HOUSEHOLD BACTERIOLOGY 57 



Mildew is a form of mold found on 'moist clothes that have not been 

 exposed to the fresh air. Mustiness is an indication of mold. Ringworm 

 is due to this species of organism, which gets under the skin and causes 

 inflammation. 



Yeasts 



The illustrations do not show a third kind of plant micro-organism 

 which, especially in the country, is often present in house dust. That 

 organism is yeast, which also is a single cell but which is reproduced by 

 little buds that swell out from the parent cell and may or may not break 

 off later. Those that float freely in the air, both inside and outside the 

 house, are called " wild yeasts." So far as shape, size, and method of 

 reproduction are concerned, these are little different from the cultivated 

 yeast plants used to raise bread or to give the " sparkle " to sweet fer- 

 mented liquids, such as beer. 



As the invisible yeast plants can remain alive for a long time without 

 moisture, we may have them furnished to us in dried cakes as well as 

 in the fresh compressed form. 



To-day, even with the cultivated yeasts, the housewife who mixes 

 her sponge in a dusty room, in dusty utensils, with old yeast or with 

 everything clean and fresh, if she lets the sponge rise too long or keeps 

 it too hot is likely to have sour bread. Bacteria can grow well when 

 and where yeast cannot, so that acid will be made from the alcohol that 

 the yeast makes from sugar. The yeast plants grow best at a medium 

 temperature, about 75 to 90 F., which is an average " summer heat." 

 In a temperature above 90 F. yeast cannot grow so well, but bacteria 

 grow better. 



The little yeast plant, although so small and simple in structure, is 

 endowed with many of the powers of trees and vegetables and other 

 higher plants. It requires food, has a certain range of temperature in 

 which it grows best, and is injured or killed by too high or too low tem- 

 perature or by too little moisture. If it be given favorable conditions it 

 will .feed, grow rapidly, and reproduce itself by swelling out one part 

 into a bud, which may or may not break away from the mother cell. The 

 most favorable temperature for the rapid growth of the yeast plant, as 

 already stated, is 75 to 90 F. Below that temperature the plant will 

 not grow rapidly and therefore cannot do much work; at a temperature 

 much above 90 it will be killed, and a dead plant cannot work any more 

 than a dead animal can. 



The work of the yeast plants is to change the sugar in bread sponge into 

 two substances alcohol, and a gas called carbon dioxid. The millions 

 of little bubbles in the sponge cannot break through the sticky gluten 



