HAEMOPHILUS INFLUENZAE: CHARACTERISTICS, CULTURE, PATHOGENESIS, TREATMENT - BIOLOGY - 2025

Some strains of Haemophilus influenzae are capsulated and others are not. The capsules are typifiable according to the type of carbohydrate they have in the capsule. 6 different types are known designated by the letters a, b, c, d, e and f.

At the laboratory level they can be differentiated by using antisera agglutinating antibodies against the corresponding polysaccharide.

Capsulated strains are pathogenic. Those of type b are the most invasive and frequently isolated in serious infectious processes. While the noncapsulated are considered habitual microbiota and, although they can also cause infections, these are not usually invasive and do not represent a greater danger.

At the laboratory level, they are difficult to isolate, as they require highly enriched media for optimal development, such as chocolate agar or Levithal agar.

That is why this microorganism falls into the group of bacteria called demanding from the nutritional point of view, although some authors prefer to call them annoying microorganisms.

Your reaction to catalase and oxidase tests is positive. It is an aerobic and facultative anaerobic microorganism that grows well at 35-37 ° C, at pH 7.6 and with 5% CO ₂. From a biochemical point of view, this bacterium is classified into 8 biotypes as proposed by Kilian.

The classification is based on the results of certain tests, such as indole, ornithine, and urease.

Taken from the source: Pantigozo P, Aguilar E, Santivañez S, Quispe M. Haemophilus influenzae, serotyping and biotyping of patients with ARFs from Hospital EsSalud-Cusco. SITUA 2006, 15 (1,2): 31-36. Available at: sisbib.unmsm.

Morphology

Microscopic characteristics

Haemophilus influenzae is very similar to the rest of the species of the genus Haemophilus, since they are very small coco bacilli and their size ranges between 0.2-0.3 µm wide and 0.5-0.8 µm long.

However, Haemophilus influenzae seen under the microscope are very pleomorphic, that is, they can take different forms. These can be seen as very short bacilli (coccobacilli) or slightly more elongated bacilli, and even filamentous. With the Gram stain they stain red, that is, they are Gram negative.

Gram of a direct sample with Haemophilus influenzae and polymorphonuclear coccobacilli. Source: Microman12345

Morphologically they are very similar to the genus Pasteurella, but the latter have a bipolar coloration that differentiates them. They are not spore formers. They also do not have flagella, therefore they are immobile.

Macroscopic characteristics

After a 24-hour incubation on chocolate agar, very small colonies of convex shape 0.5 to 0.8 mm in diameter, granular in appearance, transparent and opaque develop.

At 48 hours of incubation, the colonies have grown somewhat more (1 to 1.5 mm). It should be noted that the colonies of the strains that have capsules tend to be more mucoid and much larger, measuring up to 3.0 mm.

Habitat

Haemophilus influenzae strains colonize the nasopharynx of healthy children and adults by 50-75%. It is also possible to find this strain in the genital tract of some individuals.

Lifecycle

Haemophilus influenzae is an exclusive human pathogenic bacterium, whose natural habitat is represented by the upper respiratory tract of the human being. There the bacteria can grow and thrive without causing harm to the host.

Reproduction is asexual by fission, in this type of reproduction, an organism first duplicates its genetic material, then forms two new cells from an original stem cell.

Culture

Haemophilus influenzae cultured on blood agar. Taken and edited from: See page for author.

The term Haemophilus is a compound word, it comes from two Greek words, 'haemo', which means blood, and 'philus' which comes to be affinity.

This bacterium lives up to its name, as it grows excellently on heated blood agar (chocolate agar). It can also grow on blood agar, but needs to be in the company of a S. aureus strain. The latter, being beta-haemolytic, breaks the erythrocytes and releases the factor V that Haemophilus needs.

This is how Haemophilus colonies can develop close to the S. aureus strain. This phenomenon is known as satelliteism and is frequently used as a diagnostic strategy.

Satellite test (growth of H. influenzae around the striatum of S. aureus on blood agar. Source: CDC-PHIL

It should be noted that the Haemophilus influenzae strain does not produce hemolysis on blood agar, which differentiates it from other Haemophilus species, such as H. haemolyticus and H. parahaemolyticus.

Diseases (pathogenesis)

This microorganism is transmitted by secretions, mainly respiratory (saliva and mucus) emitted by sick people or carriers of the bacteria.

Bacteria travel in secretions expelled when the patient sneezes or coughs. The bacteria spread in the environment and these are inhaled by the susceptible individual.

Haemophilus influenzae is a pyogenic microorganism, which is why it generates purulent secretions.

Among the main pathologies that it causes are meningitis, septicemia, pneumonia, epiglottitis, conjunctivitis and otitis, among others.

Septicemia

When bacteria enter the bloodstream it is called bacteremia and is the crucial step for the spread of the microorganism to other organs or tissues. When the microorganism multiplies in the blood it is called septicemia, this condition compromises the general condition of the patient.

Meningitis

Meningitis is a serious disease that causes a stiff neck, headache, vomiting or changes in behavior, in some cases leading to death. This infection is common in children.

Pneumonia

Magnification (1000x) of sputum from a sick person with pneumonia, caused by Haemophilus influenzae. Taken and edited from: Microman12345.

It presents as a serious complication of a previous respiratory tract infection, such as bronchitis or acute febrile tracheobronchitis. It presents with a high fever, dyspnea, or a productive cough with purulent sputum. It can coexist with bacteremia. This involvement is more common in older adults.

Conjunctivitis

Conjunctivitis presents with redness of the conjunctiva, burning, swelling of the eyelids, presence of purulent discharge or hypersensitivity to light (photophobia).

Sinusitis

It is an infection of the paranasal sinuses that causes nasal congestion and abundant discharge. The discharge may be liquid or thick, greenish or yellowish, and with or without blood. Other symptoms include: cough, fever, sore throat, and swollen eyelids. This involvement is generally caused by non-capsulated strains.

Epiglottitis

It has a severe and sudden sore throat, fever, muffled voice or inability to speak, drooling, among other signs. This occurs due to obstructive laryngeal edema generated by the infection. It can cause death by suffocation.

Diagnosis

The best way to make the diagnosis is through culture. The sample will depend on the infectious process.

If meningoencephalitis is suspected, a sample of cerebrospinal fluid should be taken by lumbar puncture for cytochemical study and culture. In the event of septicemia, blood samples will be taken to perform a series of blood cultures.

If the process is conjunctivitis, secretion emitted by this mucosa will be taken. In the case of pneumonia, a sample of sputum or bronchial lavage is cultured.

The samples are seeded on chocolate agar and incubated aerobically with 5% CO ₂ for 48 hours of incubation.

Identification can be done using manual biochemical tests or also by automated systems such as the VITEK 2.

Serotyping is carried out through the agglutinating antisera. Haemophilus influenzae strains that do not react to any antiserum are classified as noncapsulated or nontypeable.

Levithal agar allows one to distinguish between capsulated and non-capsulated strains.

Treatment

Haemophilus influenzae can be treated with beta-lactams, such as ampicillin, ampicillin / sulbactam, amoxicillin / clavulanic acid, piperacillin / tazobactam. In severe infections, third-generation cephalosporins are usually used, such as: ceftriaxone, cefotaxime and cefoperazone or carbaperazones.

It is important to note that ampicillin is no longer being used because currently most of the isolated strains are resistant to this antibiotic, due to the production of a beta-lactamase.

Macrolides and quinolones can also be used.

However, the most advisable thing is to carry out the antibiogram and place treatment according to the reported sensitivity.

Prevention

After the introduction of the vaccine against Haemophilus influenzae type b, the decrease in cases of meningitis caused by this microorganism decreased significantly.

Currently the capsular antigen type b (polyribosyl-ribitol-phosphate) of H. influenzae is included in the pentavalent vaccine that also prevents against diphtheria, tetanus, pertussis and hepatitis B.

The vaccine is given in 3 or 4 doses. The 4-dose schedule is as follows:

The first dose is started at 2 months of age. Then two more doses are given every two months (that is, at 4 and 6 months of age). Finally the fourth dose is placed 6 or 9 months after placing the third. The last dose represents the boost.

Contraindication of the vaccine

The vaccine is contraindicated in:

- Patients who have presented a severe anaphylactic (allergic) reaction against the components of the vaccine.

It is important not to confuse side effects with an anaphylactic reaction. Anaphylactic reactions are serious immune responses that compromise the life of the patient. Meanwhile, the adverse effects can be local redness at the puncture site and fever.

- It is also contraindicated in patients who are sick or who have low defenses. In these cases, it is necessary to wait for normal conditions to recover in order to administer the vaccine.

- Finally, vaccination is not recommended in children under 6 weeks of age.

References

1. Sakurada A. Haemophilus influenzae. Rev. chil. infectol. 2013; 30 (6): 661-662. Available in: scielo
2. Nodarse R, Bravo R, Pérez Z. Haemophilus Influenzae meningoencephalitis in an adult. Rev Cub Med Mil 2000, 29 (1): 65-69. Available at: scielo.sld
3. Toraño G, Menéndez D, Llop A, Dickinson F, Varcárcel M, Abreu M et al. Haemophilus influenzae: Characterization of isolates recovered from invasive diseases in Cuba during the 2008-2011 period. Vaccimonitor 2012; 21 (3): 26-31. Available in: scielo.
4. Leivaa J, and Del Pozo J. Slow-growing gram-negative bacilli: HACEK, Capnocytophaga and Pasteurella group. Enferm Infecc Microbiol Clin. 2017; 35 (3): 29-43. Available at: Elsevier.
5. Valenza G, Ruoff C, Vogel U, Frosch M, Abele-Horn M. Microbiological evaluation of the new VITEK 2 Neisseria-Haemophilus identification card. J Clin Microbiol. 2007; 45 (11): 3493–3497. Available at: ncbi.nlm.
6. Pantigozo P, Aguilar E, Santivañez S, Quispe M. Haemophilus influenzae, serotyping and biotyping of patients with ARFs at Hospital EsSalud-Cusco. SITUA 2006, 15 (1,2): 31-36. Available at: sisbib.unmsm.