Streptococcus bacteria and macrophage. Computer artwork of Streptococcus sp. bacteria (yellow) and a macrophage white blood cell (green). The macrophage, when activated, will engulf (phagocytose) the bacteria and destroy them as part of the body's immune response. 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.