Bacteriology of the Household. 721 



The rapidity of reproduction depends upon warmth, moisture and 

 food supply. Some species produce a new generation every half hour; 

 thus, a single bacterium, if its growth were totally unchecked, in twelve 

 hours might become an ancestor of sixteen million descendants. In 

 two days they would fill a pint measure ; in three days they would equal 

 8,000 tons. This does not occur because there are countless checks to 

 the life of any species of bacteria. 



We may form some idea of their minuteness when we know that the 

 length of a single bacterium of some species is 1-25,000 of an inch. Many 

 million may be packed into the space of a grain of sugar, and if one falls 

 into .a minute wrinkle of the hand it is as though it had fallen into a 

 deep ditch. 



Molds. — Molds are also micro-organisms. A colony of mold organisms 

 growing upon some substance forms a velvety pile, having a dark center. 

 We often see long threads budding and branching to form a network 

 over food. Each head produces thousands of dust-like spores. Some 

 molds grow with less moisture than bacteria and some flourish in the 

 light. They are frequently found in bread, on meat, leather and sugary 

 liquids. They increase very rapidly after rainstorms, and wind affects 

 them less than it does bacteria. 



Mildew is a form of mold found on moist clothes which have not 

 been exposed to the fresh air. Mustiness is a proof of mold. Ring- 

 worm and molds on the skin are due to this species of organism which 

 gets under the skin and causes inflammation. 



Yeast. — The pictures do not show a third kind of plant which, especi- 

 ally in the country, is often present in house dust. This is yeast — also 

 a single cell, but reproducing by little buds which swell out from the 

 parent cell .and may or may not break off later. Those which float 

 freely in the air, both inside and outside of the house, are called " wild 

 yeasts." So far as shape, size and method of reproduction is concerned, 

 they are little different from the cultivated yeast plants which are used 

 to raise bread or to give the " sparkle " to sweet fermented liquids, 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. The bacteria can grow well 

 when and where the yeast cannot, so that acid will be made out of the 



