[Recognition and identification of unknown infectious agents].

The recognition and the identification of previously unrecognized infectious agents require a multidisciplinary approach to specify the nosologic entity of the disease and the epidemiological data, especially the modes of transmission and the risk factors, as well as to discover the microorganism in the laboratory. In the past 20 years, significant breakthroughs have been achieved in cellular cultures (growth factors), in immunology (monoclonal antibodies), and moreover in molecular biology, which have been widely used in the field of infectious diseases. Whereas the classical methods used to grow microorganisms remain of major interest in many cases, innovating strategies have been recently designed to identify previously unknown pathogens. The genomic amplification by polymerase chain reaction (PCR) of highly conserved bacterial genes (as those coding for ribosomal RNA), from tissue biopsies for example, allow to recognize unknown bacteria. The evolutionary distance between a newly recognized pathogen and known microorganisms can be calculated through sequencing of these genes, as described for Rochalimaea henselae or Tropheryma whipplelii. The constitution of cDNA banks from infected tissues is also a novel approach allowing to clone and sequence viral genes, such as those from hepatitis C or from hepatitis E. In the near future, noteworthy improvements will be achieved to rapidly detect microorganisms with highly sensitive and specific tests using monoclonal antibodies, molecular probes (including branched DNA) and with PCR (including Q beta replicase and ligase chain reaction), and to determine the genetic diversity of microbial pathogens by new methods as pulse field gel electrophoresis or arbitrarily primed PCR. This will result in a better knowledge of the pathophysiology of infectious diseases, in a better recognition of a typical, previously unrecognized clinical expression of pathogenicity, and also in a more precise assessment of the actual impact of a given pathogen in human populations by highly sophisticated diagnosis tests.