Genus Streptococcus: habitat, morphology, culture and biochemical characteristics

Gaurab Karki — Sat, 09 Dec 2017 06:48:24 +0000

The genus streptococcus include a large number of species of Gram +ve, catalase –ve cocci arranged in chain of varying length and also in pairs.
Streptococcus form chains during growth due to successive cell division occurring in one plane only and daughter cells failing to separate completely.

Habitat:

Some species are part of normal flora of Human and animals. Streptococcus salivarius
some are important human pathogens eg. Streptococcus pyogens, S. pneumoniae, S agalactiae.

The individual cocci are spherical or ovoid, 0.5-1 µm in diameter and are arranged in chains.
The length of chain vary widely with cultural conditions. Larger chain are formed in liquid or broth than in solid agar.
Streptococci are gram positive but they may become gram negative in ageing when bacteria die.
Streptococci are non-motile and non-spore forming.
Most species possess capsule. Group A and group C streptococci possess hyaluronic acid capsule while group B and D possess polysaccharide capsule.

Streptococci are aerobic or facultative anaerobes.
They grow best at temperature 37°
Streptococci grow well in media containing blood and sugar. 10 % CO2 promotes growth and haemolysis.
Selective media containing aminoglycosides or 1: 500,000 crystal violet selectively permit growth of Streptococci by inhibiting other bacteria.
On blood Agar: colonies are small, circular, semi-transparent, convex with small clear zone of hemolysis. Growth and hemolysis increases y 10% CO2. Streptococci are classified on the basis of hemolysis, α-hemolysis, β-hemolysis and no-hemolysis group.
Streptococci are non-halophilic but can tolerate up to 6.5% NaCl (halotolerant)
Tolerates 0.1% methylene blue and bile esculin agar.
Streptococci ferment most sugars such as sorbitol. Trehalose, lactose maltose,mannitol etc with production of acid but no gas.
Catalase (–) negative