What are bacterial cells that grow in pairs called?

Bacterial cells that grow in pairs are referred to as diplococci. This term specifically indicates that the bacteria are spherical in shape (cocci) and have arranged themselves in pairs due to their division process. Pairing occurs when the bacteria divide and remain attached to each other, which is characteristic of this specific grouping.

In the context of the other choices, spirilla are spiral-shaped bacteria, describing a different morphology; streptococci refer to chains of spherical bacteria, which reflects a different growth arrangement; and bacilli are rod-shaped bacteria, not related to the pairing of cocci. Therefore, diplococci is specifically and accurately the term for bacteria that grow in pairs, providing clarity on their structural arrangement.