PHYSIOLOGICAL FUNCTIONS OF BACTERIA 



393 



Photomicrograph of spirilla. 



of a millimeter in length or about one twenty-five thousandth 



of an inch. Bacteria usually vary from 0.5 to 5 microns in length. 

 There is probably a very large 



group of disease-producing agents, 



which may or may not be bacteria, 



that have never been seen with the 



microscope. They can pass 



through ordinary bacteria-proof 



filters, and are known as filter 



passers or a filterable virus. These 



infectious microorganisms may be 



found in the secretion, the excre- 

 tion, or blood of the body, and may 



be passed directly from one person 



to another, thus causing a disease. For example, the pus in the 



abscesses on the skin of a smallpox patient is a filterable virus. 



This material will generally produce smallpox if it is brought in 



contact with a well person. 



Bacteria are sometimes classified according to their relation to 



oxygen. Those that need free oxygen in order to live are the 



aerobic bacteria, and those that live only in the absence of free 



oxygen and perish if exposed to it are the 

 anaerobic bacteria. Certain forms of bac- 

 teria of decav thrive without air. Thev 

 obtain their oxygen by breaking down oxy- 

 gen-containing compounds. 



Physiological functions of bacteria. No 

 bacteria are completely independent. They 

 lack chlorophyll and are, therefore, un- 



Photomicrograph of strepto- , i i • e ^ a i 



bacuii enveloped in a muci- able to make their owu looQ, but havc to 



laginous sheath. -, , , i • p • j mi 



depend upon other organisms tor it. I hose 

 bacteria that inhabit and obtain their food from living organisms, 

 sometimes causing damage to those organisms, are the parasites. 



WH. FITZ. AD. BIO. — 26 



