Xylose is incorporated into the medium since it is fermented by practically all enterics except for the shigellae, and this property enables the differentiation of Shigella species. Lysine is included to enable the Salmonella group to be differentiated from the nonpathogens since, without lysine, salmonellae rapidly would ferment the xylose and be indistinguishable from nonpathogenic species. After the salmonellae exhaust the supply of xylose, the lysine is attacked via the enzyme, lysine decarboxylase, with reversion to an alkaline pH which mimics the Shigella reaction. To prevent similar reversion by lysinepositive coliforms, lactose and sucrose (saccharose) were added to produce acid in excess.1 To add to the differentiating ability of the formulation, an H2S indicator system, consisting of sodium thiosulfate and ferric ammonium citrate, is included for the visualization of the hydrogen sulfide produced, resulting in the formation of colonies with black centers. The nonpathogenic H2S producers do not decarboxylate lysine; therefore, the acid reaction produced by them prevents the blackening of the colonies.1 Xylose Lysine Deoxycholate Agar is both a selective and differential medium. It utilizes sodium desoxycholate as the selective agent and, therefore, it is inhibitory to gram-positive microorganisms.

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Columbia Agar Base supplemented with sheep, rabbit or horse blood derives its superior growth-supporting properties from the combination of peptones prepared from pancreatic digest of casein, peptic digest of animal tissue and beef extract. Yeast extract and corn starch are also included in the formulation and serve as energy sources with yeast extract being a supplier of the B-complex vitamins. It should be noted that Columbia Sheep Blood Agar has a relatively high carbohydrate content and, therefore, beta-hemolytic streptococci may produce a greenish hemolytic reaction that may be mistaken for alpha hemolysis. Fildes enrichment is prepared by the action of the enzyme pepsin on defibrinated sheep blood. Bacitracin is a polypeptide antibiotic that is active mainly against gram-positive bacteria.

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Mannitol Salt Agar is a nutritive medium due to its content of peptones and beef extract, which supply essential growth factors, such as nitrogen, carbon, sulfur and trace nutrients. The 7.5% concentration of sodium chloride results in the partial or complete inhibition of bacterial organisms other than staphylococci. Mannitol fermentation, as indicated by a change in the phenol red indicator, aids in the differentiation of staphylococcal species.

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The production of pyocyanin is stimulated by the magnesium chloride and potassium sulfate in the medium. Cetrimide Agar is a quaternary ammonium compound which is inhibitory to a wide variety of bacterial species including Pseudomonas species other than P. aeruginosa.

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Peptones are sources of nitrogen and other nutrients. Lactose is a fermentable carbohydrate. When lactose is fermented, a local pH drop around the colony causes a color change in the pH indicator (neutral red) and bile precipitation. Bile salts, bile salts no. 3, oxgall and crystal violet are selective agents that inhibit growth of gram-positive organisms. Agar is the solidifying agent.

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Violet Red Bile Glucose Agar contains peptone as a source of carbon, nitrogen, vitamins, and minerals. Yeast extract supplies B-complex vitamins, which stimulate bacterial growth. Glucose is a carbohydrate. Bile salts and crystal violet inhibit gram- positive bacteria. Glucose fermenters produce red colonies with red-purple halos (bile precipitation) in the presence of neutral red, a pH indicator. Agar is the solidifying agent.

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Sabouraud Dextrose Agar is used in qualitative procedures for the cultivation of pathogenic and nonpathogenic fungi, particularly dermatophytes. The medium is rendered more selective for fungi
by the addition of antimicrobics. Sabouraud Dextrose Broth and Sabouraud Maltose Agar and Broth are also used for culturing yeasts, molds, and aciduric microorganisms.

Fluid Sabouraud Medium is used for cultivating yeasts, molds, and aciduric microorganisms and detecting yeasts and molds in normally sterile materials.

Sabouraud dextrose media are peptone media supplemented with dextrose to support the growth of fungi. Media are also provided with maltose substituted for the dextrose. Peptones are sources of nitrogenous growth factors. Carbohydrates provide an energy source for the growth of microorganisms. Gentamicin is an aminoglycoside antibiotic that inhibits the growth of gram-negative bacteria. Chloramphenicol is inhibitory to a wide range of gram-negative and gram-positive bacteria, and cycloheximide is an antifungal agent that is primarily active against saprophytic fungi and does not inhibit yeasts or dermatophytes.

Lecithin neutralizes quaternary ammonium compounds, and polysorbate 80 neutralizes substituted phenolic disinfectants.

For the Sterile Pack products, the entire double-bagged product is subjected to a sterilizing dose of gamma radiation. Thus, the contents inside the outer bag are sterile. This allows the inner bag to be aseptically removed and brought into an environment- a mentally controlled area without introducing contaminants. A third sterile bag is included as a transport device. Since the agar medium has been sterilized after packaging, the presence of microbial growth after sampling and incubation can be relied upon to represent the presence of environmental contami- nants and not pre-existing microorganisms in the medium that may have been introduced during manufacture. The RODAC plates have a marked grid to facilitate counting organisms. The Sterile Pack Finger Dab Isolator plates are triple-bagged and are intended for sampling gloved hands.

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Potato Dextrose Agar is used for the cultivation and enumeration of yeasts and molds.

Potato starch and dextrose support the luxuriant growth of fungi. Lowering the pH of the medium to approximately 3.5 with sterile tartaric acid achieves the inhibition of bacterial growth. It is important, however, to avoid heating the medium after it has been acidified because this action results in the hydrolysis of the agar and impairs its ability to solidify.

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Hycheck hygiene contact slides are used for assessing the microbiological contamination of surfaces and fluids.

The combination of casein and soy peptones in TSA renders the medium highly nutritious by supplying organic nitrogen, particularly amino acids and longer-chained peptides. The sodium chloride maintains osmotic equilibrium. Agar is the solidifying agent.

Haemophilus species may be differentiated by their requirements for X and V factors. Paper strips impregnated with these factors are placed on the surface of the medium after inoculation with the test organism. Following incubation, a zone of growth around the strip indicates a requirement for the factor(s).

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These media provide a variety of enzymatic digests of proteinaceous substrates, inorganic salts and other nutrients to satisfy the nutritional requirements of these organisms. Selective agents are incorporated to improve the recovery of Burkholderia cepacia by inhibiting common contaminants. PC agar incorporates crystal violet to inhibit gram-positive cocci, especially enterococci and staphylococci, bile salts to inhibit most gram-positive cocci other than enterococci, and ticarcillin and polymyxin B to inhibit gram-negative bacilli. OFPBL Agar incorporates polymyxin B to inhibit gram-negative flora, while bacitracin inhibits the gram-positive organisms and Neisseria. 4 PC Agar contains the pH indicator phenol red to facilitate detection of Burkholderia Cepacia Selective Agar. Alkaline end products from the metabolism of pyruvate raise the pH of the medium, causing the color of the indicator to change from light orange to pink or pink-red in the area of growth. In areas of heavy growth of Burkholderia Cepacia, the pink color intensifies. OFPBL Agar contains the pH indicator bromthymol blue to facilitate detection of Burkholderia Cepacia. Acid end products from the metabolism of lactose lower the pH of the medium resulting in a yellow color change. Burkholderia Cepacia colonies will also have a yellow color.

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