Photomicrograph of Haemophilus influenzae as seen using a Gram-stain technique. Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, is a Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe.