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 (α)-, beta (β)-, or gamma (γ)- hemolytic (Figure 1). 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 acid (CNA) is a selective medium for gram-positive organisms because the antimicrobials colistin and nalidixic acid 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 nonfermenting gram-negative rods by the color of the colonial growth (pink or clear, respectively); this is shown in Figure 2. 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.
Fig1. Examples of various types of hemolysis on blood agar. A, Streptococcus pneumoniae showing alpha (α)-hemolysis (i.e., greening around colony). B, Staphylococcus aureus showing beta (β)-hemolysis (i.e., clearing around colony). C, Enterococcus faecalis showing gamma (γ)-hemolysis (i.e., no hemolysis around colony).
Fig2. MacConkey agar. A, Escherichia coli, a lactose fermenter. B, Pseudomonas aeruginosa, a nonlactose fermenter.
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 deter mining 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 bacteria and allow gram-positive bacteria and yeast to be recovered.
