A world without microbes : dream or nightmare ?

Louis Pasteur believed that life without microbes would be impossible. In the decades that followed, gnotoxenia, the science of life in a sterile or microbiologically controlled environment, was born, demonstrating that germ-free (axenic) life, contrary to Louis Pasteur's hypothesis, was possible. We even fantasized about the genetically and microbiologically pure man of tomorrow, about future axenic astronauts who would not risk the planets they visited being contaminated by Earth microbes. There was less fantasizing when we began to realize that Louis Pasteur wasn't so wrong after all. Animals, particularly mice, reared for long periods in axenic isolators were hypotrophic, suffering from severe nutritional and vitamin deficiencies, their immune systems immature and the late stages of maturation of the blood-brain barrier and brain delayed, giving rise to severe behavioral disorders. Abruptly released into contaminated environments, such animals would be in great distress and would undoubtedly die before being able to assemble even a partial microbiota. Co-evolution has therefore left the microbiota, particularly the intestinal microbiota, with the major functions of nutrition, maturation and maintenance of the immune system, and maturation of the central nervous system. This vital symbiosis has its equivalents in the plant world, the main example being the nitrogen-fixing capacity provided to legumes by bacteria expressing a key enzyme: nitrogenase. A world without microbes would certainly be viable, but so considerably depleted in diversity that its sustainability would be in question.