Adaptative ability of pathogenic fungus Paracoccidioides brasiliensis to human organism

Abstract

As all living organisms, fungi must reproduce to survive. To accomplish this, they learn to withstand the hardships of inhospitable environments by developing a series of adaptive mechanisms established by natural selection, which are usually useful within their natural habitats. However, these mechanisms can also be employed when, by accident, a different type of environment must be faced. This is the case of the thermally dimorphic fungus P. brasiliensis, which, in spite of its small size (approximately 15 microns), has adaptive systems that parallel or even surpass those of higher eukaryotes. Some of its most notable characteristics are its capacity to withstand nutritional deprivation, which forces the fungus to sporulate in order to survive. Furthermore, it is capable of changing forms (from mycelial to yeast) under the influence of temperatures above normal, thereby accommodating the conditions found in the tissues of its accidental human host. Once in the yeast form, it learns to tolerate the detrimental effects of a foreign environment, in which it may encounter oxygen concentrations lower than those expected for an aerobic microorganism. In spite of the fact that it needs iron for vital processes, P. brasiliensis has the capability of sequestering trace amounts of iron from the medium by means of its own chelants; the fungus is then able to meet additional challenges to its survival. Rather than remaining in the extracellular milieu, it prefers to be engulfed by the host’s macrophages, which should destroy it, in order to gain access to the iron needed for its transformation to the tissue form and further adaptation to the new host. These mechanisms illustrate the power of biological processes that, at the molecular level, are expressed equally by microscopic and macroscopic organisms.