CAMLPR Microbiology Practice Test

This question tests understanding of why certain bacteria need specific growth factors, namely X and V factors, to thrive. X factor is hemin and V factor is NAD; Haemophilus influenzae cannot make either factor itself, so it must obtain both from the surrounding environment. On standard blood agar, those factors aren’t readily available in the right form or amount, so growth is limited. Chocolate agar solves this by heating blood to lyse cells and release both X and V factors, allowing Haemophilus influenzae to grow. This organism is a classic example of dual-factor dependence, unlike others that don’t require both, and it helps explain why it’s identified by its growth pattern on enriched media. Sometimes, growth can occur near Staphylococcus aureus colonies on blood agar because S. aureus produces NAD, providing the V factor needed by some X-dependent strains, another useful diagnostic clue.