Apport de la biologie moléculaire au diagnostic de la tuberculose

Reducing delays in diagnosing tuberculosis should contribute to lower the lethality and to reduce the transmission. Microscopy examination is an easy to perform and rapid technique, but lacks sensitivity and specificity. Culture and biochemical identification are sensitive and specific, but require two to eight weeks DNA probes performed on solid or liquid media are very efficient for M. tuberculosis complex strains identification. Nucleic acid amplification techniques are available to directly detect the M. tuberculosis complex in clinical samples. Performances are variable. Very good results are obtained with experienced teams who scrupulously follow the specified guidelines.     

Applications de la biologie moléculaire au diagnostic de la toxoplasmose et d'autres protozooses

In medical parasitology PCR-based methods are the most frequently molecular used tools. The best results have been obtained for diagnosis of congenital toxoplasmosis by parasite DNA detection on amniotic fluid. This technique is now currently employed but a recent european study showed the need of an external quality assurance sheme because of the lack of homogeneity in results obtained by this blinded control (aliquots made with amniotic fluid spiked with Toxoplasma). For others protozoa the interest and limits of the use of PCR as diagnosis tool are in question..     

Approche moléculaire de la résistance à la méticilline de Staphylococcus aureus

Staphylococcus aureus is one of the most common species met in medical microbiology laboratories. It causes many infectious diseases, and some of them are severe. Today, it can resist to many antibiotics, that were initially fully active, such as G or A group penicillin, by producing a penicillinase. Hospital strains can also resist to M group penicillins (methicillin, oxacillin) by modifying the β-lactam target: penicillin binding protein (PBP). Indeed, these strains produce a new PBP, called PBP 2a, of which affinity for β-lactams is much lower than the one of common Staphylococcus PBP. This PBP2a is encoded by mecA gene, of which expression is under control of several regulation genes. The expression of methicillin-resistance can be homogeneous or heterogeneous. Some strains can resist to methicillin using other mechanisms (penicillinase overproduction, methicillin-specific hydrolytic enzyme, PBP modifications). Particular methods are needed for the phenotypic detection of methicillin-resistance: antibiograms on NaCl-supplemented agar or incubated at 30 °C. Molecular detection of mecA gene by PCR permits to detect that resistance on strains for which phenotypic detection has failed.