Haemophilus influenzae


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Haemophilus influenzae


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Haemophilus influenzae


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USMLE® Step 1 questions

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High Yield Notes

9 pages


Haemophilus influenzae

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USMLE® Step 1 style questions USMLE

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A 6-year-old boy is brought to the emergency department with increased work of breathing. His parents, who are at the bedside, recall that the patient developed a “stuffy nose” two days ago. This morning, he was noted to have a hoarse voice, drooling, and increased work of breathing. The patient was born full-term at home and has rarely seen a physician. Past medical history is otherwise noncontributory. Temperature is 39.4°C (103°F), pulse is 140/min, respirations are 32/min, and blood pressure is 90/50 mmHg. On physical examination, the patient is noted to be leaning forward with the neck hyperextended and chin thrust forward. The patient is drooling and has stridorous breathing. Radiographic imaging reveals the following finding:

Image reproduced from Radiopedia

Which of the following best describes the medication that would have been most effective in preventing this patient’s current symptoms?

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Haemophilus influenzae p. , 140


Haemophilus influenzae p. , 140


for Haemophilus influenzae p. , 140

Chocolate agar

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Conjunctivitis p. 553

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biofilm production p. 127

cephalosporins p. 186

chloramphenicol p. 189

culture requirements p. 124

Gram-negative algorithm p. 139

influenza p. 166

penicillins for p. 185

pneumonia p. 176

postviral infection p. 176

rhinosinusitis p. 697

vaccine p. 177

Haemophilus influenzae type B


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Otitis media

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Pneumonia p. 707

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Vaccines p. 109

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Content Reviewers

Viviana Popa, MD


Alexandru Duhaniuc, MD

Evan Debevec-McKenney

Elizabeth Nixon-Shapiro, MSMI, CMI

Haemophilus influenzae is a small Gram-negative coccobacillus which can normally colonize the human respiratory tract. There are two major categories of H. influenzae - encapsulated strains and unencapsulated strains.

Encapsulated strains are classified into six serotypes based on their capsular antigens - a, b, c, d, e and f, and unencapsulated strains are called nontypable, because they lack the polysaccharide capsule, and, consequently, capsular antigens.

The strains that cause disease in humans are most often Haemophilus influenzae type b, or Hib for short and Haemophilus influenzae nontypable.

Now, Haemophilus influenzae has a thin peptidoglycan layer, so it doesn’t retain the crystal violet dye during Gram staining. Instead, like any other Gram-negative bacteria, it stains pink with safranin dye.

And since it’s a coccobacillus, it’s shaped somewhere between round, like a coccus, and linear, like a bacillus. Haemophilus influenzae is non-motile, so it doesn’t move, and facultative anaerobic which means it can survive both in aerobic and anaerobic environments. It’s also catalase and oxidase positive which means it produces both these enzymes.

Finally, Haemophilus influenzae can be cultivated on chocolate agar, because this medium contains essential nutrients that Haemophilus influenzae needs to grow, like factor X, also called hemin, and factor V, also called nicotinamide adenine nucleotide.

Another way to grow it is to grow it with Staphylococcus aureus colonies, on blood agar, which provides factor V via red blood cells hemolysis.

On both blood agar and chocolate agar, Haemophilus influenzae grows into convex, smooth, gray or transparent colonies. Now, Haemophilus influenzae has a number of virulence factors, that are like assault weaponry that help it attack and destroy the host cells, and evade the immune system.

So first, encapsulated strains of Haemophilus influenzae are covered by a polysaccharide layer called a capsule. Now, this capsule is a major virulence factor for Haemophilus influenzae because of its antiphagocytic ability, meaning that it protects the bacteria against phagocytosis by macrophages or neutrophils.


Haemophilus influenzae, or just H. influenzae, is a gram-negative, facultatively anaerobic, non-motile coccobacillus. Haemophilus influenzae bacteria are classified into encapsulated strains and unencapsulated strains. Based on their capsular antigen type, encapsulated strains are further classified into six serotypes (serotypes a, b, c, d, e, and f). Unencapsulated strains are also referred to as nontypable because they lack the polysaccharide capsule, and capsular antigens.

Nontypeable strains of Haemophilus influenzae are known to cause relatively simple mucosal infections, like otitis media, sinusitis, and pneumonia. On the other hand, encapsulated strains such as Haemophilus influenza type b can cause more severe infections such as meningitis and epiglottitis, but such severe infections are not so common nowadays due to vaccination.


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