Which statement is not true of Haemophilus spp.?

• A. Obligate parasites
• B. Grow well on sheep blood agar
• C. Small pleomorphic gram-negative coccobacilli
• D. Many are found as normal flora in the human respiratory tract

Answer: (B)

Haemophilus species are fastidious bacteria that require specific growth factors to grow. They need X factor (hemin) and/or V factor (NAD). Ordinary sheep blood agar doesn’t supply these factors in usable form, so Haemophilus does not grow well there. They thrive on chocolate agar, where heating lyses red blood cells and releases X and V, enabling growth. Some strains exhibit satellitism around Staphylococcus aureus due to the NAD provided by the Staph. In addition, these organisms are small pleomorphic gram-negative coccobacilli, and many Haemophilus spp. are normal flora of the human upper respiratory tract. Because of these growth requirements, the statement that they grow well on sheep blood agar is not true.