The biological significance of anaerobic growth and anaerobic mating in Candida albicans

Candida albicans biology allows it to grow at its optimum in the presence of oxygen but as a facultative anaerobe it is also able to grow in its absence. By employing the Hungate technique for strict anaerobic bacteria and providing the cells with all the nutritional requirements, a defined growth medium was created suitable for characterizing the growth of Candida cells in an environment in which oxygen was substituted with nitrogen. There were four major differences between the growth of wild type cells aerobically versus anaerobically: (i) the anaerobic cells grew exclusively as mycelia at all the temperatures tested (25, 30 and 37°C); (ii) they did not produce farnesol, the quorum sensing molecule for Candida albicans; (iii) they did not respond to farnesol; and (iv) they were resistant to polyenes and azoles. This finding is particularly important because azoles and polyenes such as Amphotericin B constitute the primary antifungals used in the clinical setting. Anaerobically grown cells have dramatically altered sterol profiles: the cells accumulate large amounts of squalene and they produce 10% of the ergosterol level obtained when the cells are grown in the presence of oxygen. These changes may account for the enhanced antifungal resistance. The mode of action for Amphotericin B is to bind to the ergosterol in the plasma membrane and to create pores in the cells. This causes leakage of the intracellular ions. Azoles target the cytochrome P450 lanosterol 14-α demethylase, an enzyme that converts lanosterol to 4,4-dimethylcholesta 8,14,24 trienol. Because squalene is the major sterol precursor produced anaerobically, the carbon flow is either reduced, altered or absent in the ergosterol biosynthetic pathway. In order to get efficient killing of the anaerobically grown cells, other antifungal agents have to be taken in consideration. Candida albicans cells of opposite mating types conjugate in the human body even though opaque cells, the mating competent phenotype, are not stable at 37°C. We found that opaque cells are stable at 37°C in anaerobic defined growth conditions thus permitting mating efficiencies of up to 84%. Aerobically, farnesol prevents mating because it kills the opaque cells necessary for mating and, as a corollary, farnesol production is turned off in opaque cells. We have further applied these in vitro findings in vivo by using a mouse model and we have showed that opaque cells can mate in the anaerobic gastro-intestinal tract. From there, the mating progenies can possibly disseminate through the bloodstream to target organs such as kidneys, heart, liver, spleen and even the brain