Cellular shape, or morphology, is fundamental to classifying bacteria. Understanding the form and structure of a bacterium is the first step in identifying it and determining its potential role in health or disease. The genus Streptococcus is a significant group of bacteria. Analyzing their precise shape helps scientists understand how these common microorganisms grow, spread, and cause infection.
The Defining Shape and Arrangement
Streptococcus bacteria are spherical, classifying them as cocci (round or roughly circular bacteria). These cells are relatively small, typically ranging from 0.5 to 2.0 micrometers in diameter. The name coccus comes from the Greek word kókkos, meaning “grain,” “seed,” or “berry,” reflecting their distinct round appearance.
The most distinctive feature of the Streptococcus genus is how the cells are grouped, not the individual shape. When these bacteria reproduce, cell division occurs along a single axis. The cells remain attached after splitting, resulting in the formation of chains of varying lengths that can appear twisted or bent.
This unique arrangement is reflected in the genus’s name. The prefix Strepto- is derived from the Greek word streptós, meaning “twisted” or “pliable chain.” Therefore, Streptococcus literally means “twisted berry” or “chain of berries,” which visually describes their appearance under a microscope. The chain length depends on the species and growing conditions, sometimes appearing as long strands and other times only as pairs of cells.
How Morphology Aids Identification
The specific shape and arrangement of Streptococcus are the primary initial clues used for identification in a clinical setting. Identification typically begins with a Gram stain, which categorizes bacteria based on their cell wall structure. Streptococcus bacteria are classified as Gram-positive, meaning they retain the purple crystal violet stain due to their thick peptidoglycan cell wall.
The combination of being Gram-positive and exhibiting a spherical shape in a chain arrangement is highly specific. This morphology helps differentiate Streptococcus from other common Gram-positive cocci, such as Staphylococcus. While Staphylococcus species are also round, they divide along multiple planes, causing them to clump together in irregular, grape-like clusters.
Observing the chain formation provides a rapid, presumptive diagnosis, guiding the laboratory toward further testing. This initial morphological assessment directs the next steps, such as performing a catalase test (which Streptococcus will be negative for) or specialized serological grouping tests like the Lancefield classification system. The distinctive visual pattern is fundamental to the diagnostic workflow.
Common Streptococcus Infections
The Streptococcus genus includes numerous species; many are harmless parts of the body’s normal flora, but others cause a wide range of human illnesses. The most common infections are associated with Group A Streptococcus (S. pyogenes). These typically present as non-invasive diseases like streptococcal pharyngitis, commonly known as strep throat.
Other non-invasive infections include scarlet fever (strep throat accompanied by a distinctive red rash) and impetigo, a contagious skin infection causing crusty sores. Less commonly, some Streptococcus species cause serious invasive diseases when they enter sterile areas, such as the bloodstream or deep tissues. Examples include bacterial pneumonia, bacteremia, and necrotizing fasciitis.