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Questions and Answers
Which bacteria are lactose fermenters?
Which bacteria are lactose fermenters?
What color do E. coli colonies appear on EMB agar?
What color do E. coli colonies appear on EMB agar?
Blue black with a green metallic sheen
Salmonella spp. produces hydrogen sulfide.
Salmonella spp. produces hydrogen sulfide.
True
What color indicates a non-lactose fermenter on Hektoen Enteric Agar?
What color indicates a non-lactose fermenter on Hektoen Enteric Agar?
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How do the colonies of Pseudomonas aeruginosa typically appear on BAP?
How do the colonies of Pseudomonas aeruginosa typically appear on BAP?
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An organism showing no migration in a motility test is classified as motile.
An organism showing no migration in a motility test is classified as motile.
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What does a yellow coloration in Tube 2 of the OF test indicate?
What does a yellow coloration in Tube 2 of the OF test indicate?
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What does a K/A result in TSI indicate?
What does a K/A result in TSI indicate?
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Study Notes
Eosin Methylene Blue Agar
- Encourages the growth of gram-negative bacteria, specifically fecal and non-fecal coliforms.
- Coliforms are non-spore forming, motile or non-motile bacteria that ferment lactose to acid and gas. This is used as an indicator of sanitary quality in water and food.
- Lactose fermenters: Escherichia, Klebsiella, Enterobacter, Serratia.
- Acidify the media, reducing the pH.
- Dye produces a dark purple complex with a green metallic sheen.
- Eosin Y/Methylene blue (pH indicators) at low pH form a dark purple precipitate, inhibiting the growth of gram-positive bacteria.
- E. coli produces blue-black colonies with a green metallic sheen.
- Green metallic sheen indicates vigorous fermentation of lactose and/or sucrose, typical of fecal coliforms.
- Other lactose fermenters produce less acid resulting in brown-pink colonies.
- Non-lactose fermenters increase the pH by deaminating proteins resulting in colorless or light pink colonies.
- Salmonella spp. appear colorless or amber.
Hektoen Enteric Agar
- Designed to differentiate Salmonella and Shigella.
- Non-pathogens (lactose fermenters): produce acidic media, turning the agar yellow/orange.
- Salmonella spp. (non-lactose fermenters): produce blue/green colonies with dark centers due to the production of hydrogen sulfide (H2S).
- Shigella spp. (non-lactose fermenters): produce blue/green colonies without dark centers, as they do not produce H2S.
XLD - Xylose Lysine Deoxychocolate Agar
- Selectively promotes and differentiates Salmonella and Shigella.
- Inhibits other enteric pathogens by utilizing xylose fermentation with a phenol red indicator, sucrose, and lactose (in excess).
- Detects lysine decarboxylation
- Detects hydrogen sulfide.
- Non-pathogens (lactose fermenters) that do not produce lysine decarboxylase appear yellow/orange.
- Salmonella spp. (non-lactose fermenters) produce red colonies with black centers.
- Shigella spp. (non-lactose fermenters) appear colorless or red.
Pseudomonas aeruginosa - Cultural Characteristics
- Pseudomonas aeruginosa has a distinct fruity, grape-like, or corn-tortilla-like odor.
- On MacConkey agar, it appears as a non-lactose fermenter.
- On blood agar, it displays large, irregular colonies with serrated edges.
- Confluent growth forms a metallic (blue/green) sheen with a scaling appearance (alligator skin morphology).
- It also has a gun-metal color, spreading, flattened topology, and beta-hemolytic activity.
- It can produce a turquoise-blue diffusible pigment called pyocyanin.
Pseudomonas aeruginosa - Identification
- Motile, with a single polar flagellum.
- Oxidase-positive.
- Catalase-positive.
- Nitrate-positive, reducing nitrate beyond nitrite to nitrogen gas.
- Arginine dihydrolase (ADH) positive.
- Citrate-positive.
- OF test: non-fermenter (oxidizer of carbohydrates).
- TSI: Alk/Alk or K/K (2nd tube).
- Growth at 42°C: positive.
Motility Medium
- Contains TTC (2,3,5-triphenyltetrazolium chloride) indicator.
- Motility can be difficult to observe in NFB as they grow in the upper (most aerobic) portion of the tube.
- TTC detects motility as organisms that reduce TTC appear red along the stab line, and in the area the cells have migrated to, making it easier to identify.
- Motile: red stab line and red upper portion where cells have migrated.
- Non-motile; red along the inoculation line with no migration.
OF Test
- Hugh-Leifson oxidative fermentative (OF) media.
- Tube 1: overlaid with mineral oil, excluding oxygen exposure.
- Tube 2: open to atmospheric oxygen.
- Acid (yellow) on top portion of tube 2.
- Green seen on the bottom portion of tube 2 (unable to oxidize in the absence of oxygen).
- Organisms that are capable of oxidizing glucose but incapable of fermenting glucose will be yellow on the top of tube 2 and green on the bottom.
TSI (Triple Sugar Iron)
- Tube 1: K/A (alkaline slant/acid butt) - fermentation of glucose, no fermentation of lactose (NLF, Enterobacteriaceae).
- Tube 2: K/K (alkaline slant/alkaline butt) - NFB, unable to utilize lactose, glucose, or sucrose in TSI.
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Description
Test your knowledge on Eosin Methylene Blue Agar, a selective medium for gram-negative bacteria. This quiz covers the characteristics, fermentation patterns, and the visual indicators related to coliforms and their fermentation abilities. Dive into the details of how this agar plate works as an indicator for sanitary quality in food and water.
