Cocci

Cocci

Cocci are a type of bacteria that are spherical or oval in shape. They are one of the three primary shapes of bacteria, the others being bacilli (rod-shaped) and spirilla (spiral-shaped). Cocci can exist as single cells or can aggregate in various arrangements, which are often characteristic of specific genera or species.

Classification

Cocci are classified based on their arrangement and the number of planes in which they divide:

• Diplococci: These cocci appear in pairs. An example is Streptococcus pneumoniae.
• Streptococci: These cocci form chains. An example is Streptococcus pyogenes.
• Staphylococci: These cocci form grape-like clusters. An example is Staphylococcus aureus.
• Tetrads: These cocci form groups of four.
• Sarcinae: These cocci form cubical packets of eight or more cells.

Gram Staining

Cocci can be further classified based on their reaction to the Gram stain:

• Gram-positive cocci: These have a thick peptidoglycan layer in their cell wall and retain the crystal violet stain, appearing purple under a microscope. Examples include Staphylococcus and Streptococcus.
• Gram-negative cocci: These have a thinner peptidoglycan layer and do not retain the crystal violet stain, appearing pink after counterstaining. An example is Neisseria gonorrhoeae.

Pathogenicity

Many cocci are pathogenic to humans and can cause a variety of diseases:

• Staphylococcus aureus is known for causing skin infections, pneumonia, and sepsis.
• Streptococcus pyogenes is responsible for strep throat, scarlet fever, and rheumatic fever.
• Neisseria gonorrhoeae causes gonorrhea, a sexually transmitted infection.
• Neisseria meningitidis can cause meningitis and septicemia.

Laboratory Identification

Cocci are identified in the laboratory using a combination of morphological, biochemical, and molecular techniques:

• Microscopy: Observing the shape and arrangement of cocci under a microscope.
• Gram Staining: Determining whether the cocci are Gram-positive or Gram-negative.
• Biochemical Tests: Such as the catalase test and coagulase test for Staphylococcus species.
• Molecular Methods: Including PCR and sequencing for precise identification.

Treatment

The treatment of infections caused by cocci depends on the specific organism and its antibiotic susceptibility:

• Antibiotics: Penicillin, cephalosporins, and vancomycin are commonly used against Gram-positive cocci.
• Resistance: Some cocci, like Methicillin-resistant Staphylococcus aureus (MRSA), have developed resistance to multiple antibiotics, necessitating alternative treatments.

See Also

• Bacteria
• Gram Staining
• Antibiotic Resistance

References

• Todar, K. (2020). Todar's Online Textbook of Bacteriology. University of Wisconsin-Madison.
• Ryan, K. J., & Ray, C. G. (Eds.). (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill.