H. influenzae – Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes

H. influenzae - Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes


Haemophilus influenzae (often abbreviated as H. influenzae) is a small, Gram-negative coccobacillus bacterium that can cause a variety of infections in humans. Despite its name, it does not cause influenza (the flu); influenza is caused by a virus. The name originated from initial isolation attempts during the 1892 influenza pandemic, as it was mistakenly believed to be the causative agent. Here are some key points about this bacterium:

  1. Types: H. influenzae can be categorized into two main types based on the presence or absence of a polysaccharide capsule:
    • Encapsulated (Typeable): There are six identifiable types (a through f) based on their capsules. Type b (Hib) was the most common cause of invasive disease before the introduction of vaccines.
    • Non-encapsulated (Nontypeable or NTHi): These strains lack the capsule and are not typable by conventional methods. They commonly cause respiratory tract infections but rarely cause invasive diseases.
  2. Diseases: H. influenzae can cause various diseases, including:
    • Meningitis: Before the introduction of Hib vaccines, H. influenzae type b was a major cause of bacterial meningitis in children.
    • Epiglottitis: An inflammation of the epiglottis that can obstruct the airway.
    • Pneumonia
    • Otitis media: Middle ear infection.
    • Sinusitis
    • Septic arthritis: Infection in the joints.
  3. Vaccination: The Hib vaccine, which protects against H. influenzae type b, has greatly reduced the number of cases of invasive disease caused by this strain. It’s now a routine childhood vaccine in many countries.
  4. Transmission: H. influenzae is spread from person to person through respiratory droplets. Individuals can carry the bacterium in their nose and throat without being ill, serving as carriers.
  5. Treatment: Treatment typically involves antibiotics. However, antibiotic resistance, particularly to ampicillin, has been reported, so susceptibility testing can be important.
  6. Risk Groups: While anyone can get an H. influenzae infection, certain groups are at higher risk, such as young children (especially for Hib), the elderly, and those with certain chronic conditions or compromised immune systems.


Haemophilus influenzae is a bacterium with distinct morphological characteristics that help in its identification in a microbiology lab setting. Here’s a brief overview of its morphology:

  1. Shape and Arrangement:
    • Shape: H. influenzae is a coccobacillus, meaning it has a shape that is intermediate between cocci (spherical bacteria) and bacilli (rod-shaped bacteria). So, it’s often described as a small, short rod.
    • Arrangement: It typically occurs singly or in pairs. It doesn’t form long chains or clusters.
  2. Gram Stain:
    • H. influenzae is a Gram-negative bacterium. When subjected to a Gram stain, it will not retain the violet crystal stain and will instead take up the counterstain, appearing pink or red under the microscope.
  3. Size:
    • It is a small bacterium, usually about 0.5 to 1.0 micrometers in width and 1.0 to 2.0 micrometers in length.
  4. Capsule:
    • Certain strains (like H. influenzae type b, Hib) possess a polysaccharide capsule which can be visualized using special stains, like the Quellung reaction. The presence of a capsule is clinically significant because it contributes to the bacterium’s virulence and its ability to evade the host’s immune system.
  5. Growth Requirements:
    • H. influenzae has specific growth requirements and grows best on chocolate agar, a medium that contains factors X (hemin) and V (NAD or NADP), which the bacterium needs. On blood agar, satellite phenomenon can be observed where H. influenzae grows around colonies of Staphylococcus aureus, which provides the necessary factor V.
  6. Colony Morphology:
    • On chocolate agar, colonies of H. influenzae are typically small, smooth, shiny, and translucent to opaque. The appearance can vary depending on the strain and specific growth conditions.


The pathogenicity of Haemophilus influenzae (H. influenzae) is determined by various factors that allow it to cause disease in humans. The bacterium can cause a range of illnesses, from mild respiratory infections to more severe conditions like meningitis. Here are some of the primary pathogenic factors and mechanisms:

  1. Capsule: The most virulent strain of H. influenzae is type b (Hib) because of its polysaccharide capsule. The capsule:
    • Protects the bacterium from phagocytosis by white blood cells.
    • Prevents complement-mediated lysis.
    • Provides resistance against the host’s immune responses.
  2. Endotoxin: Being a Gram-negative bacterium, H. influenzae has an outer membrane containing lipopolysaccharide (LPS). LPS acts as an endotoxin that can stimulate a strong inflammatory response in the host.
  3. Pili and Non-pilus Adhesins: These structures enable the bacterium to adhere to the mucosal surfaces of the host, especially in the respiratory tract. Adherence is a critical first step for colonization and subsequent infection.
  4. IgA Protease: H. influenzae produces enzymes called IgA proteases that cleave human immunoglobulin A (IgA). This is significant because IgA is a primary defense mechanism on mucosal surfaces. By cleaving IgA, the bacterium can evade this part of the immune response.
  5. Biofilm Formation: Nontypeable H. influenzae (NTHi) can form biofilms, which are structured communities of bacteria embedded in a self-produced matrix. Biofilms enhance resistance to antibiotics and the host’s immune defenses, often contributing to chronic infections like otitis media and chronic bronchitis.
  6. Iron Acquisition: For growth and survival, H. influenzae requires iron, which is scarce within the human body. The bacterium has evolved multiple mechanisms to scavenge iron from the host, including producing siderophores (molecules that chelate and transport iron) and extracting iron from host proteins like hemoglobin and transferrin.
  7. Invasion and Dissemination: H. influenzae can invade human epithelial and endothelial cells, which may contribute to its ability to disseminate and cause systemic infections like bacteremia.
  8. Antigenic Variation: NTHi can alter its surface structures, helping it evade the immune system. This ability to change surface antigens ensures prolonged colonization and potential recurrence of infection.
  9. Resistance to Antibiotics: Over time, some strains of H. influenzae have developed resistance to certain antibiotics, especially ampicillin. This resistance can make treatment more challenging.

Lab Diagnosis

The laboratory diagnosis of Haemophilus influenzae (H. influenzae) infection involves a combination of specimen collection, culture techniques, microscopy, biochemical testing, and sometimes molecular methods. Here’s a systematic approach to the lab diagnosis:

  1. Specimen Collection:
    • Depending on the suspected infection, appropriate clinical specimens are collected. For instance, cerebrospinal fluid (CSF) for suspected meningitis, blood for bacteremia, sputum or bronchoalveolar lavage for pneumonia, and throat swabs for respiratory infections.
    • Proper collection and transportation of the specimen are crucial to maintain the viability of the bacterium.
  2. Microscopy:
    • A Gram stain of the specimen can provide an initial clue. H. influenzae appears as small, Gram-negative coccobacilli, often in pairs.
    • In CSF samples, the presence of Gram-negative coccobacilli with an increase in white blood cells (pleocytosis) is highly suggestive of H. influenzae meningitis.
  3. Culture:
    • H. influenzae requires specific growth factors (X and V factors). Therefore, it grows best on chocolate agar, which supplies both factors.
    • The bacterium might not grow on standard blood agar unless there are “satellite” colonies around other bacteria (like Staphylococcus aureus) that provide the required V factor.
    • The colonies on chocolate agar are usually small, grey, and translucent to opaque.
  4. Biochemical Testing:
    • After isolation, several biochemical tests can help identify H. influenzae, including:
      • Growth requirement for X and V factors.
      • Oxidase test (usually positive).
      • Porphyrin test.
    • Commercial biochemical kits are also available for rapid identification.
  5. Serotyping:
    • If invasive disease is suspected, serotyping can be performed to determine the capsular type, especially to differentiate type b (Hib) from other types. This is often done using slide agglutination with type-specific antisera.
  6. Molecular Methods:
    • Polymerase chain reaction (PCR) can be used to detect specific genes of H. influenzae. This method is particularly useful for direct detection in clinical specimens without the need for culture, and it’s highly sensitive and specific.
    • PCR can also differentiate between typeable and nontypeable strains or detect antibiotic resistance genes.
  7. Antibiotic Susceptibility Testing:
    • Given the rise of antibiotic resistance in some strains, especially resistance to ampicillin, it’s essential to perform susceptibility testing to guide appropriate antibiotic therapy. This is typically done using disc diffusion or broth microdilution methods.


Treatment of infections caused by Haemophilus influenzae (H. influenzae) primarily involves antibiotics. The choice of antibiotic and the duration of therapy depend on the type and severity of the infection, the patient’s overall health, and potential antibiotic resistance patterns of the bacterial strain.

  1. Antibiotics:
    • Ampicillin or Amoxicillin: Historically, ampicillin or amoxicillin was the drug of choice for many H. influenzae infections. However, resistance to ampicillin due to beta-lactamase production by some strains of H. influenzae has been observed.
    • Amoxicillin-Clavulanate, Cefuroxime, and Ceftriaxone: These are beta-lactam antibiotics effective against many beta-lactamase producing strains of H. influenzae. Ceftriaxone is often used for more severe infections, like meningitis.
    • Azithromycin and Clarithromycin: These macrolide antibiotics can be used, especially when beta-lactam antibiotics are contraindicated. However, resistance to macrolides has been reported in some areas, so they should be used based on susceptibility patterns.
    • Fluoroquinolones (e.g., Levofloxacin, Ciprofloxacin): These are used primarily for adults, especially in cases of resistance to other antibiotics or in patients with allergies to first-line agents. They should be used with caution and based on susceptibility testing.
    • Trimethoprim-Sulfamethoxazole (TMP-SMX): Previously used for respiratory infections caused by H. influenzae, its use has diminished because of increasing resistance.
  2. Duration and Route of Treatment:
    • Meningitis: Treatment usually involves intravenous (IV) ceftriaxone or cefotaxime for 7-10 days.
    • Epiglottitis: Hospitalization and IV antibiotics, such as ceftriaxone or cefotaxime, followed by oral antibiotics.
    • Otitis Media, Sinusitis, or Pneumonia: Typically treated with oral antibiotics like amoxicillin or amoxicillin-clavulanate for about 10 days, but severe pneumonia might require hospitalization and IV antibiotics.
  3. Antibiotic Susceptibility Testing:
    • Given the potential for antibiotic resistance, it’s crucial to conduct antibiotic susceptibility testing when H. influenzae is isolated from a patient. This ensures the most effective antibiotic is used.
  4. Supportive Care:
    • In addition to antibiotics, supportive care might be required, especially in severe infections. This could involve pain relief, fever management, hydration, and respiratory support in cases of epiglottitis.
  5. Vaccination:
    • While not a treatment, it’s crucial to note the importance of the Hib vaccine in preventing H. influenzae type b infections. Routine Hib vaccination in children has dramatically reduced the incidence of invasive Hib disease.


Prevention of Haemophilus influenzae (specifically type b or Hib) infections is primarily achieved through vaccination, along with a few standard precautions. Here’s how the infections can be prevented:

  1. Hib Vaccine:
    • The Hib vaccine is the most effective method for preventing H. influenzae type b infections. Before the introduction of the vaccine, Hib was a leading cause of bacterial meningitis among children under 5 years old.
    • There are several Hib vaccines available, and they’re typically given as part of routine childhood immunization programs. The exact schedule might vary by country, but commonly, doses are given at 2, 4, 6, and 12-15 months of age.
    • Since the introduction of the Hib vaccine, the number of cases of invasive Hib disease has decreased by over 99% in countries with widespread vaccination.
    • It’s essential to ensure that children receive all recommended doses of the Hib vaccine for full protection.
  2. Prophylactic Antibiotics:
    • In certain situations where someone has been exposed to a case of invasive Hib disease (e.g., meningitis or epiglottitis), especially in household settings, prophylactic antibiotics like rifampin may be given to close contacts to reduce their risk of developing the disease. This is particularly recommended for household contacts under 4 years of age or those with certain underlying medical conditions.
  3. Standard Precautions:
    • As with many respiratory pathogens, practicing good hygiene can reduce the spread. This includes:
      • Regular hand washing with soap and water.
      • Covering mouth and nose with a tissue or the elbow when coughing or sneezing.
      • Avoiding close contact with sick individuals.
      • Regularly cleaning and disinfecting frequently touched surfaces.
  4. Treatment of Carriers:
    • In settings where there’s an increased risk of transmission, like daycare centers or schools with a case of invasive Hib disease, identifying and treating carriers might be recommended to halt the spread of the bacterium.
  5. Education:
    • Educating the public and healthcare workers about the importance of vaccination and the signs and symptoms of Hib diseases can ensure early detection and treatment, reducing the risk of complications and spread.


Haemophilus influenzae (H. influenzae):

  1. Introduction:
    • Gram-negative coccobacillus bacterium.
    • Despite its name, it does not cause influenza (flu).
  2. Types:
    • Categorized based on a polysaccharide capsule presence.
    • Encapsulated (Typeable): Six types (a-f); type b (Hib) was a significant cause of invasive disease before vaccines.
    • Non-encapsulated (Nontypeable, NTHi): Cause respiratory tract infections, rarely invasive diseases.
  3. Diseases:
    • Meningitis, Epiglottitis, Pneumonia, Otitis media, Sinusitis, and Septic arthritis, among others.
  4. Pathogenicity:
    • Capsule provides resistance against host immune responses.
    • Produces IgA protease to cleave host immunoglobulin A.
    • Can form biofilms, especially NTHi strains.
    • Acquires iron from the host for survival.
  5. Morphology:
    • Small Gram-negative coccobacilli, often in pairs.
    • Requires chocolate agar for growth due to X and V factor needs.
  6. Lab Diagnosis:
    • Microscopy: Gram-negative coccobacilli on Gram stain.
    • Culture: Grows best on chocolate agar.
    • Biochemical and molecular tests for confirmation.
  7. Treatment:
    • Antibiotics: Choice based on the infection and potential resistance patterns.
    • Common choices include ampicillin, amoxicillin-clavulanate, cefuroxime, ceftriaxone, azithromycin, and fluoroquinolones.
  8. Prevention:
    • Hib Vaccine: Critical for preventing H. influenzae type b infections.
    • Prophylactic antibiotics for close contacts in certain situations.
    • Good hygiene practices.
  9. Miscellaneous:
    • NTHi strains are more associated with respiratory infections.
    • The emergence of antibiotic resistance in some strains, especially to ampicillin.
    • Hib vaccine has greatly reduced the burden of invasive H. influenzae disease.

Further Readings

  1. Textbooks:
    • “Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases” by John E. Bennett, Raphael Dolin, and Martin J. Blaser. This comprehensive textbook covers a wide range of infectious diseases, including detailed information on H. influenzae.
    • “Medical Microbiology” by Patrick R. Murray, Ken S. Rosenthal, and Michael A. Pfaller. This book provides a deep dive into the microbiology of various pathogens, including H. influenzae.
  2. Review Articles and Journals:
    • “Haemophilus influenzae: then and now” in the European Journal of Clinical Microbiology & Infectious Diseases. This article reviews the transformation in the understanding and treatment of H. influenzae diseases since the introduction of the Hib vaccine.
    • “Nontypeable Haemophilus influenzae as a Pathogen in Children” in the Pediatric Infectious Disease Journal. This article specifically looks at the clinical implications of NTHi infections in pediatric populations.
  3. Online Resources:
    • Centers for Disease Control and Prevention (CDC): The CDC’s website provides updated information on H. influenzae epidemiology, prevention, and clinical guidance.
    • World Health Organization (WHO): WHO provides global statistics and guidance on the prevention and control of H. influenzae type b infections.
  4. Research Databases:
    • PubMed: By searching “Haemophilus influenzae” on PubMed, you can access a vast number of research articles, reviews, and clinical studies related to the bacterium. This is especially useful if you’re interested in the latest research and developments.
  5. Vaccination:
    • “Hib Vaccination: What Everyone Should Know” on the CDC website provides a layman’s guide to the Hib vaccine, its importance, schedule, and potential side effects.
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