Streptococcus bacteria. Coloured scanning electron micrograph (SEM) of Streptococcus sp. bacteria. Streptococci are round Gram-positive bacteria. Some species are part of the normal flora of the human gut or mouth and do not cause disease in healthy individuals. However, some, such as Streptococcus pyogenes, which causes tonsillitis and pharyngitis, are pathogenic to humans.
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