Since the era ofKoch and Pasteur, medical microbiology has matured into a multi-disciplinary, increasingly high-tech science. Based on microscopic observations and culture of microorganisms, infectious diseases could initially be diagnosed and treated, whereas ultimately preventative strategies could be designed as well. Determination of the presence of microbial agents remains the key issue in medical microbiology. The optimal way of identifying pathogens in the laboratory has evolved from primitive strategies which were used by its pioneers into an ever-increasing number of elegant antigen-detecting assays. Recently, separate microbial antigens could be identified by means of immunological assays, and the use of poly-and mono-clonal antibodies allowed for the development of a large array of primarily protein-based diagnostic procedures and instruments. Laboratory expertise in biotyping, definition of susceptibility to antimicrobial agents, production of bacteriocins, protein and phage analyses, and the use of several chromatographic procedures was acquired and is available to many microbiology laboratories. During the past two decades, however, another major shift has occurred in the development of diagnostic tests: nucleic acids were discovered to be the identification targets of choice for the detection of a large number of different microbial pathogens. DNA, with its mere four building blocks and its relative chemical stability, proved to be an excellent template for a wide variety of diagnostic applications. Obviously, nucleic acids are the core substances that are found in all forms of microbial life. For this and other reasons to be discussed below, molecular diagnostics has grown in importance for the detection of many, if not most, agents of infectious diseases. The major principles, fields of application, today's trends, and future developments will be the topics for discussion herein.
Department of Medical Microbiology and Infectious Diseases

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