I02 BACTERIA, YEASTS, AND MOLDS 



the air, and the water, and are sure to find their way into 

 every kind of food or anything else that may be exposed 

 to the air. They are also much more troublesome than 

 molds for two reasons: (i) they multiply with a rapidity 

 that is quite inconceivable ; (2) they are quite invisible 

 to the naked eye, and their presence is not suspected 

 until they become numerous enough to produce undesired 

 changes in the material upon which they are growing. 

 As a result they present a vast number of problems to 

 the housewife, which she has dimly seen for years, but 

 for which science has only in the last few years begun to 

 offer solutions. They are much more difficult to handle 

 than either molds or yeasts, because they are smaller, 

 more numerous, and more vigorous, and for these reasons 

 it is almost impossible to exterminate them. It is an 

 impossibility to free a pantry from bacteria and very diffi- 

 cult tq guard food from their action. 



Shape. Bacteria are very simple, and there are such 

 slight differences between the various kinds that in many 

 cases it is quite impossible by microscopic study to dis- 

 tinguish one species from another. ' The bacteriologist 

 knows to-day that many bacteria which when studied under 

 the microscope appear absolutely identical, are totally 

 unlike in their general characters. It frequently happens 

 that perfectly harmless bacteria cannot by ordinary micro- 

 scopic study be distinguished from those that are very harm- 

 ful. For example, the bacillus which produces typhoid fever 

 cannot be distinguished microscopically from another com- 

 mon but harmless species found in water. For this reason 

 the microscopic study of these plants gives only a small 

 part of the facts that we need to know in regard to them. 



