Haemophilus Influenzae

Unencapsulated strains of Haemophilus influenzae can be normally present in the human respiratory tract [1]. However, pathogenic strains can cause diseases in all age groups, from children to adults. Some notable diseases caused by H. influenzae in children include epiglottitis, meningitis, and septic arthritis [2].

Staining and microbiologic features:

• Gram-negative coccoid bacilli [1]

File:Haemophilus influenzae Gram.JPG by Bobjgalindo is licensed under CC BY-SA 4.0.

• It can be encapsulated (containing a polysaccharide capsule) or unencapsulated [2]
• Humans are the primary reservoir (obligate human pathogen) [3]
• Factor X (hemin) and V (NAD) are required for its growth and metabolic activity [4]

H. influenzae XV by User:Graham Beards is licensed under CC BY-SA 3.0.

• H. influenzae grow on chocolate agar [4]
• It exhibits Satellite Phenomenon: H. influenzae itself does not grow well on sheep blood; however, when it is near colonies of Staphylococcus aureus, it shows better growth in the region surrounding the staphylococcal colonies. This enhanced growth is because Staphylococcus aureus releases NAD (nicotinamide adenine dinucleotide) into the surrounding medium, which H. influenzae can utilize as a cofactor for its metabolic processes. [1]

Hinfluenzae satillite by Deminorwood is licensed under CC BY-SA 4.0.

Virulence:

•  Polysaccharide capsule contains polyribitol phosphate, and this capsule shields them from phagocytosis unless specific antibodies are present to coat (opsonize) them [5].
• Exotoxin is absent [1]
• Both the typeable and nontypeable strains of H.influenza can lyse IgA antibodies by producing IgA1 protease [5]
• It uses pili and nonpilus adhesins to colonize the upper respiratory tract [5]
• Some strains can produce β-lactamase. [6]

Transmission:

Respiratory (through respiratory secretions). [3]

Susceptible groups:

• Individuals with some immunodeficiencies or depletion of the complement system [5]
• Patients with asplenia/splenectomy [5]
• Children between six months and two years of age: Because maternal antibodies are protective for the first six months, and it can take around three to five years for children’s immune systems to develop their protective antibodies against H.influenza. [2]

Diseases and complications:

• Non-typeable/unencapsulated strains cause most H.influenza infections [7]
• Please note that Flu is caused by influenza virus (not by H.influenza) [7]
• Meningitis: H. influenzae type b can cause meningitis. The affected patient can present with confusion, lethargy, fever, headaches, nausea, vomiting, increased sensitivity to light, neck stiffness, presence of Kernig and Brudzinski signs [8]. Neck stiffness can be absent in children. [2]
• Following antibiotic treatment, bacterial lysis and the release of lipopolysaccharide lipid A may lead to an aggressive immune reaction, causing neuronal damage and potentially resulting in neurological deficits. Some studies indicate that administering steroids before antibiotic treatment may help mitigate the inflammatory response and reduce the risk of neurological deficits. [2]
• Epiglottitis: Patients have a high fever, severe sore throat stridor, drooling, and breathing difficulty due to swelling and subsequent obstruction of the upper airway. It is a medical emergency and may require intubation. A neck X-ray will show a thumbprint sign. [7, 9]

File:Epiglottitis.jpg by Med Chaos is marked with CC0 1.0.

• Septic arthritis: In infants, H. influenzae type b is one of the leading causes of septic arthritis. The affected patient can present with fever, joint swelling, joint pain, and limited range of motion. [9]
• Sepsis
• Pneumonia
• Non-typeable/unencapsulated strains of H.influenzae type b can cause otitis media and sinusitis [10]

Diagnostic testing:

• Gram staining and culture: It shows optimum growth at 37°C and high CO₂. [10]

Haemophilus influenzae by Stefan Walkowski is licensed under CC BY-SA 4.0.

Haemophilus influenzae colony morphology on chocolate agar by Ajay Kumar Chaurasiya is licensed under CC BY-SA 4.0.

• Antigen detection using ELISA or latex particle agglutination test [10]
• Quellung test [10]

References:

1. Jawetz, Melnick, & Adelberg’s Medical Microbiology Twenty-Seventh Edition (page no: 263)
2. CMMRS edition 6, 2016-17 (page no: 96)
3. CMMRS edition 6, 2016-17 (page no: 100)
4. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 244)
5. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 245)
6. Jawetz, Melnick, & Adelberg’s Medical Microbiology Twenty-Seventh Edition (page no: 265)
7. First Aid for USMLE step 1, 2021 edition (page no: 142)
8. Step-Up to Medicine, 6th edition (page no: 373)
9. CMMRS edition 6, 2016-17 (page no: 97)
10. CMMRS edition 6, 2016-17 (page no: 101)