The year 2010 marks the 100th anniversary of Robert Koch’s death. His name is well known as one of the founders of modern microbiology, the discoverer of the etiologic agents of anthrax, tuberculosis, cholera and wound infection. What is equally important, he developed the methodologies and concepts that made microbiology a scientific discipline. He is best known for having developed the rules, or postulates, that determine the specific agent of an infectious disease. Koch’s postulates were a landmark in medical microbiology because they could be used to prove beyond any doubt that specific bacteria were the cause of an infectious disease. However, significant limitations to the postulates were soon recognized and restricted their wider scientific application. Now, over a century later, a more rigorous method to test causability still has to be developed. Technological advances led to the discovery of viruses, prions and new classes of bacteria that cannot fulfill Koch’s postulates. Advances in molecular microbiology have demonstrated that virtually all microorganisms can be considered opportunistic. They constantly turn their virulence genes on and off in a context-dependent manner to acquire unique phenotypes. They have evolved sophisticated mechanisms to sense changes in the environment and respond accordingly when their survival is threatened. Local environmental cues are gathered by bacteria at the membrane level and transmitted do deeper circuits within their cytoplasm, where they are processed by another sensing system, known as the quorum sensing signaling system. As a result of the processing of environmental information, the bacteria shift their phenotype to adjust better to the local conditions, resulting in either pathogenic or non-pathogenic behavior. Infectious pathogenesis is dictated by the chemical crosstalk between the complex network of bacteria- -bacteria, host-bacteria, and host-host. The ability of bacteria to intercommunicate and process information about resource availability and the health status of the host has led to the concept of bacteria behaving like a social group. The study of social networks developed among bacteria in their natural habitats is called sociomicrobiology.
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