coccobacillus coccobacillus a coccobacillus (plural coccobacilli), or bacilluscocco, is a type of bacterium with a shape intermediate between cocci (spherical bacteria) and bacilli (rod-shaped bacteria). coccobacilli, then, are very short rods which may be mistaken for cocci. coxiella burnetii haemophilus influenzae, gardnerella vaginalis, and chlamydia trachomatis are coccobacilli. aggregatibacter actinomycetemcomitans is a gram-negative coccobacillus prevalent in subgingival plaques. acinetobacter strains may grow on solid media as coccobacilli. bordetella pertussis is a gram-negative coccobacillus responsible for causing whooping cough. yersinia pestis, the bacterium that causes plague, is also coccobacillus. coxiella burnetii is also a coccobacillus. bacteria from the genus brucella are medically important coccobacilli that cause brucellosis. haemophilus ducreyi, another medically important gram-negative coccobacillus, is observed in sexually transmitted disease, chancroid, of third world countries.