MICROBIOLOGY

PRINCIPLES OF DIAGNOSTIC MEDICAL MICROBIOLOGY CONTD... IDENTIFYING BACTERIA USING 16S RRNA The 16S rRNA of each species of bacteria has stable (conserved) portions of the sequence.  Many copies are  present in each organism.  Labeled probes specific for the 16S rRNA of a species are added,  and the amount of  label on the double-stranded hybrid is measured.  This technique is widely used for the rapid identification of  many organisms.  Examples include the most common and important Mycobacterium species,   C immitis,  Histoplasma capsulatum ,  and others.  Portions of the 16S rRNA are conserved across many species of microorganisms.  Amplifying the 16S rRNA using  primers to these conserved regions allows isolation and sequencing of the variable regions of the molecules.  These variable sequences are genus- or species-specific markers that allow identification of microorganisms.   Pathogens that are difficult or impossible to culture in the laboratory have been identified using thi

PRINCIPLES OF DIAGNOSTIC MICROBIOLOGY... CULTURE SYSTEMS:  For diagnostic bacteriology ,  it is necessary to use several types of media for routine culture,  particularly when  the possible organisms include aerobic,  facultatively anaerobic,  and obligately anaerobic bacteria.  The  specimens and culture media used to diagnose the more common bacterial infections.  The standard medium for specimens is blood agar ,  usually made with 5% sheep blood.  Most aerobic and facultatively anaerobic organisms will grow on blood agar .  Chocolate agar ,  a medium containing heated blood  with or without supplements,  is a second necessary medium; some organisms that do not grow on blood agar, including pathogenic Neisseria and Haemophilus ,  will grow on chocolate agar.  A selective medium for enteric  gram-negative rods (either MacConkey agar or eosin-methylene blue [EMB] agar) is a third type of medium  used routinely.   Specimens to be cultured for obligate anaerobes must be plated on