Streptococcus

Streptococci are gram positive bacteria that are arranged in chains. Cell division takes place along a single axis without separation of daughter cells. Hence they get arranged in chains.  They are part of the normal flora of humans. But some of them are pathogens.  Most important among them is Streptococcus pyogenes causing pyogenic infections that tend to spread along the tissue planes. (This is in contrast to Staphylococcus infections that tend to be localised.)