Discuss the difference between Bacillus and bacilli, and the difference between Streptococcus and streptococci. Solution Bacillus is a rod shaped bacterium. Bacillus is singular(single bacterium) while bacilli is plural (a group of bacillus bacterium). Streptococcus is spherical shaped bactarium. Streptococcus is singular (single bacterium) while streptococci is plural (a group of streptococcus bacterium)..