Peptone and soy protein base agar with yeast extract, dextrose, and buffers. Addition of hemln, l-cystine, and 5% blood enriches Jor anaerobes
Nonselective medium for the recovery of anaerobes and aerobes
Peptone base broth. Sodium selenite toxic for most Enterobacteriaceae
Enrichment of isolation of Salmonella spp.
Skirrow agar i
Peptone and spy protein base agar with lysed horse blood. Vancomycin inhibits gram-positive organisms; polymyxin B and trimethoprim inhibit most gram-negative organisms
Selective for Campylobacters^?.
* Streptococcal selective agar (SSA)
Contains crystal violet, colistin, and trimethoprim-sulfamethoxazole in 5% sheep blood agar base
Selective for Streptococcus pyogenes and Streptococcus agalactiae *
Tetratfilonate broth J
Peptone base broth. Bile salts and sodium thiosulfate Inhibit gram-positive organisms and Enterobacteriaceae
Selective for Salmonella and Shigella spp.
Thayer-Martin agar U
Blood agar base enriched with hemoglobin and supplement B; contaminating organisms inhibited by colistin, nystatin, vancomycin, and trimethoprim
Selective for N. gonorrhoeae and AT. meningitidis
Pancreatic digest of casein, soy broth, and glucose enrich growth of most microorganisms.
Supports growth of anaerobes, aerobe?, mlcroaerophilic, and fastidious microorganisms
Thiosulfate citrate-bile salts (TCBS)agar
Peptone base agar with yeast extract bile salts, citrate, sucrose, ferric citrate, and sodium thiosulfate. Bromthymol blue acts as indicator
Selective and differential for vibrios
r Todd-Hewitt broth supplemented with antibiotics
Todd-Hewitt, an enrichment broth for streptococci, is supplemented with nalidixic acid and gentamicin or colistin for greater selectivity Thioglycollate and agar reduce redox potential
Selection and enrichment for Streptococcus agalactiae in female genital specimens
Trypt'case soy broth (TSB)
All-purpose enrichment broth that can support the growth of many fastidious and nonfastidious bacteria
Enrichment broth used fbrsubculturing various bacteria from primary agar plates
Xylose lysine desoxycholate (XLD)agar
Yeast extract agar with lysine, xylose, lactose, sucrose, and ferric ammonium citrate. Sodium desoxycholate inhibits gram-positive organisms; phenol red as indicator
Isolation and differentiation of Salmonella and Shigella spp. from other gram-negative enteric bacilli
Of the dozens of available media, only those most commonly used for routine diagnostic bacteriology are summarized in this discussion. Part VII discusses which media should be used to culture bacteria from various clinical specimens. Similarly, other chapters throughout Part III discuss media used to identify and characterize specific organisms.
Brain-Heart Infusion. Brain-heart infusion (BHI) is a nutritionally rich medium used to grow various microorganisms, either as a broth or as an agar, with or without added blood. Key ingredients include infusion from several animal tissue sources, added peptone (protein), phosphate buffer, and a small concentration of dextrose. The carbohydrate provides a readily accessible source of energy for many bacteria. BHI broth is often used as a major component of the media developed for culturing a patient's blood for bacteria (see Chapter 52), establishing bacterial identification, and for certain tests to determine bacterial susceptibility to antimicrobial agents (see Chapter 12).
Chocolate Agar. Chocolate agar is essentially the same as blood agar except that during preparation the red blood cells are lysed when added to molten agar base. This lysis releases intracellular nutrients such as hemoglobin, hemin ("X* factor), and the coenzyme nicotinamide adenine dinucleotide (NAD or *V factor) into the agar for utilization by fastidious bacteria. Red blood cell lysis gives the medium a chocolate-brown color from which this agar gets its name. The most common bacterial pathogens that require this enriched medium for growth include Neisseria gonorrhoeae, the causative agent of gonorrhea, and Haemophilus spp., which cause infections usually involving the respiratory tract and middle ear. Neither of these species are able to grow on sheep blood agar.
Columbia CNA with Blood. Columbia agar base is a nutritionally rich formula containing three peptone sources and 5% defibrinated sheep blood. This supportive medium can also be used to help differentiate bacterial colonies based on the hemolytic reactions they produce. CNA refers to the antibiotics colistin (C) and nalidixic acid (NA) that are added to the medium to suppress the growth of most gram-negative organisms while allowing gram-positive bacteria to grow, thus conferring a selective property to this medium.
Gram-Negative (GN) Broth. A selective broth, gram-negative (GN) broth is used for the cultivation of gastrointestinal pathogens (i.e.. Salmonella spp. and Shigella spp.) from stool specimens and rectal swabs. The broth contains several active ingredients, including sodium citrate and sodium desoxycholate (a bile salt)
that inhibit gram-positive organisms and the early multiplication of gram-negative, nonenteric pathogens. To optimize its selective nature, GN broth should be subcultured 6 to 8 hours after initial inoculation and incubation. After this time, the nonenteric pathogens begin to overgrow the pathogens.
Hektoen Enteric (HE) Agar. Hektoen enteric (HE) agar contains bile salts and dyes (bromthymol blue and acid fuchsin) to selectively slow the growth of most nonpathogenic gram-negative bacilli found in the gastrointestinal tract and allow Salmonella spp. and Shigella spp. to grow. The medium is also differential because many nonenteric pathogens that do grow will appear as orange to salmon-colored colonies. This colony appearance results from the organism's ability to ferment the lactose in the medium, resulting in the production of add, which lowers the medium's pH and causes a change in the pH indicator bromthymol blue. Salmonella spp. and Shigella spp. do not ferment lactose, so no color change occurs and their colonies maintain the original blue-green color of the medium. As an additional differential characteristic, the medium contains ferric ammonium dtrate, an indicator for the detection of H2S, so that H2S-produdng organisms, such as Salmonella spp., can be visualized as colonies exhibiting a black predpitate (Figure 7-6).
MacConkey Agar. MacConkey agar is the most frequendy used primary selective and differential agar. This medium contains crystal violet dye to inhibit the growth of gram-positive bacteria and fungi, and allows many types of gram-negative bacilli to grow. The pH indicator, neutral red, provides this medium with a differential capadty. Bacterial fermentation of lactose results in add production, which decreases medium pH and causes the neutral red indicator to give bacterial colonies a pink to red color. Non-lactose-fennenters, such as Shigella spp,, remain colorless and translucent (see Figure 7-4).
Phenylethyl Alcohol (PEA) Agar. Phenylethyl alcohol (PEA) agar is essentially sheep blood agar that is supplemented with phenylethyl alcohol to inhibit the growth of gram-negative bacteria. Five percent sheep blood in PEA provides nutrients for common grampositive coca such as enterococd, streptococci, and staphylococd (see Figure 7-3),
Sheep Blood Agar. Most bacteriology specimens are inoculated to sheep blood agar plates because this medium supports all but the most fastidious clinically significant bacteria. Additionally, the colony morphologies that commonly encountered bacteria exhibit on this medium are familiar to most clinical microbiologists.
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