Simplified medium for ampicillin susceptibility testing of Haemophilus influenzae

Recent reports of bona fide ampicillin resistance among strains of Haemophilus influenzae have emphasized the need for improved methods of susceptibility testing of clinical isolates. A simplified medium composed of only Mueller-Hinton medium plus supplement C (yeast autolysate and hematin) was used successfully for ampicillin susceptibility testing of 20 recent clinical isolates of H. influenzae, including six strains with confirmed ampicillin resistance. With this medium, susceptible strains were inhibited by less than 1 mug of ampicillin per ml, whereas resistant strains had minimal inhibitory concentrations of 8 mug or greater per ml. This medium can be used for either disk diffusion, tube dilution, or agar plate dilution methods of susceptibility testing. It has the practical advantage of simplicity of preparation and retains the light color and transparency of Mueller-Hinton medium. This medium can also be used for preparation of an inoculum suspension that can be adjusted to a standardized turbidity.