Streptococcus

Streptococcus is a large genus of gram-positive, spherical bacteria.

These microscopic organisms belong to the family Streptococcaceae. They are part of the order Lactobacillales, which includes many lactic acid bacteria. The name comes from Ancient Greek words meaning "twisted grain." This name describes their unique physical structure. Unlike staphylococci, which divide in many directions to form grape-like clusters, streptococci divide along a single axis. This specific way of dividing causes them to grow in long, bent, or twisted chains.

Scientists classify these bacteria using their hemolytic properties. Hemolysis describes how the bacteria affect red blood cells when grown on blood agar. There are three main types of hemolysis. Alpha-hemolytic species cause partial or incomplete hemolysis. They oxidize the iron in hemoglobin, which turns the agar a dark greenish color. This is why the viridans group is named after the Latin word for green. Beta-hemolytic species cause complete lysis, or the total rupture, of red blood cells. This creates clear or yellow transparent areas around the bacterial colonies.

Finally, gamma-hemolytic species cause no hemolysis at all.

Beta-hemolytic bacteria use specific tools called exotoxins to break down cells. One such enzyme is called streptolysin. There are two main types: Streptolysin O and Streptolysin S. Streptolysin O is an oxygen-sensitive cytotoxin. It interacts with cholesterol in the membranes of eukaryotic cells, such as red and white blood cells. Streptolysin S is an oxygen-stable cytotoxin. It affects immune cells like lymphocytes and helps the bacteria avoid being cleared by the host's immune system. Some species, like Streptococcus agalactiae, show special properties in the CAMP test when grown with Staphylococcus.

To organize these many species, scientists use the Lancefield grouping system. This method is a serotype classification based on specific carbohydrates found on the bacterial cell wall. Rebecca Lancefield developed this system while working at Rockefeller University. The groups are named with letters from A to W, though E, I, and J are excluded. In medical settings, doctors focus on specific groups. They look for alpha-hemolytic S. pneumoniae and the viridans group. They also watch for beta-hemolytic Lancefield groups A and B.

Group A streptococcus, or GAS, includes the species Streptococcus pyogenes. This bacterium is a major human pathogen. It can cause noninvasive infections like strep throat, impetigo, and scarlet fever. However, it can also cause invasive, severe infections. These include necrotizing fasciitis, which is sometimes called "flesh-eating" infection, and toxic shock syndrome. GAS is estimated to cause more than 500,000 deaths every year globally. Sometimes, the immune system's response to GAS can cause problems. For example, untreated infections can lead to rheumatic fever, where antibodies attack the body's own heart valves and joints.

Group B streptococcus, or S. agalactiae, presents different risks. It is a leading cause of meningitis and sepsis in newborns and the elderly. Because these bacteria can live in the female reproductive tract, they can be passed to a baby during birth. To prevent this, medical guidelines recommend testing pregnant women between 35 and 37 weeks of gestation. If a woman tests positive, doctors provide antibiotics during labor. This protocol can prevent 65% to 70% of early-onset sepsis cases in infants. Vaccines against Group III polysaccharides have also been shown to help prevent transmission.

Many streptococci are not harmful and are actually quite useful. Many species are part of the commensal human microbiota. This means they live naturally in our mouths, skin, intestines, and respiratory tracts without causing disease. They are even a necessary ingredient in making Emmentaler, or Swiss, cheese. Other groups, like Group C and Group G, are considered zoonotic pathogens. This means they can be passed from animals to humans. For example, S. equi can cause strangles in horses, while other species affect cattle. Understanding these diverse roles helps scientists manage both food production and public health.