Facts about Cocci

Cocci are spherical bacteria having a diameter of 0.5 to 1.0 micrometer. They often occur in pairs (diplococci). Staphylococci (grape-like clusters) and streptococci (long chain-like patterns) are their other two forms. Read this BiologyWise article for some interesting facts about cocci.
BiologyWise Staff
Did You Know?
Bacteria have a simple cellular structure, as they do not have a true nucleus or membrane-bound organelles like mitochondria. They are believed to be among the first living beings to appear on Earth.
Although all bacteria seem to be identical, that is definitely not the case. When viewed under the microscope, a single bacterium has different shapes varying from rod (bacilli) to spherical (cocci). Unlike bacilli that have a large diameter, cocci are among the smallest and show several distinct arrangements.

The word coccus derives itself from the Greek word for berry. Cocci are one of the two basic bacterial forms, the other being bacillus. This classification, as you know, is based on their shape. Cocci are further classified on the basis of their aggregations; i.e. the numbers and patterns in which they occur. While some cocci cause diseases in humans and animals, some are useful. This article discusses cocci in greater detail, with the help of some important facts about them, pertaining to their structure, organization, and reproduction, and also gives you examples of harmful and useful bacteria.
  • Cocci are spherical and sometimes elongated oval in shape and have a diameter ranging from 0.5 to 1.0 micrometers.
  • Flat, elongated, and oval-shaped means a larger surface area, which allows them to easily absorb more nutrients.
  • Although a single coccus can survive in isolation, it is often found attached to other cocci, forming a specific pattern. The arrangement may vary depending upon the number of cocci in the group.
  • When spherical bacteria are present in pairs (joined together), they are referred to as diplococci. Whereas, a tetrad arrangement is a group of four, forming a square shape. Sarcinae refers to group of eight that form a cube. Streptococci have a chain-like pattern, while the staphylococci are visible as grape-like clusters under the microscope.
  • Streptococci that form long chains of spherical cells and staphylococci that make grape-like clusters of round cells are the two most prevalent forms of cocci.
Common Forms
Cocci bacteria
Reproduction in cocci occurs through a process referred to as binary fission in which the DNA of the parent cell is replicated into two. The replication causes enlargement of the parent cell, which eventually separates into two daughter cells (cocci), each containing the same DNA as that of the parent cell.
Disease-causing Cocci
Streptococcus Pyogenes
  • This gram-positive bacterium is responsible for causing strep throat, a painful throat infection that makes it difficult to swallow food and is often marked by fever and headache.
  • It can also cause localized skin infections such as impetigo that manifests in the form of blisters and sores that usually appear on the neck, face, and hands.
  • The period of incubation of these bacteria varies from 1 to 3 days.
Streptococcus Pneumoniae
  • This bacterium has been identified as the primary cause of pneumonia, a lung infection that causes breathing problems and a productive cough.
  • Other respiratory problems that can be attributed to these bacteria include sinusitis, bronchitis, and rhinitis.
  • Middle ear infections such as otitis media and common eye problems that include conjunctivitis also occur due to infection by these bacteria.
  • The bacteria can also cause other serious infections including meningitis, sepsis, peritonitis, and endocarditis.
Staphylococcus Aureus
  • About 15 to 40% of healthy individuals have these gram-positive bacteria on the skin and in the mucous membrane lining of the respiratory tract without showing any symptom of the infection.
  • The bacterium can trigger skin infections such as boils and abscess, and respiratory problems such as sinusitis.
  • People with severe atopic dermatitis, poorly controlled diabetes and those suffering from malnutrition and alcoholism are at a greater risk of getting this bacterial infection.
  • The bacterium is also capable of generating 7 different toxins that have often been associated with food poisoning.
Staphylococcus Epidermidis
  • Staphylococcus epidermidis, a gram-positive bacterium is one of the many species of the genus Staphylococcus that usually reside on the skin without causing any hurt to their host.
  • However, people with an impaired immune system have chances of developing infection from the bacteria.
  • The bacteria can also cause biofilm formation in indwelling implants such as catheters, which can lead to infections.
Neisseria Gonorrhoeae
  • These gram-negative bacteria occur in pairs (diplococci) and are the cause of gonorrhea, a sexually transmitted infection.
  • These coffee bean-shaped diplococci spread from one person to another through sexual contact.
  • The bacterium can also cause eye infection (conjunctivitis), throat infection (pharyngitis), proctitis (inflammation of the anus) prostatitis (inflammation of the prostate), and urinary tract infection such as urethritis.
Useful Cocci
Streptococcus salivarius
  • Streptococcus salivarius is a friendly gram-positive bacterium. They occupy the oral cavity and the upper respiratory tract in babies soon after birth.
  • They are harmless and known for their probiotic activity. In fact, they are viewed as a source of probiotics and have been found to be effective in reducing the frequency of the occurrence of throat infections caused by streptococcal pyogenes.
  • Also, quite a few commercial products that are advertised as promoters of healthy oral bacteria contain the microorganism Streptococcus salivarius as an active ingredient.
  • Streptococcus salivarius can also inhibit action of bacteria that have been linked to tooth decay.
Streptococcus thermophilus
  • Streptococcus thermophilus is gram-positive and classified as a lactic acid bacterium.
  • These bacteria occur in fermented milk products and are used in making yogurt.
Cocci, like other bacteria, can be gram-positive or gram-negative. The gram-positive cocci are usually harmless but become active and cause infections when they gain access to the internal body sites. They are notorious for causing skin and lung infections. Gram-negative cocci are obligate pathogens, which means they need to cause an infection, without which they cannot spread from one host to another. Neisseria meningitidis and Neisseria gonorrhoeae are some examples of this type.