A grouping of four bacterial cells is referred to as what?

A grouping of four bacterial cells is referred to as a tetrad. This term specifically describes the arrangement of cocci (spherical bacteria) that divide in two planes, resulting in four cells that remain attached to each other in a square-like formation.

In contrast, other terms such as diplococcus refer to a pair of cocci that remain together after cell division, while staphylococcus describes bacteria that form irregular, grape-like clusters due to division in multiple planes. Streptococcus indicates a chain of cocci that remain attached after division in one plane. Therefore, the term tetrad is precise for defining a group of four bacteria, making it the correct answer.