Definition
The term streptococci refers to a type of bacteria that have certain common morphological and functional characteristics. Most streptococci are harmless and belong to the normal human flora. Only a few can cause infections.
What groups of streptococci are there?
Streptococci are divided into three groups. First, the so-called alpha-hemolytic streptococci are distinguished from the beta-hemolytic streptococci. The differentiation is made by how they break down the red blood pigment haemoglobin, i.e. haemolyse.
The pneumococci and Viridans pneumoniae, which also belong to the streptococci, belong to the alpha-hemolytic streptococci. The beta-haemolytic streptococci are further subdivided into the A-, B- and D-streptococci. This further subdivision is done by different sugar chains embedded in the bacterial wall.
The A-streptococci belong to the beta-hemolytic streptococci. The best known pathogen of the A-streptococci is the Streptococcus pyogenes. This pathogen triggers infections especially in the nasopharyngeal area.
Examples of this are acute tonsillitis (angina tonsillaris), i.e. an inflammation of the tonsils, scarlet fever, inflammation of the middle ear or various infections of the soft tissues. The most prominent pathogen from the group of B-streptococci is Streptococcus agalactiae. It is a typical trigger of meningitis in newborns (neonatal meningitis) or neonatal sepsis, colloquially known as blood poisoning.
In adults, Streptococcus agalactiae can cause wound and bone infections, as well as inflammation of the urethra (urethritis). Streptococcus mutans belongs to the group of alpha-hemolytic streptococci. It is one of the main causes of caries.
This pathogen has some special properties that help it to form bacterial plaques on the tooth surface. For example, the bacterium can convert carbohydrates into lactic acid. This makes the environment around the tooth more acidic, which is harmful to the tooth substance.
Furthermore, Streptococcus mutans can produce certain proteins that reduce the local immune defence in the mouth, so that the body cannot fight the bacterium effectively. Only a few streptococci can cause infections in humans. How dangerous they are then depends, among other things, on the stage of life and the strength of the immune system. Among the human pathogenic streptococci, i.e. those that can cause disease in humans, are the pneumococci, the viridans streptococci, streptococcus pyogenes, streptococcus agalactiae and the enterocococci, which strictly speaking also belong to the streptococci.