PLACENTAL AGAR 31 



and found to hold back ordinary bacteria. The first half-hour's nitrate is usually 

 found to be perfectly sterile. To facilitate this filtration the cylinder of the filter 

 is filled with fine, clean, sterile sand until the candle is completely covered. The 

 filtrate is either tubed or added to 2% sterile, previously melted agar at 40 to 41 C., 

 mixed, and slanted. No titration or other preparation is necessary, except that 

 the medium is placed at a temperature of 40 C. for two days to inactivate the comple- 

 ment, as suggested by Bass in the use of human blood cultures. Fresh human 

 placenta contains over 30% of the hydrolytic products of protein digestion, and will 

 therefore secure growths of strictly parasitic or feebly vegetative bacteria, and possi- 

 bly protozoa, that are grown with great difficulty or not at all on ordinary media. 

 For instance, the acid-fast organisms from bits of leprous tissue, either of human 

 or rat origin, grow on this medium so readily that microscopic growth can be dis- 

 cerned in from five to seven days. From human tuberculous glands, urine, or cerebro- 

 spinal fluid the same method will give a growth of B. tuberculosis that can be distin- 

 guished in from seventy-two hours to a few days. 



