in pairs, chains, or grapelike clusters.


The most common of the 3 shapes of bacteria is cocci, or round bacteria. An example of a bacteria that appears round is Streptococcus. There are several variations of Streptococci, and they are distinguished in part by their arrangement. Some Streptococci exist in pairs, such as Streptococcus pneumonia, which can cause pneumonia. Others are joined in chains. One of the most well-known Streptococci is Group A Beta hemolytic Streptococci. Beta hemolysis appears as a clear area surrounding the colony growing on a blood agar plate. The hemolysis occurs because of a hemolysin the organism emits which breaks down red blood cells. This is useful in distinguishing Group A Beta Strep from other less virulent strains of Streptococci.

Another cocci, which can be serious, is Staphylococcus aureus. Staph aureus appears in grape-like clusters under a microscope and has become resistant to many types of antibiotics. A variant of Staph aureus is called methicillin-resistant Staph aureus, which is often found in hospital acquired infections and has mutated due to overuse of antibiotics. It is very difficult to treat methicillin-resistant Staph aureus (MRSA).


Another of the 3 shapes of bacteria is bacilli. These are rod shaped, gram positive organisms. One of the most commonly known bacilli is Bacillus antracis, which is the bacteria that can cause anthrax. Bacillus antracis, as well as other strains of Bacillus, are typically found in soil and are usually not harmful to humans.

Lactobacillus is another common gram positive bacilli found in many probiotics. It is helpful for the digestive system, providing what are considered “good” bacteria for healthy digestion. There are a number of varieties of Lactobacilli found in probiotics, such as Kiefer.


Spirillum are spiral shaped bacteria that stain gram negative. They move with tufts of flagella located at both ends. One of the most common spirillum that affects humans is Treponema pallidum, which causes syphilis. Most spirilla affect animals; however, there is a variety that can cause rat bite fever in humans.

Living in Harmony With Microorganisms

Bacteria are prolific in our world, having an important role. For humans who are healthy and have strong immune systems, bacteria are more helpful than harmful. There are a few species which can cause disease. The best defense is hygiene, so simply using soap and water can prevent the spread of most bacterial infections.