The term “cocci,” derived from the Greek word kokkos meaning “berry” or “seed,” refers to bacteria with a spherical or nearly spherical shape. This morphology distinguishes them from rod-shaped (bacilli) and spiral-shaped (spirilla) bacteria. Cocci are single-celled organisms, typically measuring around one micrometer in diameter, and are abundant in nearly every environment, colonizing soil, water, and the human body. Their simple, round form is the starting point for microbiologists seeking to identify and classify them.
How Spherical Bacteria Are Classified by Arrangement
Spherical bacteria are categorized based on how they group together following cell division, a pattern visible under a microscope. The plane of division and whether daughter cells remain attached determines the resulting arrangement. One common grouping is the diplococcus, where cells divide along a single plane and remain paired, such as the bacteria that cause certain forms of pneumonia.
When division occurs in one plane and the cells stick together sequentially, the result is a chain-like structure known as a streptococcus arrangement. This pattern is characteristic of the Streptococcus genus, which includes agents responsible for strep throat. If the cell divides randomly in multiple planes, the resulting growth pattern is an irregular, three-dimensional cluster called a staphylococcus.
This staphylococcus grouping is often likened to a bunch of grapes. Less common arrangements include the tetrad, a square-like grouping of four cells, and sarcina, a cubical packet of eight cells. These distinct patterns serve as an important initial clue for laboratory identification.
The Diagnostic Difference: Gram-Positive vs. Gram-Negative Cocci
Beyond their physical arrangement, spherical bacteria are categorized by the chemical structure of their cell wall, revealed through the Gram stain procedure. This diagnostic tool separates bacteria into Gram-positive and Gram-negative groups based on the thickness of the peptidoglycan layer, a polymer providing structural integrity.
Gram-positive cocci possess a thick, multi-layered cell wall composed primarily of peptidoglycan, up to 80 nanometers thick. When stained with crystal violet dye, this dense structure retains the purple color, even after an alcohol wash. Major genera like Staphylococcus and Streptococcus fall into this category, appearing purple under the microscope.
Gram-negative cocci have a much thinner peptidoglycan layer, measuring about 8 to 10 nanometers. This thin layer is sandwiched between an inner membrane and an outer membrane containing lipopolysaccharides (LPS). Since the thin peptidoglycan layer cannot hold the crystal violet dye after the alcohol wash, these cells are subsequently colored pink or red by a counterstain, such as safranin. The genus Neisseria, which includes the bacteria causing gonorrhea and meningitis, represents the most medically relevant Gram-negative cocci.
Primary Roles and Impacts on Human Health
Spherical bacteria act as both beneficial residents and agents of disease in human biology. Many cocci are commensal, residing on the skin and mucous membranes as part of the normal human flora without causing harm. For example, Staphylococcus epidermidis helps prevent more harmful bacteria from colonizing the area, a phenomenon known as colonization resistance.
Commensal cocci also contribute to immune system maturation and assist in the breakdown of complex carbohydrates in the gut. However, this balance can shift, and these organisms can become opportunistic pathogens if they enter a site where they do not normally reside or if the host’s immune system is compromised.
Certain cocci are widely recognized as primary causes of serious infections. Staphylococcus aureus, a Gram-positive coccus, commonly causes skin infections, abscesses, pneumonia, and bloodstream infections. Streptococcus pneumoniae is a frequent cause of pneumonia, ear infections (otitis media), and meningitis, particularly in children and the elderly. The Gram-negative coccus Neisseria meningitidis is capable of causing life-threatening meningococcal meningitis. Understanding the specific shape, arrangement, and Gram status of these bacteria is essential for medical professionals to quickly identify the organism and select the appropriate antibiotic treatment.