1 This is a preliminary, unpublished version (pre-print) of a book chapter. It may be used and cited according to the description presented below, informing the corresponding link and the date you accessed the work. MARTINS, Roberto de Andrade. Fictitious microorganisms: Lacerda and the discovery of Bacillus beribericus. Pre-print, June 2022. Available at: https://doi.org/10.5281/zenodo.6614989 However, if you are willing to cite this work, it is advisible to check whether the final version of the same is available. To see whether the work has already been printed, in order to use the final version, with the formatting and pagination that were used in publication, you just have to click the above link. FICTITIOUS MICROORGANISMS: LACERDA AND THE DISCOVERY OF BACILLUS BERIBERICUS Roberto de Andrade Martins Abstract: In the late 19th century, beriberi was a serious disease in Brazil. Several different causes were ascribed to that malady, including miasma infection, undernourishment, food poisoning, contagion, and changes of weather. In 1883, the Brazilian physician João Batista de Lacerda (1846-1915) began experimental investigation of microorganisms as possible cause of beriberi. He attempted to follow Pasteur’s and Koch’s method. Lacerda reported the finding of a microorganism he called Bacillus beribericus in the blood of beriberi patients. He claimed that he had been able to cultivate those microorganisms and to produce beriberi in healthy animals by inoculating that culture. Lacerda’s work was criticized both in Brazil and abroad. Other researchers claimed similar discoveries, including Cornelis Pekelharing, in India. The paper discusses the method used by Lacerda. It is shown that he was unable to follow the rigorous method prescribed by Robert Koch. Keywords: Lacerda, João Batista de; beriberi; microbe theory; Koch’s method 1. INTRODUCTION The second half of the 19th century witnessed the first great successes of the microbiological theory of diseases. It seemed that at last, after thousand years of darkness, Medicine had found a safe, scientific method for its development. Excessive optimism and exaggerated confidence on the microbe theory led, however, to serious mistakes. Careful researches (such as those of Robert Koch) had shown that some diseases were produced by microorganisms. This led, however, to a widespread belief that all diseases are caused by microbes. To many physicians of the late 19th century, it seemed that medical research had been reduced to the search for pathological microorganisms (together with their eventual vectors) and the development of vaccines. Nowadays we know that some diseases are caused by nutritional deficiency 1 . It is not enough to eat a large amount of food – it must be of the right kind. A suitable equilibrium of protein, 1 For a general review of the development of nutritional research up to the beginning of the 20th century, see Mccollum (1957) and Santos (1989).