The Role of Microorganisms in Disease

      The importance of microorganisms in disease was not immediately  obvious to people, and it took many years for scientists to establish the connection between microorganisms and illness.Recognition of the role of microorganisms depended greatly upon the development of new techniques for their study. Once it
became clear that disease could be caused by microbial infections,microbiologists began to examine the way in which hosts defended themselves against microorganisms and to ask how disease might be prevented. The field of immunology was born.

Recognition of the Relationship
between Microorganisms and Disease

                        Although Fracastoro and a few others had suggested that invisible organisms produced disease, most believed that disease was due to causes such as supernatural forces, poisonous vapors called miasmas, and imbalances between the four humors thought to be present in the body. The idea that an imbalance between the four humors  led to disease had been widely accepted since the time of the Greek physician Galen . Support for the germ theory of disease began to accumulate in the early nineteenth century. Agostino Bassi  first showed a microorganism could cause disease when he demonstrated in 1835 that a silkworm disease was due to a fungal infection. He also suggested that many diseases were due to microbial infections.In 1845 M. J. Berkeley proved that the great Potato Blight of Ireland was caused by a fungus. Following his successes with the study of fermentation, Pasteur was asked by the French government  to investigate the pébrine disease of silkworms that was disrupting the silk industry. After several years of work, he showed that the disease was due to a protozoan parasite. The disease was controlled by raising caterpillars from eggs produced by healthy moths.Indirect evidence that microorganisms were agents of human disease came from the work of the English surgeon Joseph Lister  on the prevention of wound infections. Lister impressed with Pasteur’s studies on the involvement of microorganisms in fermentation and putrefaction, developed a system of antiseptic surgery designed to prevent microorganisms from entering  wounds. Instruments were heat sterilized, and phenol was used on surgical dressings and at times sprayed over the surgical area. The approach was remarkably successful and transformed surgery after Lister published his findings in 1867. It also provided strong indirect evidence for the role of microorganisms in disease because phenol, which killed bacteria, also prevented wound infections.The first direct demonstration of the role of bacteria in causing disease came from the study of anthrax by the German physician Robert Koch . Koch  used the criteria proposed by his former teacher, Jacob Henle, to establish the relationship between Bacillus anthracis and anthrax, and published his findings in 1876.Koch injected healthy mice with material from diseased animals, and the mice became ill. After transferring anthrax by inoculation through a series of 20 mice, he incubated a piece of spleen containing the anthrax bacillus in beef serum. The bacilli grew, reproduced, and produced spores. When the isolated bacilli or spores were injected into mice,anthrax developed. His criteria for proving the causal relationship between a microorganism and a specific disease are known as    Koch’s postulates.