306 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



highly virulent and in many eases can overcome resistance 

 readily. 



Certain species of cocci, particularly organisms belonging 

 to the genus Staphylococcus, are abundant upon the skin. 

 These bacteria, together with certain species of Strepto- 

 coccus, are found frequently in the mouth and intestines. 

 In the alimentary tract, particularly in the colon, a con- 

 siderable number of species of bacteria grow normally. 

 Most important and most common in this group is the 

 organism known as the Bacterium coli. Under certain 

 abnormal conditions the bacterial flora of the intestines may 

 contain a great variety of other kinds of bacteria. 



How Bacteria Gain Admission to the Tissues of the 

 Body. Infection Atria.— Microorganisms capable of caus- 

 ing disease gain admission to the body in a variety of ways. 

 In some cases they enter as a result of traumata, that is, 

 through direct mechanical injury to the skin or mucous 

 membranes. Bacteria, for example, frequently gain admis- 

 sion through wounds, giving rise to inflammation, pus 

 production and sometimes generalized infection. A special- 

 ized type of traumatic infection is to be found in the bites 

 of certain insects. The mosquito, for example, when 

 infected from the blood of a person having malaria may 

 later inject the organisms capable of causing this disease 

 into another individual. In a few cases organisms may 

 apparently gain access to the body through the unbroken 

 skin or mucous membrane and produce disease. Certain of 

 the molds, for example, capable of causing diseases such as 

 ringworm and favus apparently do not require any injury 

 to the skin in order to begin their growth. A very frequent 

 channel of infection is the aUmentary tract. Many diseases 

 are contracted as a result of swallowing the causal micro- 

 organisms. Typhoid fever is a disease of this type. In 

 certain cases the respiratory tract constitutes the infection 

 atrium. One may inhale, for example, the organism 

 capable of causing pneumonic plague or other respiratory 



