108 BACTERIA AND FOOD 



small bacteria ; each of these grows and divides into two daugh- 

 ter cells. Soon a group or colony forms, the individual cells 

 huddling together in irregular masses or clinging together in 

 chains or other definite forms. 



When food is scarce bacteria cease to divide and to form new 

 cells, but many of them develop spores within themselves. 

 Spores are hardy and can survive intense cold, 

 strong heat, drought, and lack of food (Fig. 46). 



When the bacteria die the spores are set free. 

 FIG. 4 6. Bacteria, They are very light and are blown far and wide 

 the white spots by the wind and start new colonies in widely 



scattered places. Bacteria do not all form 

 spores; those which do are troublesome to get rid of, those 

 which do not are more easily controlled. 



How bacteria cause decay. Green plants manufacture 

 food for themselves from materials taken from the air and earth. 

 Bacteria cannot make their own food from inorganic matter, 

 but they get nourishment from ready-made foods such as 

 meat, milk, cheese, grains, fruits, vegetables. Bacteria se- 

 lect from the meat or milk some substances as food and 

 reject others, and as a result alter its character. If you burn 

 a match you see that smoke and gases escape from it and 

 that a black crumbling mass remains. A chemical change 

 has taken place in the wood ; some of the substances of 

 which it was composed passed into the air, others remained 

 in the charred black mass. Bacteria cause chemical changes 

 in the substances on which they feed. When they get into 

 food they decompose it; that is, they destroy its chemical 

 nature and break it up into other substances. The various 

 new substances which result from the action of bacteria are 

 called decomposition products. 



When bacteria get on the surface of meat they decompose it, 

 changing it into substances that are different from fresh meat in 



