A six-month-old male child was brought to the opd with the chief complaint of fever, inability to feed and seizures since the morning. On examination, the child was found to have altered sensorium and neck rigidity. A lumbar puncture was carried out and CSF sent for examination. The cytology and biochemistry results were suggestive of pyogenic meningitis. Microscopic examination revealed the presence of Gram-negative coccobacilli. On culture on blood agar , the microbe shows the following phenomenon:
High-Yield Explanation
H.influenzae is a small (1.0 x 0.3 µm), Gram-negative, non-motile, non-sporing bacillus, exhibiting considerable pleomorphism. In sputum, it usually occurs as clusters of coccobacillary forms, while in CSF from meningitis cases, long, bacillary and filamentous forms predominate. Cells from young cultures (18- 24 hours) are usually coccobacillary, while older cultures are distinctly pleomorphic. Strains isolated from acute infections are often capsulated.
The bacillus has fastidious growth requirements. The accessory growth factors, named X and V, present in blood are essential for growth.
It is aerobic but grows anaerobically also. The optimum temperature is 3 7°C. It does not grow below 20°C. Some strains require 10% CO2, especially for primary isolation from the clinical specimen. It grows on blood agar if a source of the V factor is also provided.
When S.aureus is streaked across a plate of blood agar on which a specimen containing H.infiuenzae has been inoculated, after overnight incubation, the colonies of H.influenzae will be large and well developed alongside the streak of staphylococcus, and smaller farther away. This phenomenon is called satellitism and demonstrates the dependence of H. influenzae on the V factor, which is available in high concentrations near staphylococcal growth and in smaller quantities away from it. This is a routine test in clinical bacteriology for the identification of H. influenzae.
Reference: Ananthanarayan Panekar microbiology 8th ed page no 333,334