New pathogens and old resistance genes.

In the past fifty years the treatment and ecology of infectious diseases has evolved greatly. The major changes have come as the development of "new" pathogens which are usually "old" pathogens in new guises. The chief environmental factor responsible for this change is the use of antimicrobial agents. It is estimated that more than 10(9) kg of antibiotic have been produced and used since the early 1959's. In spite of this threat, bacterial pathogens have not only survived, but even flourished. It is now too late to reverse this trend, since the treatment of infectious diseases relies heavily on antibiotics and there is a well-established pool of antibiotic resistance genes in nature. Hospital are unique, varied ecological niches for microbial diversity, and we know little about where resistance genes come from or the pathways by which these genes are passed around the bacterial population. Since less than 10% of the bacterial species on earth can be identified, we are ignorant of a large component of the factors responsible for health, disease and antibiotic resistance. What can we look forward to? There has been much talk of reemerging pathogens such as Mycobacterium tuberculosis, Listeria monocytogenes, enterococci and various fungi. These organisms have surfaced because of their ability to survive in hostile environments (antibiotics, antiseptics). We should anticipate the reappearance of other such organisms, and clinicians and the pharmaceutical industry should be prepared for this. Now it is the time to get profit from the lessons of the past, which, together with better knowledge of microbial biochemistry and ecology, must be applied to the problems of infectious diseases.