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.
This allows Haemophilus influenzae to escape destruction and attach to epithelial cells in the airways. Also, on the capsule, there are pili, which are hair-like extensions, and adhesion proteins such as HMW1 and HMW2 that help the bacteria attach to host cells.
Both encapsulated and unencapsulated strains have an outer membrane - which consist of lipooligosaccharides, or LOS.
LOS inhibits mucociliary clearance, or the self-clearing mechanism of the bronchi, that would normally sweep the bacteria out of the respiratory tract. With the help of LOS, Haemophilus influenzae can colonize the respiratory tract.
What is more, both encapsulated and unencapsulated strains make IgA protease, a toxic protein that destroys Immunoglobulin A, or IgA.
IgA is an immune system protein found in the nasopharyngeal mucosa secretions that normally opsonizes invading bacteria, meaning it tags them so neutrophils can recognize and destroy them. So IgA protease neutralizes the first line of mucosal defense!
Interestingly, unencapsulated strains have two other abilities that allow it to evade the immune system. First, these strains can change the oligosaccharides they express on their outer membrane each time they infect a cell, and this process is called phase variation.
Because of phase variation, the immune system can’t remember the infecting strain, so it can’t mount a quick specific immune response against it when it encounters the same strain in the future.
Finally, unencapsulated strains have the ability to produce biofilms. A biofilm is basically a layer of goop-like material made of exopolysaccharides or EPS, within which Haemophilus influenzae bacteria live and reproduce.
Comparing a biofilm to strawberry jam, the seeds would be the bacteria and the rest of the jam would be the EPS. The bacteria in the biofilm hide from the host's immune system and antibiotics.
Ok, now, encapsulated strains tend to cause more invasive disease on account of their capsule. One way is to spread directly from the nasopharynx to the epiglottis, causing epiglottitis, or to the soft skin tissues in the face, causing cellulitis.
Encapsulated strains can also breach through the epithelium of the nasopharynx and invade the blood capillaries directly, causing bacteremia.
From the bloodstream, it can spread to distant sites, like the meninges, causing meningitis, the bones, causing osteomyelitis or the joints, causing septic arthritis.
Nonencapsulated strains of Haemophilus influenzae, on the other hand, are less invasive, and they mostly cause mucosal infections by direct extension.
For example, they may go up the eustachian tubes and reach the middle ear, causing otitis media. If they spread to the paranasal sinuses, they cause sinusitis.
Finally, if they go down the respiratory tract, they may cause bronchitis, which is the inflammation of the bronchi, or pneumonia, which is the inflammation of the lungs.
Now, nonencapsulated strains of Haemophilus influenzae colonize the nasopharynx of 40 to 80% of children and adults, whereas encapsulated strains, such as Hib, colonizes 3 to 5% of children aged 2 to 5 years.
Either way, Haemophilus influenzae is mostly transmitted via respiratory droplets and respiratory secretions. Risk factors vary depending on the infecting strain.
