The effect of drying upon the viability of bacteria

The curve of viability, measuring the rate of decrease among bacteria exposed to an unfavorable environment, has been determined for many conditions. Drying is known to cause a rapid elimination, but no exact quantitative determinations have been made. In the experiments here reported, colon bacilli were used. One cubic centimeter of a 48-hour broth culture was mixed with 2½ grams of sea sand spread out in an ordinary Petri dish. Duplicate dishes prepared in this way were covered with porous earthenware tops and kept for various periods of time from four hours to ten days. At the end of each period nine cubic centimeters of sterile water were added to the sand in one of the dishes and well mixed by careful agitation. Portions of this water were then plated in duplicate on agar in the usual manner.

The results of eight series of tests are shown in the table below expressed in percentages of the original number of bacteria present. The original numbers in different series varied from 4,150,000 to 117,300,000 per gram of sand, and the final number remaining after ten days in Series II and III were 20,700 and 18,900 per gram, respectively. The average temperature and humidity of the room in which the plates were kept is shown for the last five series at the bottom of the table.

The results are quite concordant when the dryness of the atmosphere is taken into consideration. Series VI showed a more rapid reduction than the others during the first 7 hours and the relative humidity was 60 per cent. Then came Series V and VIII with a relative humidity of 72 per cent., while in Series IV and VII with an atmospheric humidity of 90 per cent., nearly 70 per cent. of the bacteria were alive after 8 hours.