The same bacterial genus gives us strep throat and Swiss cheese
Streptococcus thermophilus helps turn milk into yoghurt and Emmentaler. A close relative, Streptococcus pyogenes, causes strep throat and is estimated to kill more than 500,000 people a year. Most members of the genus simply live in our mouths and guts, and a few switch between harmless neighbour and dangerous invader.
The Viennese surgeon Albert Theodor Billroth coined the name in 1877 from Greek words meaning twisted and grain. Because streptococci divide along a single axis, they grow in pairs or chains that can look bent, unlike staphylococci, which divide in several directions and form grape-like clumps. In 1984 many former members were moved into the new genera Enterococcus and Lactococcus, leaving more than 50 recognised species. A 2018 study by Patel and Gupta used 134 molecular signatures to sort the genus into two main clades and 14 subclades.
Hospital laboratories still sort them by how they treat red blood cells on blood agar. Alpha-haemolytic species turn the agar greenish by converting haemoglobin to biliverdin; this group includes S. pneumoniae, a leading cause of bacterial pneumonia, and the viridans streptococci, whose name comes from the Latin for green. Beta-haemolytic species burst red cells completely, leaving clear zones, thanks to toxins called streptolysins. Gamma-haemolytic species leave the cells alone.
Beta-haemolytic types are subdivided by Rebecca Lancefield's system, developed at Rockefeller University, which reads carbohydrates on the cell wall; 21 groups run from A to W. Group A infections are usually mild, like strep throat, impetigo and scarlet fever, but invasive forms include toxic shock syndrome and necrotising fasciitis. Untreated infection can also trigger rheumatic fever, in which antibodies made against the bacterium cross-react with the body's own heart valves, joints and kidneys.
Group B, S. agalactiae, is the most common cause of meningitis in babies aged one to three months and can pass from mother to newborn. The United States screens pregnant women by culture, while the United Kingdom treats on risk factors, a policy that gives antibiotics during labour to 15 to 20 percent of women and prevents 65 to 70 percent of early-onset infections. Group C includes S. equi, the cause of strangles in horses.
Source: Streptococcus
