Haemophilus influenzae


  • Haemophilus influenzae is a non-motile Gram-negative, coccobacillary, facultatively anaerobic bacterium, and it’s Oxidase and Catalase postive.
  • It was mistakenly considered to be the cause of influenza and so it kept its name since that time “bacterial influenza”
  • There are two major groups H.influenzae:
  1. The encapsulated group. and this group consists of six types, they are (a,b,c,d and f),The capsule has a role in virulence as it gives protection from phagocytosis, so the non-capsulated strains of H.influenzae  are usually less invasive
  2. Non-capsulated group, The non-encapsulated strain of H. influenzae is present in the nasopharynx of approximately 75 percent of healthy children and adults so H. influenzae cultured from the nasopharyngeal cavity or sputum would not indicate H. influenzae disease, because these sites are colonized in disease-free individuals. However, H. influenzae isolated from cerebrospinal fluid or blood would indicate H. influenzae infection.
  • H. influenzae type b (Hib) is the most common bacterium that cause disease such as bacteremia, pneumonia, epiglottitis and acute bacterial meningitis
  • The bacterial transmission spread person-to-person by direct contact or through respiratory droplets like coughing and sneezing.
  • Clinical diagnosis of H. influenzae is typically performed by bacterial culture or latex particle agglutinations.
  • Haemophilus influenzae requires X(Hemin) and V(NAD) factors for growth so H. influenzae culture is performed on chocolate agar, which contain  X (hemin) and V (NAD) factors and the plate is placed at 37°C in a CO2-enriched incubator.
  • H. influenzae  can’t grow on Blood agar as it lacks the growth factors X and V but in special case the growth is only achieved as a satellite phenomenon around other bacteria for example H. influenzae will grow in the hemolytic zone of Staphylococcus aureus on blood agar plates; the hemolysis of cells by S.aureus releases factor V which is needed for its growth. H. influenzae will not grow outside the hemolytic zone of S.aureus due to the lack of nutrients such as factor V in these areas.
  • Colonies of H. influenzae appear as convex, smooth, pale, grey or transparent colonies.
  • Use of antibiotics prior to sample collection greatly reduces the isolation rate by killing the bacteria before identification is possible
Haemophilus influenzae on Chocolate Agar

Haemophilus influenzae on Chocolate Agar

  • Gram-stained and microscopic observation of a specimen of H. influenzae will show Gram-negative, rod shaped, with no specific arrangement
  • Non-encapsulated organisms from sputum are pleomorphic and often exhibit long threads and filaments. The organism may appear Gram-positive unless the Gram stain procedure is very carefully carried out.
Haemophilus influenzae gram stain - pink pleomorphic rods

Haemophilus influenzae gram stain – pink pleomorphic rods


  • Other Methods of H.influenzae diagnosis:
  • The latex particle agglutination test (LAT) is a more sensitive method to detect H. influenzae than culture. Because the method relies on antigen rather than viable bacteria, the results are not disrupted by prior antibiotic use.
  • (PCR) assays have been proven to be more sensitive than either LAT or culture tests, and highly specific.


  • There’s a vaccine that can prevent disease caused by Hib (Type B), but not the other types of Haemophilus influenzae bacteria and it’s recommended for children under age 5 and patients with splenectomy.


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