HAEMOPHILUS

The Haemophilus genus represents a large group of Gram-negative rods that like to grow in blood. The blood medium provides two factors which many Haemophilus species require for growth: X factor and V factor. Sometimes Haemophilus is cultured using something called the "Staph streak" technique. In this procedure, both Staphylococcus and Haemophilus organisms are grown together on blood agar. Haemophilus colonies will usually form small colonies called "satellites" around Staphylococccus colonies because Staph can provide the necessary factors required for optimium Haemophilus growth. Morphologically, Haemophilus bacteria usually appear as tiny coccobacilli under the microscope, but they are included with the Pleomorphic bacteria because of the many shapes they assume. A methylene blue stain of a smear can also help with identification. Haemophilus species are classified by their capsular properties into six different serological groups, (a-f). Species which possess a type b capsule are clinically significant because of their virulent properties.

H. influenzae

(under the microscope)
Infection by H. influenzae is common in children and its name may lead you to conclude that it is the causative agent of the flu. Actually, this bacterium causes a secondary respiratory infection that usually inflicts those who already have the flu. This species may exist with or without a pathogenic polysaccharide capsule. Although both strains occur as natural flora of the nose and pharynx, they can confer severe illness in patients that are immunosuppressed or that have pre-existing respiratory ailments. Strains that lack the capsule usually cause mild localized infections (otitis media, sinusitis), as opposed to the type b encapsulated strain of H. influenzae that can cause several serious infections. Most of these infections occur in unvaccinated children less than five years of age because they have not yet formed antibodies against the bacterium. H. influenzae infection can lead to a variety of diseases:

• Meningitis- H. influenzae is the most common cause of bacterial meningitis in children between the ages of five months and five years. The initial respiratory infection can spread to the blood stream and eventually the central nervous system. A stiff neck, lethargy, and the absence of the sucking reflex are common symptoms in infected babies. A vaccine is available, but is not always effective in very young children. Adult meningitis is much less common and usually only occurs in those predisposed to illness.
• Epiglottitis- H. influenzae is the number one cause of this potentially fatal disease, which may cause airway obstruction in children between the ages of 2 and 4.
• Haemophilus infection has also been associated with chronic bronchitis, pneumonia, bacteremia, conjuctivitis, and a host of other illnesses.

OTHER SPECIES

H. aegyptius

H. ducreyi