Coccobacillus

Coccobacillus is a shape descriptor used in microbiology to refer to bacteria whose cells are intermediate between cocci (spheres) and bacilli (rods). The term denotes appearance rather than a taxonomic group, and coccobacilli can range from very short rods to oval or peanut-shaped cells. In practice, most clinically encountered coccobacilli are Gram-negative, though shape alone is not sufficient for identification.

Morphology can be pleomorphic and may vary with growth conditions, so laboratory reports often note coccobacillary forms rather than a definite coccoid or rod. Staining and culture characteristics, along with biochemical and molecular tests, are used to identify the organism to genus and species.

Common medically important coccobacilli include Haemophilus influenzae, Bordetella pertussis, Pasteurella multocida, Brucella spp., and Acinetobacter species. Haemophilus influenzae is Gram-negative and requires X (hemin) and V (NAD) factors for growth on enriched media. Bordetella pertussis is a strictly aerobic, fastidious organism that typically needs specialized media such as Regan-Lowe for culture. Pasteurella multocida grows on routine blood or chocolate agar and is associated with animal bites. Brucella spp. are small, slow-growing, Gram-negative coccobacilli that are zoonotic and require appropriate biosafety containment. Acinetobacter species are short Gram-negative rods or coccobacilli frequently implicated in nosocomial infections.

Clinical relevance varies by genus but the descriptor highlights organisms that present as short, oval, or rod-like cells, guiding initial laboratory interpretation and subsequent identification.