PROBLEM I. Hoiv We Are Frotected Aiyabist Microor{rLm'mns 



because after the first or second steam- 

 ing the spores became ordinary bacterial 

 cells and were killed in the next heating. 



Bacteria, like all other cells, are also 

 destroyed by strong chemicals. Such 

 substances are known as disbifectmns. 

 When strong enough, disinfectants kill 

 even the spores. The chemicals known 

 as antiseptics do not as a rule kill bacte- 

 ria; they produce conditions which are 

 unfavorable so that the bacteria do not 

 increase in numbers. They are also less 

 injurious to tissue cells. For this reason 

 antiseptics are used on cuts and ^\'ounds. 



Lack of moisture is another unfavor- 

 able condition. Dryness prevents the bac- 

 teria from getting food, growing, and 

 reproducing; in general it does not kill 

 them. 



How bacteria enter the body. Bacteria 

 constantly surround the animal. How do 

 they enter? They never enter by them- 

 selves. They are always brought in, 

 sometimes attached to particles of dust 

 or to some food, sometimes floating in a 

 hquid. All kinds of bacteria enter in this 

 way, pathogenic as well as harmless ones. 

 If the conditions are favorable to the 

 bacteria, they multiply; if not, they die. 

 But the right conditions for entering and 

 multiplying in the body are not usually 

 present. 



The body is protected in a number of 

 ways against the entrance of bacteria. 

 The openings into the body, such as the 

 nose and mouth, have a sticky coating of 

 mucus which catches the particles of 

 dust and the bacteria which may be at- 

 tached. Cilia on the epithelial cells lining 

 some of the air passages sweep these par- 

 ticles out of the body. Those bacteria 

 that are swallowed and reach the stom- 



315 



ach come in contact with juice contain- 

 ing hydrochloric acid and are usually 

 kiHed.' 



Covering the outside of the body is the 

 skin, through which bacteria cannot pass. 

 To be sure, sometimes there are breaks 

 in the skin; these may be very small, but 

 tin\' as they may be they present a wide 

 gateway to bacteria. The skin may be 

 pierced by an animal, even a small insect, 

 which may introduce microorganisms 

 when it stings. Once past the barrier of 

 the skin the invading bacteria may enter 

 the other tissues where there is moisture, 

 food, and a suitable temperature for 

 growth. 



The second line of defense. The first 

 line of defense is composed of the skin 

 and the devices, such as hairs in the nose, 

 cilia, tears, and moist membranes, that 

 catch bacteria-laden dust particles and 

 carry them away from the tissues. The 

 second line of defense exists within the 

 body itself. You read earlier about the 

 phagocytes that gather at the point of 

 infection and how pus is formed. With 

 certain acute infections large numbers 

 of phagocytes and other types of white 

 corpuscles appear in the blood. Phago- 

 cytes may also engulf and digest animal 

 parasites and foreign particles of all sorts. 



There are other cells in the body that 

 engulf bacteria just as the phagocytes do. 

 These cells are scattered about in organs 

 and tissues, among them the liver, spleen, 

 lungs, and bone marrow. Those that are 

 found in the general connective tissue of 

 the body are particularly active. These 

 go to the place of infection together 

 with the phagocytes. To recall what you 

 learned about white corpuscles, do Exer- 

 cise 9. 



