Overview
The genus Haemophilus comprises small, non-motile, non-sporing, oxidase-positive, pleomorphic Gram-negative bacilli that are parasitic on humans or animals. The name literally means 'blood loving' organisms, reflecting their absolute dependence on growth factors found in blood.
Haemophilus influenzae is the most clinically significant species, historically a major cause of childhood bacterial meningitis before the introduction of the Hib conjugate vaccine. Recognising its fastidious growth requirements — X and V factors — and the classic 'satellitism' phenomenon are essential laboratory skills.
Learning Objectives
After this lesson you will be able to…- Describe the morphology of Haemophilus influenzae.
- Discuss the cultural characteristics, including X and V factor requirements and satellitism.
- Describe the pathogenesis of H. influenzae, distinguishing invasive and noninvasive disease.
- Explain the antigenic classification of H. influenzae into capsular types a–f.
- Explain the laboratory diagnosis of H. influenzae infections, including specimen handling and culture.
Clinical Story
Why This MattersA 14-month-old infant is brought to the emergency department with high fever, irritability, neck stiffness and photophobia. A lumbar puncture is performed and CSF is sent to the lab. Gram stain shows pleomorphic, poorly-staining Gram-negative coccobacilli. The technologist plates the CSF onto chocolate agar and, mindful that this organism requires special growth factors, also streaks a strain of Staphylococcus aureus across a blood agar plate — after overnight incubation, small colonies appear larger near the staphylococcal streak, a phenomenon called satellitism, confirming Haemophilus influenzae meningitis.
Core Concepts
H. influenzae is a small, Gram-negative, non-motile, non-sporing bacillus exhibiting marked pleomorphism. In sputum, it typically occurs as clusters of coccobacillary forms. The bacilli are relatively difficult to stain; staining for 5–15 minutes with Leoffler's methylene blue or dilute carbol fuchsin gives good results.
H. influenzae has fastidious growth requirements — growth factors X and V, present in blood, are essential. It is aerobic but also grows anaerobically. Optimum temperature is 37°C; it does not grow below 20°C.
The classic identification test is satellitism: when Staphylococcus aureus is streaked across a blood agar plate inoculated with a specimen containing H. influenzae, colonies of H. influenzae grow large and well-developed near the staphylococcal streak (where V factor is released) and smaller farther away.
When blood agar is heated to 80–90°C or boiled briefly, V factor is released from erythrocytes, making chocolate agar superior for primary isolation, yielding copious growth. Capsulated strains show translucent, iridescent colonies on Levinthal's agar.
Glucose and xylose are fermented with acid production, but not lactose, sucrose or mannitol. Catalase and oxidase reactions are positive; nitrates are reduced to nitrites. H. influenzae is a delicate organism, destroyed by heating at 55°C for 30 minutes, by refrigeration, drying and disinfectants. In culture, cells die within 2–3 days due to autolysis; cultures may be preserved about a month on chocolate agar slopes.
Major surface antigens are the capsular polysaccharide, outer membrane protein, and lipooligosaccharide. Capsulated strains are classified into six capsular types, a to f, based on the polysaccharide antigen. Major acute invasive infections belong to type b (Hib), which has a unique capsular structure containing ribose and ribitol instead of hexoses/hexosamines.
Strains lacking a capsule cannot be typed and are called nontypable strains; next to Hib, these are the most clinically relevant in day-to-day infections.
H. influenzae is an exclusively human pathogen. Invasive disease occurs when the bacillus acts as a primary pathogen, spreading via blood, protected from phagocytes by its capsule (mostly capsulated type b strains, most common in children). Haemophilus meningitis is the most important invasive infection; others include laryngoepiglottitis, conjunctivitis, bacteremia, pneumonia, arthritis, endocarditis and pericarditis.
Noninvasive disease spreads by local invasion along mucosal surfaces, causing secondary infections usually of the respiratory tract — otitis media, sinusitis, and exacerbations of chronic bronchitis and bronchiectasis. These are usually seen in adults and often caused by noncapsulated strains.
Growth Requirements & Key Data
Identification Panel⚠️ Values summarised from standard microbiology references; always confirm with your laboratory's SOPs.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Satellitism around Staphylococcus streak on blood agar | Confirms V-factor dependence, typical of H. influenzae | Proceed to confirm with X/V factor disc test and biochemical panel |
| Small opaque colonies on chocolate agar from CSF | Presumptive H. influenzae meningitis, especially in children | Perform latex agglutination/CIE for capsular antigen; urgent clinical correlation |
| Iridescent colonies on Levinthal's medium | Indicates capsulated strain | Type strain (a–f); type b strains are most invasive |
| Nontypable, noncapsulated isolate from sputum | Suggests noninvasive respiratory tract infection (otitis media, sinusitis, bronchitis) | Correlate with clinical picture; typically less severe than invasive disease |
Common Errors & How to Avoid Them
Cause: H. influenzae is very sensitive to low temperatures and viability drops quickly if the specimen is refrigerated.
Prevention: Plate CSF promptly on blood agar or chocolate agar and incubate immediately in 5–10% CO2 with high humidity.
Cause: Without exogenous V factor from lysed staphylococcal colonies, H. influenzae may fail to grow or grow poorly on blood agar.
Prevention: Always streak a staphylococcus strain across the plate, or preferentially use chocolate agar, which already contains released V factor.
Cause: H. influenzae stains poorly and is easily missed or misclassified with standard Gram technique.
Prevention: Use Leoffler's methylene blue or dilute carbol fuchsin staining for 5–15 minutes for clearer visualization.
Laboratory Tips from the Bench
Whenever pleomorphic, poorly-staining Gram-negative coccobacilli are seen in a pediatric CSF specimen, think Haemophilus influenzae and set up satellitism testing alongside chocolate agar culture.
Remember that capsular polysaccharide antigen can be detected directly in CSF (meningitis) or urine (systemic infection) by latex particle agglutination — useful when culture is negative due to prior antibiotic therapy.
Remember X and V factors: X = heXamine-related hemin (heat-stable), V = NAD (the letter V looks like the branched NAD molecule, and it is heat-laVile / destroyed by heat).
Important Notes
Widespread Hib conjugate vaccination has dramatically reduced invasive type b disease; laboratories should remain alert to nontypable strains, which remain a common cause of respiratory infections.
CSF for suspected H. influenzae meningitis should never be refrigerated before inoculation, as the organism is highly sensitive to cold temperatures.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeHigh fever (39.5°C), irritability, neck stiffness, photophobia and a bulging anterior fontanelle for one day. Not fully vaccinated per records.
The CSF findings of neutrophilic pleocytosis, low glucose, high protein, and pleomorphic poorly-staining Gram-negative coccobacilli are classic for bacterial meningitis due to Haemophilus influenzae, particularly in an incompletely vaccinated infant.
- →Incomplete Hib vaccination remains a key risk factor for invasive H. influenzae disease.
- →CSF should never be refrigerated before culture due to the organism's cold sensitivity.
- →Satellitism testing and chocolate agar culture are the classic confirmatory laboratory approaches.
Frequently Asked Questions
Its thin cell wall and pleomorphic, small coccobacillary form make it difficult to visualize clearly with standard Gram stain; special stains like Leoffler's methylene blue or dilute carbol fuchsin give better results.
No — nontypable (noncapsulated) strains are actually the most common cause of everyday infections like otitis media and sinusitis, while type b is specifically associated with severe invasive disease such as meningitis.
Isolation is improved by incubating plates in an environment of 5–10% CO2 with high humidity, which supports optimal growth of this fastidious organism.
Quick Revision
10-Minute ReviewKey Takeaways
- Haemophilus influenzae is a fastidious, pleomorphic Gram-negative coccobacillus requiring X and V growth factors.
- Satellitism around Staphylococcus aureus colonies is the classic confirmatory laboratory phenomenon.
- Capsular type b (Hib) is responsible for most invasive disease including meningitis, while nontypable strains cause noninvasive respiratory infections.
- Chocolate agar, incubated in CO2-enriched humid conditions, is the preferred isolation medium.
- CSF specimens must never be refrigerated before culture due to the organism's cold sensitivity.
- Latex agglutination for capsular antigen offers rapid diagnosis directly from CSF or urine.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Microbiology Module — Lesson 29: Haemophilus.
- Collee JG, et al. Mackie & McCartney Practical Medical Microbiology, 14th ed.
- Centers for Disease Control and Prevention. Haemophilus influenzae Disease.