Epidemiology of nosocomial candidiasis: the importance of molecular typing

Modern epidemiology studies now require that nosocomial pathogens be characterized to the subspecies level whenever possible to better define infectious processes and modes of transmission. In general, if isolates are classified as different by at least one molecular typing method, they may be assumed to represent different strains and to reflect independent infections. If the isolates are the same, it may be assumed that cross infection has occurred or that the patients were infected by exposure to a common source. Typing methods may also be used to address clinical problems related to distinguishing reinfection versus relapse of an infection, and to examine the development of antifungal resistance among fungal pathogens during the course of antifungal therapy. Determining DNA fingerprints of sequential isolates from patients undergoing antifungal therapy has been useful in demonstrating the potential for the development of antifungal resistance in previously susceptible strains and for detecting the substitution of a more resistant strain for a more susceptible strain in the face of intense antimicrobial pressure. In order to be useful as an epidemiological typing method, a DNA fingerprinting system must effectively distinguish between genetically unrelated strains, identify the same strain in separate samples, and reflect genetic relatedness or unrelatedness (genetic distance) among strains.