184 VETERINARY BACTERIOLOGY 



therefore, which are readily transmitted by contact, direct or 

 indirect, are said to be contagious. All contagious diseases are 

 necessarily infectious, but the reverse is not true; for example, 

 malaria in man and Texas fever in cattle cannot be regarded as 

 contagious, as they are transmitted only through the bite of cer- 

 tain insects. 



Bacteria Normally Present in the Body or on its Surface. 

 Many microorganisms are found in the healthy body. In some 

 cases they are harmless commensals; in others, they merely await 

 a favorable opportunity to invade the tissues. The care used in 

 surgical operations is made necessary by the presence of these 

 organisms. 



Bacteria of the Skin. Some bacteria are found quite constantly 

 upon the skin and hair, others may be isolated only occasionally. 

 Staphylococci and streptococci are generally present, and many 

 of them are capable of causing pus infection when introduced 

 under the skin. Certain organisms, such as the Bacillus smegmatis, 

 are found commonly in the axillae, where the skin is moist and 

 frequently greasy. 



Bacteria of the Mouth. Microorganisms to the number of sixty- 

 two species were found by Miller in the human mouth. Of these 

 he found ten quite constantly present. The flora of the mouth in 

 domestic animals has not been accurately determined, but large 

 numbers of bacteria are always present. Some of these are 

 pyogenic cocci. 



Bacteria of the Stomach. The acidity of the gastric juice in 

 man destroys many of the microorganisms which enter the stomach. 

 In cases where the normal acidity is absent many bacteria will 

 multiply. The flora of the stomach in animals is not well under- 

 stood. Bloat is due to the development of gas-producing bacteria 

 in the ingested carbohydrate-rich food. The organisms respon- 

 sible are probably those normal to the intestinal tract. 



Bacteria of the Small Intestine and Colon. The flora of the 

 intestines in man has been studied at length by bacteriologists, 

 but that of animals is not so well understood. The organisms 

 most common are those belonging to the so-called intestinal 

 group, such as Bacillus coli and Bacillus lactis aerogenes. Strep- 

 tococci are generally present in feces. Certain putrefactive 



