A SELECTIVE MEDIUM FOR COUNTING FUSIFORM ORGANISMS IN DENTAL SPECIMENS

¹ Department of Microbiology, School of Dentistry, Tokyo Medical and Dental University, Tokyo, Japan

1. In order to obtain information about the role fusiform organisms play in a human mouth, quantitative measurements of these organisms were made on a selective medium.

2. This selective medium was composed of 1 per cent polypeptone, 1 per cent Ehrlich meat extract, 1 per cent glucose, 0.05 per cent cystine, 0.05 per cent sodium glutamate, 0.25 per cent sodium chloride, 0.01 per cent sodium azide, 1:80,000 crystal violet, and 0.3 per cent agar at pH 8.0. Use of a banjo-shaped culture flask was recommended for counting colonies.

3. Inclusion of a small amount of agar was important in allowing growth of fusiforms in the presence of glucose and crystal violet. Three-tenths per cent agar was used when culture was made in the flask, and 1.0 per cent agar when the culture was carried out in a Petri dish under strict anaerobic conditions. This latter method of culture produces more colonies from a given inoculum.

4. Addition of azide is necessary to inhibit the growth of aerobic oral bacteria and small gram-negative cocci. Addition of glucose, cystine, and sodium glutamate enhances growth. Tryptophane is also stimulatory but it was omitted from the above formula on account of its cost.

5. The degree of selectivity of this medium is fairly satisfactory for estimating the relative number of fusobacteria in the mouth. It is very doubtful that the Leptotrichia of Bøe will develop in this medium.