Primary culture media are divided into several categories. The first are nutritive media, such as blood or chocolate agars. Nutritive media support the growth of a wide range of microorganisms and are considered nonselective because, theoretically, the growth of most organisms is supported. Nutritive media can also be differential, in that microorganisms can be distinguished on the basis of certain growth characteristics evident on the medium. Blood agar is considered both a nutritive and differential medium because it differentiates organisms based on whether they are alpha (a)-, beta (P)-, or gamma (y)- hemolytic (Figure 5-2). Selective media support the growth of one group of organisms, but not another, by adding antimicrobials, dyes, or alcohol to a particular medium. MacConkey agar, for example, contains the dye crystal violet, which inhibits gram-positive organisms. Columbia agar with colistin and nalidixic add (CNA) is a selective medium for gram-positive organisms because the antimicrobials colistin and nalidixic add inhibit gram-negative organisms. Selective media can also be differential media if, in addition to their inhibitory activity, they differentiate between groups of organisms. MacConkey agar, for example, differentiates between lactose-fermenting and nonfetmenting gram-negative rods by the color of the colonial growth (pink or dear, respectively); this is shown in Figure 5-3. In some cases (sterile body fluids, tissues or deep abscesses in a patient on antimicrobial therapy), backup broth (also called supplemental or enrichment broth) medium is inoculated, along with primary solid (agar) media, so small numbers of organisms present may be detected; this allows detection of anaerobes in aerobic cultures and organisms that may be damaged by either previous or concurrent antimicrobial therapy. Thioglycollate (thio) broth, brain-heart infusion broth (BHIB), and tryptic soy broth (TSB) are common backup broths.
Selection of media to inoculate for any given specimen is usually based on the organisms most likely to be involved in the disease process. For example, in deteimining what to set up for a CSF specimen, one considers the most likely pathogens that cause meningitis (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis, Escherichia coli, group B Streptococcus) and selects media that will support the growth of these organisms (blood and chocolate agar at a minimum). Likewise, if a specimen is collected from a source likely to be contaminated with normal flora, for example, an anal fistula (an opening of the surface of the skin near the anus that may communicate with the rectum), one might want to add a selective medium, such as CNA, to suppress gram-negative organisms and allow grampositive organisms and yeast to be recovered.
Routine primary plating media and direct examinations for specimens commonly submitted to the microbiology laboratory are shown in Table 5-1. Chapter 7 on bacterial cultivation reemphasizes the strategies described here for selection and use of bacterial media.
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