Selective and Differential Media Flashcards
What are Mannitol salt agar (MSA) plates used to isolate?
Halophiles, specifically Enterococcus faecalis and Staphylococcus sp.
What does Staphylococcus aureus look like on MSA plates?
- On MSA, pathogenic Staphylococcus aureus produces small colonies surrounded by yellow zones.
- The reason for this change in color is that S. aureus ferments the mannitol, producing an acid, which, in turn, changes the pH indicator dye (phenol red) from red to yellow.
Eosin methylene blue (EMB) agar is a differential medium used for the detection and isolation of what?
ashwin is a bitch
- Eosin methylene blue (EMB) agar is a differential medium used for the detection and isolation of Gram-negative intestinal pathogens.
- Differentiates b/w coliforms and enterics
On EMB plates, a combination of eosin and methylene blue is used as an indicator and allows differentiation between organisms that ferment ______ and those that do not.
On EMB plates, a combination of eosin and methylene blue is used as an indicator and allows differentiation between organisms that ferment lactose and those that do not.
Why is sucrose is also included in the medium of EMB plates?
Sucrose is also included in the medium because certain members of the family Enterobacteriaceae (enteric bacteria) or coliforms (a subset of enterics) ferment sucrose more readily than they ferment lactose.
On EMB plates, methylene blue acts as an inhibitor for what?
- Methylene blue acts as an inhibitor to Gram-positive organisms (so EMB agar is selective, too).
What does E. coli look like on EMB plates? Why?
- Colonies of E. coli normally have a dark center and a greenish metallic sheen.
- Vigorous fermentation of sucrose, as E. coli ferments sucrose and/or lactose faster than E. aerogenes
What does Enterobacter aerogenes look like on EMB plates?
Enterobacter aerogenes are usually pink, mucoid, and much larger than colonies of E. coli.
On EMB plates, what do organisms that do not ferment lactose or sucrose look like?
Produce colonies that retain their natural color.
- Example: Salmonella; creamy colored colonies
What is MacConkey’s aga used for?
- MacConkey’s agar is a differential plating medium used in the detection and isolation of all types of dysentery, typhoid and paratyphoid organisms.
- It is generally used for differentiating strains of Salmonella typhosa from members of the coliform group.
- However, the medium supports the growth of all Salmonella and Shigella strains and gives good differentiation between these enteric pathogens and the coliform group.
What does E. coli look like on MacConkey’s agar? Why?
- When grown on MacConkey’s medium, colonies of E. coli (a fecal coliform) are reddish or deep fuschia in color.
- This is due to acidification of the medium via the formation of acidic fermentation products from lactose fermentation.
- Neutral red dye is a pH indicator that turns red below a pH of 6.8.
- The acid end-products of lactose fermentation produced by E. coli also precipitate bile salts, and neutral red is absorbed by the precipitated salts.
- Thus, the medium around E. coli has a pink cloudy appearance.
On MacConkey’s agar plates, non-fecal coliforms ferment lactose to acid products similar to E. coli. However, they do not produce what?
- Non-fecal coliform bacteria can ferment lactose to acid products too, but don’t precipitate bile salts.
- They appear pink.
On EMB plates, dysentery, typhoid, and paratyphoid bacilli do not ferment lactose. What do they do instead?
- Dysentery, typhoid and paratyphoid bacilli do not ferment lactose but give an alkaline reaction when grown on the medium.
- Colonies of these organisms retain their normal creamy color.
How is the growth of Gram-positive organisms inhibited on MacConkey’s agar?
- The growth of Gram-positive organisms is inhibited because of the crystal violet and bile salts in the medium.
- So, MacConkey’s agar is selective as well as differential.
