A Selective Medium for the Primary Isolation of Haernophilus pertussis and Haemophilus parapertussis

SUMMARY: A description is given of a selective medium which supports the growth of Haemophilus pertussis and Haemophilus parapertussis and largely inhibits the growth of Haemophilus inpuenxae. The medium contains horse-meat extract, horse blood, salt agar, 2 pg. 4: 4’-diamidino-diphenylamine-dihydrochloride/ml. and 0.3 unit penicillin/ml. The medium is of value in the primary isolation of H. pertussis and H. parapertussis. Dawson, Farnworth, McLeod & Nicholson (1951) reported that a medium containing beef extract and horse blood was able to support the growth of Haernophilus pertussis while considerably inhibiting the growth of H. inJuenxae. This inhibitory effect was shown to be associated with the horse blood, and we have since shown that an even greater inhibitory effect can be obtained by using horse-meat extract instead of beef extract. During the course of this work Lncey (1951) described a medium which was highly selective for H. pertussis containing 0.3 units penicillin/ml. and 12 pg. 4: 4’-diamidino-di-phenylamine-HCl/ml. In view of Lacey’s findings we investigated the possibility of incorporating 4: 4‘-diamidino-diphenylamine-HCI in our medium. It was shown that the growth of H. injuenzae was completely suppressed by one-sixth of the concentration of the diamidine used by Lacey, while the growth of H. pertussis and H. parapertussis was practically unaffected. As a result of this finding we prepared a meat extract medium containing the diamidine and penicillin and tested its value for the primary isolation of H. pertussis. EXPERIMENTAL Bordet-Gengou medium A mixture of 100 g. thinly sliced potato, 8 g. glycerol and 192 ml. distilled water was steamed for 2 hr. and filtered through lint. The extract was added to S g. agar (Davis Standard-from Thomson, Skinner & Hamilton Ltd., 12 Cadogan Street, Glasgow, C.2) dissolved in 600 ml. 0.6 % NaCl an