The Inhibition of Haemophilus Influenzae By Certain Agar and Peptone Preparations

Proteose-peptone agar was defective for the culture of H. influenzae from small inocula when sterilising temperatures exceeded 116°C. All 23 strains tested grew poorly or not at all on the medium sterilised at temperatures of 121°C or more for 30 min. The inhibition was more marked when 5% CO2 was present during incubation. The inhibitory effect disappeared after the medium had been stored at 4°C for 7 days before inoculation, or could be abolished by the addition of a reducing agent such as sodium dithionite 100 μg per ml.

The growth of H. influenzae may also be adversely affected by agar. Of three commercial agars tested, one batch of a particular brand was found to be inhibitory when used from a freshly opened bottle. Inhibition could not be demonstrated after the agar was washed or in the presence of sodium oleate (4 μg per ml).