Medical microbiology deals with the causative agents of infectious diseases of man, his reactions to them and the methods of protection against such diseases.Disease and death have always attracted the attention of the human mind. Ancient man ascribed them to divine wrath and other supernatural forces. Later, other concepts such as the influence of environment, of bodily constitution and of faulty diet were proposed.
There have been, from very early times, occasional suggestion that diseases may result from invasion of the body by external contagion. Varo and Columella in the first ccntury B.C. postulated that diseases ere earned by invisible beings (Antmalta rlnx’a), inhaled or ingested. Fracastonus of Vtrcna (1546) proposed a contagtum mum as £e possible cause of infectious disease.
Leeuwenhoek the world of ‘little animalcules represented only a curiosity of nature. It was only some two centuries later that their importance in medicine and in other areas of biology came to be recognized.
The earliest discovery of a pathogenic microorganism was probably made by Atigusiino Bassi (1835), who showed that the muscardinc disease of silk worms was caused by n fungus. Dnvainc and Pollcndcr (1850) observed anthrax bacilli in the blood of animals dying of the disease. In fact, even before the microbial aetiology of infections had been established, this was evident to some observant physicians. Oliver Wendell Holmes in the U.S.A. (1843) and IgnazScmmclwcisin Vienna (1846) had independently concludcd that puerperal sepsis was transmitted by the contaminated hands of obstetricians and medical students and demonstrated the efficacy of simple measures such as washing hands in an antiseptic solution.
The development of bacteriology as a scientific nondiscrimination Pasteur (182*45). Though trained as a chemist, his studies on.fermentation led him to take an interest in microorganisms. He established that fermentation was ihejesult-of microbial activity andjhai different ‘types of fermentation were associated which” activity of .different Jsinds _of_ microorganisms (1857). The basic principles and techniques of bacteriology were evolved by Pasteur during his enquiry into the origin of microbes.
This was then the subject of much controversy. Needham, an Irish priest, tud in 1745 published experiments purporting the spontaneous generation (abio-enesis) of microorganisms in putrcscible fluids. Tliis view was opposed by Spallanzani, an Italian abbot (1769). In a series of classic experiments. Pasteur proved conclusively that all forms of life, even microbcs. prose only from their like and not dc novo. In the course of tKcsc studies, he intro-duccdlcchniquesjif sterilisation ,and_devclopcd *thc steam steriliser, hot-air ovenand autoclave.
It also established ihe’dftforing growth needs of different bacteria. His work attracted such attention and he attained such eminence in the world of science that not only Francc but all Europe looked to him to solve major problems in various fields^ Thus started his. studies, on pebrine, anthrax, chicken cholera, and hydrophobia. An accidental observation that chicken cholera bacillus cultures left on the bench for several weeks lost their pathogenic property but retained their • ability to protect the birds against subsequent . infection by them led to the discovery of the process of attenuation and the development of live vaccines.
He attenuated cultures of the anthrax bacillus by incubation parasympathetic 43C and proved that inoculation of such cultures in animals induced specific protection against anthrax The success of such immunization was dramatically demonstrated by a public experiment on a farm at Pouilly-lc-Fort (1881) during which vaccinated sheep, goats and cow* were challenged with a virulent anthrax, bacillus culture. All the vaccinated animals survived the challenge while an equal number of vaccinated control animals succumbed to it. It was Pjstcur who coined the term vaccine for such prophylactic preparations to commemorate the first of such, preparations, namely,underemployed Jenny-ncr for protection against smallpox.
Pasteur’s development of a vaccine for hydrophobia And History of Medical Microbiology
The greatest impact in medicine was made by Pasteur’s development of a vaccine for hydrophobia. This was acclaimed throughout the world. The Pasteur Institute, Paris, was built by public contribution and similar institutions were established soon in many other countries for the preparation of vaccines and for the investigation of infectious diseases.
An immediate application of Pasteur’s work to these, a microorganism can be accepted as the causative agent of an infectious disease only if the following conditions are satisfied:
1. The bacterium should be constantly associated with the lesions of the disease.
2. It should be possible to isolate the bacterium in pure culture from the lesions.
3. Inoculation of such pure culture into suitable laboratory animals should reproduce the lesions of the disease.
4. It should be possible to isolate the bacterium in pure culture from the lesions produced in the experimental animals.
An additional criterion introduced subsequently requires that specific antibodies to the bacterium should be demonstrable in the scrum of patients suffering from the disease. Though it may not always be possible to satisfy all the postulates in every case, they have proved extremely useful in sifting doubtful claims made regarding the causative agents of infectious diseases.
By the oegtnning of the twentieth century, many infectious diseases had been proved to have been caused by bacteria. But there remained a large number of diseases such as smallpox, chi-ckenpox, measles, influciwa and the common cold for which no bacterial cause could be established. During his investigation of rabies in dogs. Pasteur had suspccted that the disease could be causcd by a microbe too small to be seen even under the! microscope.
The existence of such ultramicro-scopic microbcs was proved when Ivanovsky (1892) reproduced mosaic disease in the tobacco plant, by applying to healthy leaves juice from the diseased plants from which all bacteria had been removed by passage through fine filters. Beijerinck (1898) confirmed these findings. Loef-flcr and Frosch (1898) observed that the foot and mouth disease of cattle was causcd by a similar filter-passing vims. The first human disease proved to have a virus aetiology was yellow fever. The U.S. Army Commission under Walter Reed, investigating yellow fever in Cuba (1902), established not only that it was causcd by a filterable-virus but also that it was transmitted through the bite of infectcd mosquitoes. Landsteincr and
Popper (1909) showed that poliomyelitis was caused by a filterable virus and transmitted the disease experimentally to monkeys. Investigation of viruses and the diseases causcd by them was rendered difficult as viruses could not be visualised under light microscopes or grown in culture media. Though the larger viruses could be seen after appropriate staining under the light microscope, detailed study of their morphology had to wait till,the introduction of the clcctron microscope by Ruska (1934) and subsequent refinements in electron microscopic techniques. Cultivation of viruses was possible only in animals or in human volunteers till the technique ot growing them on chick embryos was developed by Goodpasture in the 1930s. The application of tissue culture in virology expanded the scope of virological techniques considerably.
The possibility that virus infection could lead to malignancy was first put forth by Ellerman and Bang (1908) Peyton Rous (1911) isolated a virus causing sarcoma in fowls. Several viruses have since been isolated which cause natural and experimental tumours in animalsand birds. Viruses also cause malignant transformation of infectcd cells in tissue culture. The discovery of viral and ccllular oncogenes has shed light on the possible mechanisms of viral oncogenesis. After many decades of futile search, positive proof of the viral causation of human malignancy was established when the virus of human T-cell leukaemia was isolated in 1980.
Twort (1915) and d’Herclle (191?) independently discovered a lytic phenomenon in hectic cultures. The agents responsible were termed bacteriophages — viruses shirt attack bacteria. Early hopes that bacteriologist may have therapeutic applications had to be abandoned, but these viruses have paid unexpected scientific dividends. The essentia! part of viruses is their core of nucleic acid which acts as the carrier of genetic information in the same manner as higher organisms. The discipline of molecular biology owes its origin largely to studies on the genetics of bacteriophages anil bacteria.
It had been noticed from very early days that persons surviving an attack of smallpox did not develop the disease when exposed to the infection subsequently. This observation had been applied for the prevention of the disease by producing a mild form of smallpox intentionally (variolation). This practice, prevalent in India, China and other ancient civilisations from time immemorial, was introduced into England by Lady Mary Wortley Montague (1718) who had observed the custom in Turkey. Variolation was effective but hazardous.
Jenner observing the immunity to smallpox in milkmaids who were liable to occupational cowpox infection introduced the technique of vaccination using cowpox material (1796). This was the first instance of scientific immunisation and, though introduced empirically, has stood the test of time. Jenner’s vaccination paved the way for the ultimate eradication of smallpox.
The next major discovery in immunity was Pasteur’s development of vaccines for chicken cholera, anthrax and rabies. While the techniques introduced by him were successful, the mcchanism of protection afforded by them remained obscure. The explanation of the underlying mcchanism came from two sources. Nuttall (1888) observed that defibrinated blood had a bactericidal effect, and Buchner (1889) noticed that this effect was abolished by heating the sera for one hour at 55°C. The heat labile bactericidal fuctor was termed *alcxinc\ A specific humoral factor or‘antibody’wasdescribed by von Behring and Kitasato (1890) in the scrum of animals which h3d received sublethal doses of tetanus toxin. Pfeiffer (1893) demonstrated bactericidal effect in vivo by injecting live cholera vibrios mtraperitoneally in guinea pigs previously injected with killed vibrios.
The vibrios were shown to undergo lysis. The humoral nature of such lytic activity was proved by Bordet (1895), who defined the two components participating in the reaction, the first being heat stable and found in immune sera (antibody or substance sensibitica-rrtce) and the second being heat labile and Identi-\ cal with Buchner’s alexine, subsequently named Vqmptcrncotl^oon a number of other ways were demonstrated in which antibodies react with antigens, such as agglutination, precipitation, complement fixation and neutralisation.
Metchnikoff (1883) discovered the phenomenon of phagocytosis and proposed the phagocytic response as the prime defense against the microbial invasion of tissues. This led to the cellular concept of immunity. Polemics regarding the significance of the cellular and humoral mechanisms of immunity were largely put to rest with the discovery by Wright (1903) of opsonisation, in which antibodies and phagocyte cells act in conjunction.
Prior experience with a microorganism or other antigen did not always result in the beneficial effect of immunity or protection. At times it caused the opposite effect. Koch (1890) had noticed that when the tubercle bacillus or its protein was injected into a guinea pig already infected with the bacillus, an exaggerated response took place—a hypersensitivity reaction known as Koch’s phenomenon. Portier and fikhct-(1902). studying the effect of the toxic extracts of sea anemones in dogs made ‘the paradoxical observation that dogs which had prior contact with the toxin were abnormally sensitive to even minute quantities of it subsequently. This phenomenon was termed ’anaphylaxis’. Later, many similar reactions were observed, both experimentally and in nature, of injury, disease or even death resulting from repeated contacts with antigens. The importance of this phenomenon, in the pathogenesis of many human diseases, led to the development of the discipline of’af/ergy’.
The characteristic feature of immunity, whether it is protective, or destructive as in allergy, is its specificity. As the mediators of humoral immunity (antibodies) are globulins, the explanation for the exquisite specificity of the immunological reaction had to await the advances in protein chemistry. The pioneering work of Landsteiner laid the foundations of immunochemistry. Chemists dominated the study of immunity for several dccadcs, and theories of antibody synthesis were postulated b>
ore than anything else, to unravelling the genetic code and other mysteries of biology at the molecular level. They have made available information and techniques that could be used for gcncticmanipulation and molecular engineering.The number of Nobel Laureates in Medicine and Physiology, awarded the prize for their work in microbiology, listed below, is evidence of the positive contribution made to human health by the science of microbiology.
