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a genus of bacteria of the family Lactobacteriaceae. They are chiefly spherical bacteria, which are found in pairs as a result of division in a single plane; at times they form short chains. They are gram positive. The genus has only one species, D. pneumoniae (pneumococcus), which has more than 80 serotypes. Diplococcus is one of the causative agents of lobar pneumonia. Certain Diplococcus— those that are gram negative and the pisiform bacteria—are classified in the genus Neisseria, family Neisseriaceae.