Which of the following is NOT a shape of bacterial cells?

Bacterial cells can exhibit a variety of shapes, among the most common being spherical (cocci), spiral (spirilla or spirochetes), and rod-shaped (bacilli). These shapes are fundamental to identifying and categorizing different types of bacteria.

Spherical bacteria, known as cocci, can exist as single cells, in pairs, chains, or clusters, which is an important characteristic in microbiology taxonomy. Spiral bacteria have a twisted shape that allows them to move in a corkscrew motion, aiding in their mobility and helping them navigate through viscous environments. Rod-shaped bacteria, or bacilli, are often found in chains, and this shape can influence their function and ecological niche.

Hexagonal shapes, while found in some organisms such as certain algae or fungi, are not typical for bacterial cells. Thus, in the context of microbial morphology, hexagonal is not recognized as a standard bacterial shape. This makes it the correct choice for the question, as it does not fit within the common classifications of bacterial cell shapes.