Haemophilus influenzae (formerly called Pfeiffer's bacillus or Bacillus influenzae) is a Gram-negative, non-motile, coccobacillary, facultatively anaerobic, capnophilic pathogenic bacterium of the family Pasteurellaceae. The bacteria are mesophilic and grow best at temperatures between 35 and 37℃. H. influenzae was first described in 1893 by Richard Pfeiffer during an influenza pandemic when he incorrectly described Haemophilus influenzae as the causative microbe, which is why the bacteria retain the name "influenza". H. influenzae is responsible for a wide range of localized and invasive infections, typically in infants and children, including pneumonia, meningitis, or bloodstream infections. Treatment consists of antibiotics, however H. influenzae is often resistant to the penicillin family but augmentin can be used in mild cases. The recommended form of prevention of the type b form of H. influenzae is a series of the Hib vaccine and boosters, which are most often given under the age of 5, and sometimes in conjunction with other vaccines in the form of the DTaP-IPV/Hib vaccine. This species was the first free-living microorganism to have its entire genome sequenced. Serotypes In 1930, two major categories of H. influenzae were defined: the unencapsulated strains and the encapsulated strains. Encapsulated strains were classified on the basis of their distinct capsular antigens. The six generally recognized types of encapsulated H. influenzae are: a, b, c, d, e, and f. H. Influenzae type b, also known as Hib, is the most common form, recognizable by its polyribosyl ribitol phosphate (PRP) capsule, and found mostly in children. Types a, e, and f have been isolated infrequently, while types d and c are rarely isolated. Unencapsulated strains are more genetically diverse than the encapsulated group. Unencapsulated strains are termed nontypable (NTHi) because they lack capsular serotypes; however, they can be classified by multilocus sequence typing. NTHi strains are considered to be part of the normal human flora in the upper and lower respiratory tract, genitals, and conjunctivae (mucous membranes of the eye). Physiology Structure H. Influenzae is a small Gram-negative bacterium, approximately 0.3 micrometer to 1 micrometer. Like other Gram-negative bacteria, H. influenzae has a thin peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharide. Some types of H. influenzae contain a polysaccharide capsule around the outer membrane to aid in protection and colonization. The bacteria are pleomorphic, meaning the shape of the bacterium is variable, however it is typically coccobacillus or rod-shaped. H. Influenzae contains pili, which are specialized to