(Janssen / circa 1590’s)

Although the original microscope was a rather simple device, its ability to magnify the range of human vision paved the way for a revolutionary discovery of microbial life. Credit for the initial device has historically been given to a Dutch craftsman, Zacharias Janssen, working in the Netherlands late in the 16th century as a spectacle maker. These tradesmen, considered to be the world’s finest, had been using concave lenses for more than a century to correct far-sightedness. Janssen subsequently extrapolated this approach to visual amplification by creatively attaching two such lenses at opposite ends of hand-held tube. Janssen’s device was unquestionably primitive, but the microscope (from Greek words meaning "to see small"), would shortly reveal a hitherto unimagined realm of life.

(Leeuwenhoek & Hooke / circa 1660-1680’s)

Yet another Dutchman, Antonie van Leeuwenhoek, was the first to use magnifying lenses for the study of microbial life, thereby introducing a new life form which he duly characterized as "animalcules." Leeuwenhoek’s investigative efforts initially stemmed from his work as a textile merchant, for which he apparently used his magnifying lens to study the quality of textile weaves. Lacking formal academic training, and far more inclined to commercial endeavor than investigative research, his initial letters to England’s newly formed Royal Society which carefully documented his findings nonetheless captured an immediate, and undoubtedly astonished, audience with many of the world’s most prominent scientists. Leeuwenhoeks microscope was deceptively simple, with a single, nearly spherical lens (his records indicated he ground more than 400 such lenses in his lifetime) affixed to a back plate whose distance from an opposing mounting pin was coarsely adjusted with focusing screws. Balanced against this simplicity, though, his device provided magnifications ranging from 50 to 300 diameters (roughly one-third the typical maximum provided by a standard contemporary microscope). The realm newly revealed beneath this tool offered an astounding array and abundance of life, leading him to comment that, "there are more animals living in the scum of the teeth in a man’s mouth, than there a men in an entire kingdom." Indeed, the world depicted within Leeuwenhouk’s extremely accurate drawings covered an astounding range of "animalcules" which we now recognize as bacteria, protozoa, algae, and fungi. His seminal efforts launched a following wave of microscopic studies and publications, including Robert Hooke’s "Micrographia" (1665;

Unraveling Spontaneous Generation

(Redi® Spallanzini ® Pasteur / circa 1665® 1860)

Having discovered the surprising existence of these ‘cells,’ considerable debate then developed over the next several centuries in regard to their underlying origin. Since ancient times, and extending well into the Renaissance, the concept of spontaneous generation had been widely accepted with some forms of life, including weeds and vermin. The key fundamental premise of this theory was that that non-living matter evolved into viable life forms through some form of unknown, spontaneous transformation. On the other hand, Leeuwenhoek offered a competing suggestion that these microbes were formed from "seeds" or "germs" released by his "animalcules."

This latter theory was initially reinforced, at least in rudimental form, by an Italian physician, Francesco Redi (circa 1665), who studied the growth of maggots on putrefying meat. As opposed to spontaneous growth, his findings showed that these maggots actually represented the larval stages of flies that laid eggs on these unprotected surfaces. By comparison, meats held either in a closed vessel or covered with fine gauze (protected from direct fly contact) exhibited no such growth. Even stronger evidence to debunk the ‘spontaneous’ premise was then collected by the Italian naturalist Lazzaro Spallanzini roughly five decades later, using heat to prevent the appearance of animalcules in closed, hermetically sealed, infusions. Based on this success, and spurred by a prize of 12,000 francs established by Napolean Bonaparte to find a means of preserving fresh foods for his soldiers, a French inventor, Francois Appert, then used Spallanzini’s heating strategy in 1795 to develop a commercial "appertization" process for preparing canned foods.

Just over six decades later (1856), the eminent French chemist, Louis Pasteur similarly applied this knowledge about the use of controlled heating, developing his so-called ‘pasteurization’ process as a means of carefully sterilizing wines. Indeed, the success Pasteur enjoyed with pasteurization prompted his subsequent research focus on microbial metabolism, by which he finally laid the ‘spontaneous generation’ premise to rest. Using his now-famous gooseneck-flasks he conclusively proved that sterile solutions would not yield any ‘germ’ growth whatsoever unless re-exposed with contaminated air. 

(Needham -> Tyndall / circa 1740 -> 1870)

During the era of inquiry regarding ‘spontaneous generation,’ an eminent British naturalist, John Tuberville Needham, had conducted yet another sterilization study (in 1740) whose results were, at the time, considered inconsistent and inconclusive. Since his heated broths still generated new cells, some suspected his methods had been flawed. However, several years after the ‘spontaneous generation’ concept had effectively been laid to rest, an English physicist, John Tyndall (1870) belatedly discovered a previously overlooked fact that hay infusions subjected to prolonged boiling (and seemingly sterilized) could, in fact, still germinate new viable cells.

However, what he found was that these culture had not evolved through any ‘spontaneous’ means or improper handling, but rather the growth of ‘thermo-resistant’ agents. Tyndall’s work was then confirmed shortly thereafter by a German botanist, Ferdinand Cohn, who visually confirmed the presence of these heat-resistant bodies, known today as endospores. Tyndall’s subsequent work with these sporulating cells also led to his development of a unique sterilization procedure (i.e., Tyndallization) by which discontinuous heating would effectively kill all cells, even including those able to develop such spores.

 Anaerobic Metabolism

(Pasteur / circa 1857-1876)

Spallanzini and Appert’s 18th century studies with the use of sealing to negate microbial growth were roughly paralleled in time by the discovery of oxygen by Joseph Priestley and Antonie Lavoisier. Although working with mice as opposed to microbes, it was their perception that oxygen played an absolutely vital role for life, and this belief extended well into the 19th century. However, subsequent studies on alchohol fermentation would eventually disprove this premise. Starting in 1837, several scientists (Cagniard-Latour, Schwann, and Kutzing) began to connect the processes of alchohol fermentation with microbial agents, as opposed to the more conventional (albeit erroneous) view that these reactions were purely chemical mechanisms.

The advocates of this latter ‘chemical’ scheme included Liebig, Berzelius, and Wohler, several of the era’s leading chemists. Starting in 1857, and extending through to 1876, Pasteur subsequently verified the ‘metabolic’ mode of fermentation, and noted that the various end-products of this metabolic process were generated by different pathways maintained by this action. During the course of specifically studying butyric acid fermentation, Pasteur serendipitously found that oxygen actually had an inhibitory impact on these reactions, leading to his confirmation that anaerobic life was truly possible.

(Snow ® Pasteur ® Hansen ® Koch / circa 1850 ® 1873)

Although Giralamo Fracastoro had raised the first concerns about infectious disease in 1546, the connection between human disease and microbes was still unknown midway through the 19th century. As early as 1813, fungal diseases had been found with wheat and rye plants and by 1845 Berkeley had proven that the Great Potato Blight in Ireland was the result of yet another fungal disease. Bassi similarly proved in 1836 that fungal diseases could attack silkworms, and shortly thereafter Schonlein concluded that even human’s could incur fungal infections.

Pasteur’s own work with the pasteurization of wines had also touched on what he referred to as ‘diseases’ of beer and wine. However, when cholera struck again in England during the 1850’s, there was absolutely no awareness of its bacterial origin. While studying the onset and transmission of this disease in London’s Soho district, though, Dr. John Snow launched the first so-called ‘epidemiological’ study of disease. Snow traced this outbreak to a specific well (just outside the door of a pub now named in his honor) and carefully developed a clear correlation between local victims and their use of this well. At that point, he then took the world’s first public health measure towards disease control by simply removing the well pump handle. At much the same time, Pasteur’s work also began to focus on the transmission of contagious human disease. In 1862 he summarized his findings on the ‘germ theory of disease,’ and by most accounts, this publication represents the most important single advancement in the history of medicine. Just over a decade later, Gerhard Henrik Armauer Hansen made the first connection in 1873 between a particular disease and specific bacteria. Hansen, a Norwegian physician who oversaw a leper hospital, considered this organisms to be the source of leprosy.

Perhaps more commonly, though, credit for this seminal connection between human diseases and microbial agents is given to Robert Koch, who in 1876 conclusively demonstrated the bacterial source of anthrax. This disease had been the subject of intensive study for several decades, and as early as the 1850’s and 1860’s several researchers had noted the consistent presence of tell-tale, rod-shaped elements in the blood its victims. Koch, working in his own home, subsequently established the specificity of anthrax as being attributable to "only one kind of bacillus," while at the same time establishing the related importance of this organism’s sporulating abilities.

Six years later, Koch published yet another revolutionary finding for a second, disease-causing bacillus, which in this case was responsible for tuberculosis. Quite serendipitously, Pasteur was at the same time shifting his research focus towards human disease concerns, and together with J. Joubert they independently confirmed Koch’s findings with anthrax.

From this point forward, Pasteur’s and Koch’s research institutes (respectively located in Paris and Berlin) subsequently opened the so-called "golden age of medical microbiology". Pasteur’s organization addressed the mechanisms of infection and subsequent processes of recovery and immunity, while Koch’s group took a more focused approach towards isolating, cultivating, and characterizing the specific agents responsible for major infectious diseases.

 Bacteriology as a Formal Scientific Field

(Linnaeus ® Cohn / circa 1872)

Prompted and inspired by Pasteur’s efforts, a German botanist, Ferdinand Julius Cohn, published a three-volume treatise on bacteria in 1872 which many consider to be the true origin of the field of bacteriology. This book offered several classic insights, including the first attempt at bacterial classification and the first description of bacterial spores.

The notion of using this sort of organized approach to taxonomic classification for biological forms had been established over a century earlier by the Swedish botanist, Carolus Linnaeus. This system for organizing plant and animal species, as carried in his classic Systema naturae (1735) and Genera plantarum (1737) publications, was developed and published when Linneaus was still in his twenties. Nearly three decades later, Linnaeus published yet another classic, Genera morborum (1763), which used much the same approach to classify the known diseases.

(Pasteur ® Schloesing & Muntz / circa 1862 ® 1873)

Following the discovery by Roger Bacon in the 13^th century that saltpeter could be blended with sulfur and charcoal to produce gunpowder, this nitrate-bearing salt quickly became one of the world's most strategically important chemicals. With no conception whatsoever that nitrates were biochemically produced by ammonia-oxidizing bacteria, ancient alchemists nonetheless knew that this crystalline essence could often be scraped from stones lying in the midst of animal and human waste (i.e., sources of ammonia). Over the next several centuries, the production of saltpeter subsequently evolved into a highly-refined art. By the 19^th century, the manufacture of nitrate had become one of the three leading biochemical processes in the world, along with those of alcohol fermentation and the production of bread products. The scientific search for what was perceived to be the chemical origin of nitrates had attracted many notable researchers (including Boyle and Hooke in the 18^th century as well as Liebig and Humboldt in the 19^th century) but Louis Pasteur developed an unexpected notion that a microbial reaction might be involved (1862). Pasteur specifically directed his revolutionary idea to a fellow French chemist working on agricultural research, Jean Baptiste Boussingault, but it was his assistant and demonstrator, Achille Muntz who, in collaboration with J.J. Theophile Schloesing, finally published conclusive evidence in 1873 that nitrification was a microbial process.

 Lithotrophic Metabolism

(Winogradsky / circa 1889)

After having studied a peculiar group of so-called ‘iron bacteria’, Sergei Winogradsky shift his attention to yet another group of bacteria which he found to actively metabolize sulfur. Following his first related manuscript in 1887, he published yet another article in 1889 which specifically revealed the previously unknown circumstance that these cells could actually derive their metabolic energy from inorganic nutrients. Organic substrates had, prior to this discovery, been considered the only form of oxidizable food for microbes.

 Autotrophic Metabolism

(Winogradsky / circa 1890)

At this point in time, scientists had come to appreciate the fact that inorganics (including iron, sulfur, and ammonia) could actually be oxidized by a limited number of microorganisms, and yet their attempts to isolate these responsible microbial agents on standard media surfaces (e.g., either potato slices, or the newly-developed gelatin surfaces) proved quite unsuccessful. However, using solely iorganic media Winogradsky was able to successfully grow ammonia-oxidizing bacteria, thereby proving that these microbes could use and assimilative inorganic carbon dioxide by way of an entirely autotrophic lifestyle.

 Nitrogen Fixation

(Beijerinck & Winogradsky / circa 1888 -> 1901)

Credit for the remarkable discovery of nitrogen fixation is generally split between Matinus W. Beijerinck (whose publication in 1901 carried the most important observations) and Sergei Winogradsky (who was actually first in isolating the responsible bacteria). As early as 1888, Beijerinck had reported on the symbiotic growth of certain specialized bacterial forms which grew within root nodules, but their symbiotic connection to these roots which allowed them to biochemically reduce atmospheric nitrogen gas back to ammonia not conclusively confirmed for several years thereafter.

 Virus Discovery

(Iwanowsky & Beijerinck / circa 1892 -> 1899)

A new means of filtering solutions had been developed during the 1880's at the Pasteur Institute which would effectively remove minute bacterial cells from suspensions. However, a Russian scientist, D. Iwanowsky, found that tobacco plants were still susceptible to infectious attack by a filtered suspension. This observations led to a conclusion that even smaller, and hitherto unknown, viral particles were present. Matinus W. Beijerinck independently reached a similar set of findings.

 Antibiotic Production

(Dubos & Waksman / circa 1939 -> 1954)

In 1939, an American microbiologist, Rene Jules Dubos, isolated a unique substance from a Bacillus culture that had the unexpected ability to inhibit the growth of other bacteria. He called this polypeptide compound, tyrothricin, and one year later his former teacher, Selman Waksman, discovered yet another group of biochemical products which he refered to as actinomycin and streptomycin (respectively produced from Actinomyces and Streptomyces bacteria). Waksman’s new discoveries, for which he coined the term, antibiotics, played a crucial role in finding chemical agents able to combat bacteria resistant to penicillin, including the tuberculosis bacteria which by this point had become a major source of human disease. Waksman received the Nobel Prize for medicine and physiology in 1952.