Bacilliform

Bacilliform is a descriptive term in microbiology used to denote a bacterium that is rod-shaped, or bacillus-like, in contrast to cocci (spherical) or spirilla (spiral) forms. The term derives from the Latin bacillus for "little staff" and the suffix -iform meaning "in the form of." Bacilliform bacteria are elongated, cylindrical cells; their dimensions vary by species, but typically have a length several times their width. The shape is established by microscopic examination and does not by itself identify genus or species.

The descriptor is used across bacterial taxa; it does not denote a taxonomic grouping. For example, Bartonella bacilliformis is a rod-shaped bacterium that causes Carrion's disease in humans, while Bacillus species such as Bacillus anthracis are also rod-shaped (bacillary), though belonging to different genera. Other common Gram-negative rods and Gram-positive rods share the bacilliform morphology.

In clinical and laboratory contexts, noting bacilliform morphology helps guide differential diagnoses after sample staining (e.g., Gram stain) and light microscopy. However, morphology is insufficient to determine identity; culture characteristics, biochemical tests, and molecular methods are needed for definitive classification. The term is used primarily as a descriptive aid rather than a taxonomic category, and it may appear alongside other descriptors such as coccobacillary or diplobacillary to convey variations in cell shape.