Tuberculosis was feared as the “white death” in nineteenth-century Europe. At that time, the concepts of viruses and bacteria were not widely accepted, and people blamed heredity or “bad air.” Robert Koch employed cutting-edge techniques: growing samples from lesions on solid media and examining them under the microscope. In 1882 he presented a slender rod-shaped bacterium that he consistently found in patients’ sputum and organs, and he reproduced the disease in animals, proving causation. This strongly supported the germ theory of disease and clearly showed that TB was contagious. His work accelerated the development of tuberculin testing, X-ray diagnosis and, later, antibiotic therapy, transforming public health. Koch’s experimental procedures became the gold standard in bacteriology and were applied to plague, cholera and many other infections. For this broad societal impact he received the 1905 Nobel Prize in Physiology or Medicine.

Mycobacterium tuberculosis belongs to the acid-fast group; its cell wall is rich in mycolic acids, making it resistant to ordinary Gram staining. Koch introduced a modified Ehrlich stain using fuchsin or methylene blue combined with heated phenol, which vividly revealed acid-fast bacilli. He also refined the solid media invented by Löwenstein, allowing the slow, low-oxygen growth of the organism over roughly three weeks. By inoculating rabbits and guinea pigs, he reproduced granuloma formation and caseous necrosis, creating the prototype of what later became Koch’s postulates. His 1882 lecture to the Berlin Physiological Society was a sensation, firmly linking bacteriology with clinical diagnosis. Identification of the bacillus gave scientific legitimacy to sanatorium isolation practices and drove public health measures such as ventilation, sunlight exposure and disinfection. Until antibiotics like streptomycin appeared mid-20th century, prevention and early diagnosis remained the only control strategies, and Koch’s insights guided medical policy for decades. Even today Ziehl–Neelsen staining and culture remain part of the diagnostic algorithm. Although PCR and whole genome sequencing have revolutionized the field, Koch’s experimental causal inference is still the foundation of infectious-disease science. For modern issues such as latent infection and multidrug-resistant TB (MDR-TB, XDR-TB), his work provides an indispensable historical starting point.

To concentrate soluble antigens from acid-fast bacilli, Koch combined 72 °C heat hydrolysis in hydrochloric acid with phenolic extraction, creating the preparation later known as tuberculin. Initially promoted as a vaccine, its high toxicity led to its redeployment as a diagnostic reagent. Koch’s isolation protocol evolved into the Bismuth standard medium and ultimately the Löwenstein–Jensen medium, where glycerol supplementation was found to enhance colony formation. He experimentally introduced dark-field microscopy, reporting the lack of motility in acid-fast organisms. The four criteria he systematized in his 1890 Untersuchungen paper—constant presence, pure isolation, reproduction of disease, and re-isolation—constitute the classic Koch’s postulates and still influence today’s discussions on in-vivo validation after metagenomic discovery. He also hinted that the bacillus relies on fatty-acid metabolism via the glyoxylate shunt rather than classical aerobic ATP generation, a hypothesis that later illuminated its capacity for long-term latency. Additionally, his slide-seal method and colony-picking subculture minimized contamination with atypical mycobacteria and advanced aseptic technique. Glass pipettes and flat culture plates devised during these studies inspired subsequent Petri dish and Pasteur pipette designs. In a 1901 travel report, he highlighted zoonotic transmission of M. bovis in Africa, foreshadowing the modern One Health framework. This suite of technical and conceptual innovations established bacteriology as an autonomous discipline and earned the high regard of the Nobel Committee.