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Flashcards in Bacteriological Media Deck (33)
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What are enriched media?

Media with supplements added to support the growth of fastidious organisms. Blood agar and chocolate agar are examples of enriched media.


What are selective media?

Media with substances such as bile salts, dyes, or antibiotics added to inhibit certain bacteria. Examples of selective media are CNA, PEA, EMB, MacConkey, XLD, Hektoen Enteric, Modified Thayer Martin, and Martin Lewis.


What are differential media?

Media formulated to provide distinct colonial appearance based on certain biochemical reactions, such as lactose fermentation or hydrogen sulfide production. MacConkey, EMB, XLD, and Hektoen Enteric are differential media.


What might cause blood agar plates to appear brown?

Lysis of the RBCs. This can result from exposure to temperature extremes or from age of the medium.


What can be added to media to make it selective for gram-negatives?

Crystal violet, bile salts, eosin, or methylene blue.


What can be added to media to make it selective for gram-positives?

Colistin, nalidixix acid, or phenyethyl alcohol.


What type of bacteria grow on CNA and PEA?



When growth is observed on blood agar and MacConkey agar but not on NA or PEA, what type of organism has been isolated?

A gram-negative rod.


Why is chocolate agar used?

To grow fastidious organisms such as Neisseria and Haemophilus. (Most other organisms also grow on chocolate agar.)


When is xylose lysine deoxycholate agar (XLD) used?

For the recovery of Salmonella and Shigella from stool cultures. Hektoen enteric agar or SS agar may be used instead.


A pure growth of colorless colonies is observed on a MacConkey plate. What type of colonies would be expected on the XLD plate set up from the same specimen?

Colorless or red colonies which may have black centers if the organism produces H2S.


Name four media specifically used fro the isolation of anaerobes.

CDC anaerobic blood agar, Bacteroides bile esculin agar (BBE), laked kanamycin-vancomycin blood agar (LKV), phenylethyl alcohol sheep blood agar (PEA), thioglycolate broth, and chopped meat broth.


Will anything other than anaerobes grow in thioglycolate broth?

Yes. Most aerobes grow as well. The location of the growth indicates the organism's oxygen requirements. Aerobic organisms grow at the top of the tube, strict anaerobes at the bottom, and facultative anaerobes throughout.


In the Gulf states, where one might expect to isolate Vibrio, what medium should be added to those routinely used for stool cultures?

Thiosulfate citrate bile salts sucrose (TCBS). Alkaline peptone water may be used as an enrichment broth.


Which medium is best for recovery of Legionella pneumophila from clinical specimens?

Buffered charcoal yeast extract agar (BCYE).


Which medium should be used to recover Bordetella pertussis from a nasopharyngeal specimen?

Bordet Gengou, Regan Lowe, or Jones Kendrick charcoal agar.


What media are used to isolate mycobacteria?

Lowenstein Jensen, Middlebrook 7H10 and 7H11, and broth media.


When are Thayer Martin or Martin Lewis agar used?

To isolate pathogenic Neisseria from sites with normal flora. Antibiotics is these media suppress the normal flora.


E. coli was isolated on the Thayer Martin plates from several different patients in the same day. What might account for this unexpected finding?

The antibiotics were left out of the medium or had deteriorated. This illustrates the importance of performing quality control on media.


Which media are used for fecal/rectal cultures?

A supportive medium (blood agar), a medium selective for gram-negative rods (MacConkey or EMB), a medium selective for Salmonella and Shigella (XLD or Hektoen enteric), and Campylobacter blood agar. Some labs also use an enrichment broth for SAlmonella and Shigella (GN, tetrathionate, or selenite broth). Many include MaConkey sorbital agar (SMAC) for E. coli O157:H7. In certain geographic areas, CIN agar for Yersinia enterocolitica and/or TCBS for Vibrio may be included.


In some laboratories, MacConkey plates inoculated with stool specimens are incubated at room temperature when which fecal pathogen is suspected?

Yersinia enterocolitica. The optimum temperature for Y. enterolitica is 25-30C.


When is bismuth sulfite agar used?

When Salmonella is suspected. S. typhi produces black colonies surrounded by a metallic sheen.


Name a medium that is selective for Clostridium difficile.

Cycloserine sefoxitin fructose agar (CCFA). C. difficile produces yellow ground glass colonies.


What media are inoculated for urine cultures?

Blood agar and MacConkey or EMB.


What is added to holding media to absorb fatty acids present in the specimen that could kill fragile organism such as Neisseria gonorrhaeae?



Which cultures require incubation in increased CO2?

Blood, CSF, genital, Respiratory, wound, and cultures for AFB and Campylobacter.


Which plates are routinely incubated in increased Co2 at 37C?

Chocolate, modified Thayer Martin or Martin Lewis, Human Blood Tween (HBT) and Vaginalis (V) agar.


Which plates are routinely incubated in increased Co2 at 42C?

Campylobacter blood agar plates.


Why is media heated before autoclaving?

To dissolve the agar.


How is most media sterilized?

By autoclaving for 15 minutes at 15 pounds of pressure (121C).