Cocci Bacteria - Types and Diseases

Morphology of Bacteria

Cocci are spherical or round-shaped bacteria. Bacteria are of several shapes, and based on their morphology, they can be classified into –

  • Cocci – They are typically spherical or oval-shaped
  • Bacilli – They are rod-shaped
  • Spirochaetes – They are slender and coiled

The bacteria can also be comma-shaped (vibrio), star-shaped and pleomorphic. Pleomorphic bacteria can change their size and shape based on external factors. Example – Mycoplasma pneumonia.

Cocci Bacteria

The term cocci is derived from the Greek word kokkos, meaning berry or seed. Cocci include archaeon or bacteria that are generally round, oval or spherical in shape.

Types of Cocci Bacteria

Cocci can grow in chains, clusters or pairs. Based on the arrangement of cocci cells, they can be further divided into –

  • Streptococci – cocci arranged in chains. E.g., Streptococcus mutans
  • Diplococci – cocci arranged in pairs. Eg., Neisseria gonorrhoeae
  • Monococci – single cocci
  • Staphylococci – clusters of cocci. Eg., Staphylococcus aureus
  • Tetrads – cocci in groups of four. Eg., Pediococcus
  • Sarcina – cocci in groups of eight. E.g., Sarcina ventriculi

Streptococci and diplococci mostly occur due to the non-separation of daughter cells after cell division. Sarcinae are organised in cubical pockets as cell division alternates amid three perpendicular planes.

Gram-positive Cocci Bacteria

Like any other gram-positive bacteria, they also turn positive for Gram staining (retain violet stain). They have thick peptidoglycan layers in their bacterial cell wall. Gram-positive cocci mostly cause infections in the intestine, vagina, nasopharynx region and mouth.

Staphylococci – They are gram-positive cocci that are arranged in grape-like clusters. They are non-sporing, non-motile and are approximately 1µm in diameter.

Micrococci – They are also gram-positive cocci that appear as irregular clusters. They are tetrads and differ from staphylococci in attacking sugars oxidatively.

Streptococci – They are another group of gram-positive cocci that appear in chains. They are usually a part of normal human flora. Some of them are important human pathogens that cause pyogenic infections. Their growth in the cell medium is enhanced by the addition of glucose, serum or blood.

Here are some examples of Gram-positive cocci –

  • Peptococcus
  • Parvimonas
  • Peptoniphilus
  • Blantia
  • Ruminococcus
  • Finegoldia
  • Anaerococcus

There are also some cocci that have a thin-peptidoglycan layer (gram-negative cocci). They do not retain violet stain during gram staining and instead take up counterstain like safranin and appear red in colour.

Some examples of gram-negative cocci –

  • Anaeroglobus
  • Acidaminococcus
  • Veillonella
  • Megasphaera
  • Negativococcus

Also Explore:

Difference between Gram-positive and Gram-negative Bacteria

Difference between Gram-positive and Gram-negative cell wall

Diseases Caused by Cocci Bacteria

Most cocci bacteria are disease-causing pathogens.

  • Pneumococcus – They are anaerobic gram-positive members of the Streptococcus genus. Streptococcus pneumonia (pneumococcus) colonises the nasal cavity and the respiratory tract and is the main cause of many pneumococcal infections like meningitis, pneumonia and sepsis.
  • Peptostreptococci – They are small and the most common anaerobic cocci isolated from human infections like puerperal infections, pleuropulmonary disease and brain abscess. Example – Peptostreptococcus anaerobius is responsible for puerperal sepsis.
  • Staphylococcus – Staphylococcus aureus are commensal bacterium that can also cause diseases sometimes. They mostly cause skin infections like staphylococcal scalded skin syndrome. Furthermore, this species is also responsible for staphylococcal food poisoning.

Also Check: Microorganisms Useful or Harmful

Frequently Asked Questions


What are coccobacilli?

Coccobacilli are short rod-shaped bacteria whose length is approximately equal to their width. Their shape is somewhere intermediate between bacilli and cocci. Example – Brucella.


What is coccidioidomycosis?

Coccidiomycosis is a mammalian fungal disease that has no relation to cocci bacteria. It is usually caused by the pathogenic fungus Coccidioides posadasii or Coccidioides immitis. Sometimes, coccidioidomycosis can also be a cause of community-acquired pneumonia.


What is coccidiosis?

Coccidiosis is another similar term, but it is related to a parasitic disease caused by coccidian protozoans (apicomplexans) like Isopora, Eimeria. Cryptosporidium, etc. This disease affects the intestinal tract of many animals like dogs, cats and also birds.


What are mycoplasmas?

Mycoplasmas are bacteria that are cell wall deficient and thus do not possess a stable shape. They may be oval or round bodies and can also be interlacing filaments. They are usually very small in size and can reproduce in a cell-free medium.

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