PART I
HISTORY AND SCOPE OF MEDICAL MICROBIOLOGY
Microbiology is the study of living organisms of microscopic size. The term was introduced by the French chemist Louis Pasteur.
At a very early stage man developed concept that contagious disease was caused by invisible living things. Invention of the microscope proved it to be a reality. Antony van Leeuwenhoek (1632-1723) designed a single lens microscope and demonstrated the little agents of disease, which he designated as animalcules. These animalcules are now well established entities belonging to bacteria, viruses and several other pathogens. The organisms being invisible to naked eye are known as microorganisms.
For many years it was believed that the micro-organisms arose from dead, especially decomposed organic matter. This was known as theory of spontaneous generation.
CONTRIBUTIONS OF LOUIS PASTEUR (FIG. 1.1)
Louis Pasteur (1822-1895), a French chemist generated strong evidence to show that the microorganisms did not rise de novo or spontaneously in the media but were introduced from without. Pasteur showed that these organisms were maximum in the dusty air of towns and minimum in air of hilly areas where human habitation did not exist.
Pasteur also showed that microorganisms were inactivated by:
- boiling
- at 120°C under pressure (autoclaved)
- at 170°C (hot air oven).
Pasteur's demonstration of airborne microbes prompted Joseph Lister's (1827-1912) work on wound sepsis. He introduced the practice of protecting the wounds from airborne microbes by applying antiseptic dressing and making medical and paramedical workers wash their hands with antiseptic solution before they touched any exposed part of a patient. He achieved strikingly successful results and brought down tremendously the mortality due to sepsis.
Pasteur's achievements in the field of sterilization were closely followed by many other workers. Tyndall introduced the method of sterilization by repeated heating with appropriate intervals for germination of spores between them and their subsequent destruction. The method is known as tyndallization and is practiced even today.
Pasteur developed vaccines against chicken cholera and anthrax by using attenuated suspension of bacteria. 2In order to show that the process was akin to Jenner's use of cowpox, he referred to this as vaccination. These attenuated organisms, on injection into animals, protected them from the effects of virulent bacteria. Soon anthrax immunization was widely practised with an enormous reduction in mortality amongst sheep.
However, his epoch making discovery was the development of rabies vaccine from the spinal cord of rabbits. It has been responsible for saving innumerable human lives consequent to bites by rabid animals.
The discoveries made by Pasteur can be summarised as follows:
CONTRIBUTIONS OF ROBERT KOCH (FIG. 1.2)
Robert Koch's first contribution to science was demonstration of the character and mode of growth of causative bacillus of anthrax.
In 1882, Koch discovered tubercle bacillus and in 1883 the cholera vibrio. For his manifold discoveries in bacteriology, Koch is considered as father of bacteriology.
Koch's Postulates
To confirm the claim that a microorganism isolated from a disease was indeed the cause of this, Koch postulated a set of criteria. According to these postulates, a microorganism can be accepted as a causative agent of an infectious disease only if the followings are satisfied:
- The isolate should be found in every case of the disease and under conditions which explain the pathological changes and clinical features
- It should be possible to isolate the causative agent in pure culture from the lesion
- When such pure culture is inoculated into appropriate laboratory animal, the lesion of the disease should be reproduced
- It should be possible to reisolate the causative agent in pure culture from the lesion produced in the experimental animal (Fig. 1.3).
Subsequently another criterion has been introduced which demands that specific antigens or antibodies to the bacterium should be detectable in the serum of the patient suffering from the disease.
Exceptions to Koch's Postulates
Some of the exceptions of these postulates are:
- Inability to grow Treponema pallidum and Mycobacterium leprae-known causative agents of syphilis and leprosy respectively on artificial media
- Inability to grow many viruses and rickettsial pathogens on artificial media.
The lifelong achievements of Robert Koch are summarised as under:
During the last quarter of the nineteenth century, succession of discoveries were reported which had bearing on the relation of bacteria to human and animal diseases. Table 1.1 shows some important discoveries.3
ANTIMICROBIAL AGENTS
With the identification and confirmation of bacteria as causative agents of human diseases, efforts were made to develop effective agents which could kill bacteria in the body of the man without damaging the host tissue. Pioneer work was done by Paul Ehrlich (Fig. 1.4) who is justly described as the father of chemotherapy. In the 1900s he cured one form of trypanosomiasis in rats with the dye trypan red and another form in mice with an organic arsenic compound, atoxyl.
In 1910, Ehrlich successfully treated syphilis using compound ‘606′’ (dioxydiaminoarsenobenzol dihydrochloride) and called it Salvarsan. Fleming discovered penicillin and Waksman streptomycin. Subsequently, several fungi have been used as source of antimicrobial substances.
DISCOVERY OF VIRUSES
By the end of nineteenth century many infectious diseases had been proven to have a bacterial aetiology. The trend continued in twentieth century. But yet there remained many diseases of common occurrence for which no bacterium could be demonstrated. These included smallpox, chickenpox, measles and common cold. Advent of electron microscopy in 1934 by Ruska made morphological examination of viruses possible. The first human disease proven to have a virus aetiology was yellow fever.
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In 1930s viruses could be grown in bacteria free, living chick embryo—a technique perfected by Goodpasteur. By 1940, growth in tissue culture of susceptible mammalian cells was established. The availability of well defined cell lines have now replaced tedious methods of growing viruses in the living animals.
NOBEL LAUREATES
A number of Nobel laureates in Medicine and Physiology were awarded this prize for their work in Microbiology and Immunology (Table 1.2).
Discovery of New Organisms
The discovery of new microorganisms is a continuous phenomenon. A large number of new organisms have been discovered in recent past (Table 1.3). Some of these have acquired considerable importance because of the mortality and morbidity caused by them (HIV, hepatitis B, hepatitis C) and others have the capability to cause international scare (SARS CoVirus) or even pandemic (influenza virus H5N1).