Koch's postulates

Microbiologist Robert Koch was born in 1843. Koch's postulates are a series of ground rules to determine whether a given organism can cause a given disease. Koch theorized that a pathogen must be:

• found in all cases of the disease examined
• prepared and maintained in a pure culture
• capable of producing the original infection, even after several generations in culture
• retrievable from an inoculated animal and cultured again

For all their lingering influence, Koch's postulates never anticipated the era of the human metagenome in which thousands of difficult or impossible-to-culture species of bacteria contribute to a single disease state. Koch's century-old ideas have held science back from understanding how chronic disease occurs because they make no provision for these facts.

The Marshall PathogenesisA description for how chronic inflammatory diseases originate and develop. is consistent with mounting evidence that Koch's postulates no longer apply to discerning the vast amounts of microbes in the human body.

According to TD Brock at the American Society of Microbiology,¹ attempts to rigidly apply Koch’s postulates to the diagnosis of viral diseases may have significantly impeded the early development of the field of virology. It also impeded the understanding of chronic disease.

In 1932, Razumov noted a large discrepancy between the viable plate count and total direct microscopic count of bacteria taken from aquatic habitats,² yet his work was dismissed, underestimate, or both – undoubtedly because it was not consistent with Koch's postulates.

The faithful adherence to Koch's ideas about disease has led researchers to overestimate their comprehension of how pathogens cause disease.

For most of the twentieth century, the predominant feeling about the treatment, control and prevention of infectious diseases was optimism.

Mitchell L. Cohen ³

In 1931, Henry Sigerist wrote, “Most of the infectious diseases … have now yielded up their secrets…. Many illnesses … had been completely exterminated; others had [been brought] largely under control….”⁴ Between 1940 and 1960, the development and successes of antibiotics and immunizations added to this optimism, and in 1969, Surgeon General William H. Stewart told the United States Congress that it was time to “close the book on infectious diseases.”⁵

With “victory” declared, increasing emphasis was directed at the “non-infectious” diseases such as cancer and heart disease. Often, research on infectious disease or activities on their prevention and control were de-emphasized and resources were reduced or eliminated. As recently as the 1980s, pharmaceutical companies, believing that there were already enough antibiotics, began reducing the development of new drugs or redirecting it away from antibiotics.^{6 7}

Robert Koch created a series of ground rules to determine whether a given organism can cause a given disease.

As early as the 19th century, researchers realized that viruses invalidate Koch’s postulates because they require another living cell in order to replicate.

The fact that scientists are still trying to apply Koch’s postulates to bacteria is causing an even greater array of problems. For one, bacteria in the L-form (also known as cell wall deficient bacteria) cannot be easily grown in the lab and can only be studied in conditions that mimic those of the human body. As Gerald Domingue, Professor Emeritus at Tulane University states, “When it comes to L-form bacteriaDifficult-to-culture bacteria that lack a cell wall and are not detectable by traditional culturing processes. Sometimes referred to as cell wall deficient bacteria., Koch’s postulates cannot be fulfilled because it is impossible to duplicate all the variables involved in disease expression.”

It is already widely accepted that some species of bacteria cause disease despite the fact that they do not fulfill Koch’s Postulates since Mycobacterium leprae and Treponema pallidum, (which are implicated in leprosy, and syphilis respectively) cannot be grown in pure culture medium.

Another problem with the postulates - and perhaps its most significant liability - is that the rules fail to successfully account for horizontal gene transfer. Horizontal gene transfer is a process in which organisms swap genetic material, a common biological process which blurs the distinction between one organism and the next. Recall that Koch postulated that one pathogen caused one disease.

Koch's contributions were substantial, but his ideas preceded any understanding of genetics or of horizontal gene transferAny process in which a bacterium inserts genetic material into the genomes of other pathogens or into the genome of its host. and made no provision for organisms that were not genetically distinct.

The blind adherence to Koch's postulates precludes a more nuanced understanding of disease: it is in fact a group of genetically indistinct organisms, a metagenomic microbiota, which causes and drives chronic disease.