BISC209: Selective,and/or Differential, and/or Enriched media: Difference between revisions

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<font size="+1">'''Selective / Differential / Enrichment media'''</font size="+1"><BR>
<font size="+1">'''Selective / Differential / Enrichment media'''</font size="+1"><BR>
Selective media helps select for growth of certain organisms in a mixed population by using a ingredient that inhibits the growth of other microorganisms, but not the desired species or group. Enrichment media selects for certain microorganisms by including a nutrient that the desired microorganism or group can use and its competitors can not. (Sometimes enrichment media also limits alternate sources of nutrition). Differential media does not select for any particular group by inhibiting or enhancing their growth over competitors, but it does show a visible difference between or among groups of microorganisms. Some media can be both selective and differential. <BR>
Selective media helps select for growth of certain organisms in a mixed population by using a ingredient that inhibits the growth of other microorganisms, but not the desired species or group. Enrichment media selects for certain microorganisms by including a nutrient that the desired microorganism or group can use and its competitors can not. (Sometimes enrichment media also limits alternate sources of nutrition). Differential media does not select for any particular group by inhibiting or enhancing their growth over competitors, but it does show a visible difference between or among groups of microorganisms. Some media can be both selective and differential. <BR>
For more information on the formulations and types of media available in microbiology see:  [http://www.bd.com/ds/technicalCenter/inserts/difcoBblManual.asp BD diagnostice Systems Difco catalog of media] http://www.bd.com/ds/technicalCenter/inserts/difcoBblManual.asp<BR>
==Selective for Gram positive Organisms==
==Selective for Gram positive Organisms==
'''Phenylethyl Alcohol Agar (PEA)'''
'''Phenylethyl Alcohol Agar (PEA)'''
PEA selects for the growth of Gram positive organisms by inhibiting the growth of Gram negative bacilli. The phenylethyl alcohol interfers with DNA synthesis in Gram negative organisms.  This medium is particularly useful at inhibiting the overgrowth of Gram negative ''Proteus species'' that tend to swarm (they are highly motile) and, thus, make isolation of Gram positive organisms difficult in a mixed population . <BR>  
PEA selects for the growth of Gram positive organisms by inhibiting the growth of Gram negative bacilli. The phenylethyl alcohol interfers with DNA synthesis in Gram negative organisms.  This medium is particularly useful at inhibiting the overgrowth of Gram negative ''Proteus species'' that tend to swarm (they are highly motile) and, thus, make isolation of Gram positive organisms difficult in a mixed population . <BR>  
'''Recipe''':  10g tryptose, 3g Beef extract, 5g Sodium Chloride, 2.5g Phenylethyl alcohol, 15g Agar to 1 liter distilled or deionized water pH 7.1-7.5.<BR><BR>
'''Recipe''':  10g tryptose, 3g Beef extract, 5g Sodium Chloride, 2.5g Phenylethyl alcohol, 15g Agar to 1 liter distilled or deionized water pH 7.1-7.5.<BR><BR>
'''Mannitol Salt Agar''' (also see Targeted Enrichment media list)<BR><BR>
'''Mannitol Salt Agar''' (an alternative to PEA, not used in 2010)<BR><BR>
Postive control organism:  ''Staphylococcus epidermidis''


==Selective for Gram negative Organisms==
==Selective for Gram negative Organisms==
'''MacConkey Agar''' is a differential and selective medium that is used for the separation of lactose fermenting Gram negative enteric bacteria from Gram negative bacteria that do not ferment lactose.  It is routinely used for the isolation of the pathogenic bacterial genera, ''Salmonella'' or ''Shigella'', from clinical specimens, water samples and contaminated food.<BR>
'''Eosin–Methylene Blue (EMB) Agar''' is a differential medium for the detection of Gram negative enteric bacteria.  The medium contains peptone, lactose, sucrose, dipotassium phosphate, eosin and methylene blue dyes.  Eosin and methylene blue act as indicators to differentiate between Gram negative organisms that ferment lactose and those that do not ferment lactose.  Most bacteria that ferment lactose form colonies on EMB agar that are dark blue to black with a metallic sheen due to precipitation of the dyes by the acid by-products of fermentation.  Colonies produced by lactose non-fermentors are not dark blue or black.  The growth of Gram positive bacteria is generally inhibited on EMB agar because of the toxicity of methlyene blue dye. In low concentration, the protective lipid outer membrane of Gram negative bacteria prevents entry of the toxic water soluble dye while the more porous cell wall of Gram positive bacteria without the protective outer membrane makes them more sensitive to the toxicity of methyene blue.<BR><BR>
MacConkey agar contains lactose, peptone, bile salts, sodium chloride, neutral red and crystal violet dyes as well as agar and water.  Bile salts and crystal violet inhibit the growth of Gram positive organisms.  The differential action of MacConkey agar is based on whether or not the organism ferments lactose (Table A-1). Colonies of lactose fermenting bacteria absorb the neutral red dye and thus, appear red in color.  Bile salts may also be precipitated around a lactose-fermenting colony as a red halo.  Colonies of bacteria that do not ferment lactose appear colorless and translucent.  This differentiation is most obvious when discrete, single colonies are present on the MacConkey plate.<BR>
'''Recipe''':10g peptone, 10g Lactose,  2g dipotassium phosphate, 0.4g  eosin Y, 0.065 g methylene blue 15 g Agar.  final pH 6.9-7.3 <BR><BR>
'''Recipe''':2% peptone, 1% lactose, 0.15% bile salts, 0.5% NaCl, 1.35% agar, 30 µg/ml neutral red, 1 µg/ml crystal violet<BR><BR>
 
''Table A-1.'' Colonial appearance on MacConkey Agar after18-24 hours at 35°C.  The growth of colonies on the plate indicate that the organism is most likely Gram negative and the color of the colony indicates that the organism is able to ferment lactose and/or form bile salts.<BR>
 
{| border="1"
|+
! '''Organism''' !! '''Colonial Appearance'''
|-
! ''Escherichia coli''
| pink to red colonies with bile salt precipitate
|-
! ''Enterobacter aerogenes''
| pink to red colonies
|-
! ''Proteus vulgaris''
| colorless colonies
|-
! ''Salmonella enteritidis''
| colorless colonies
|-
! ''Shigella dysenteriae''
| colorless colonies
|-
|}
 
 
'''Eosin–Methylene Blue (EMB) Agar''' is a differential medium for the detection of Gram negative enteric bacteria.  The medium contains peptone, lactose, sucrose, dipotassium phosphate, eosin and methylene blue dyes.  Eosin and methylene blue act as indicators to differentiate between Gram negative organisms that ferment lactose and those that do not ferment lactose.  Most bacteria that ferment lactose form colonies on EMB agar that are dark blue to black with a metallic sheen due to precipitation of the dyes by the acid by products of fermentation.  Colonies produced by lactose non-fermentors are not dark blue or black.  The growth of Gram positive bacteria is generally inhibited on EMB agar.<BR>
'''Recipe''':1% peptone, 0.2% dipotassium phosphate, 1.5% agar, 400 µg/ml eosin Y, 65 µg/ml methlene blue <BR>
'''Table 2.''' Colonial appearance on EMB Agar after 18-24 hours at 35°C.  A differential medium can be used to differentiate Gram negative enteric organisms based on the colony color.<BR>
'''Table 2.''' Colonial appearance on EMB Agar after 18-24 hours at 35°C.  A differential medium can be used to differentiate Gram negative enteric organisms based on the colony color.<BR>


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<BR><BR>
==Other selective media==
 
'''Salmonella-Shigella (SS) Agar''' is a selective medium used for the detection and isolation of Salmonella and/or Shigella bacterial strains from clinical specimens, water and contaminated food products. Salmonella and Shigella, as well as other organisms that do not ferment lactose, form colorless opaque, translucent or transparent colonies on SS agar.  Most lactose fermenting bacteria are inhibited, and the few that can grow form red, mucoid colonies, some of which may have black centers.<BR>
'''Recipe''': 0.5% beef extract, 0.5% peptone, 1% lactose, 0.85% bile salts, 0.85% sodium citrate, 0.85% sodium thiosulfate, 0.1% ferric citrate, 1.35% agar, 333 µg/ml brilliant green, 25 µg/ml neutral red <BR><BR>
Table 3. Colonial appearance on SS Agar after18-24 hours at 35°C.  The color of the colonies of these Gram negative enteric organisms helps select for Salmonella and /or ''Shigella species.''
 
 
{| border="1"
|+
! '''Organism''' !! '''Colonial Appearance'''
|-
! ''Enterobacter aerogenes''
| creamy pink colonies
|-
! ''Proteus mirabilis''
| colorless colonies
|-
! ''Salmonella enteridtidis''
| colorless colonies
|-
! ''Shigella flexnerii''
| colorless colonies
|-
|}
 
 
'''Bismuth Sulfite Agar''' is a highly selective medium for the isolation of ''Salmonella thyphi'' and other ''Salmonella species'' from clinical specimens, food products, water and sewage. Bismuth sulfite and brilliant green inhibit the growth of gram positive bacteria and many non-salmonellal members of the coliform group.  H<sup>2</sup>S production occurs due to the presence of sulfur in the medium.  H<sup>2</sup>S causes the iron in the media to precipitate causing colonies to turn black often with a metallic sheen.
'''Recipe:''' 0.5% beef extract, 1% peptone, 0.55 dextrose, 0.4% disodium phosphate, 0.03% ferrous sulfate, o.8% bismuth sulfite indicator, 2% agar, 25 µg/ml brilliant green <BR><BR>
 
Table 4. Colonial appearance on Bismuth Sulfite Agar after 18-24 hours, 35°C.  This selective medium uses colony color to help identify species of ''Salmonella'' and some other coliform bacteria.<BR>
 
{| border="1"
|+
! '''Organism''' !! '''Colonial Appearance'''
|-
! ''Escherichia coli''
| brown-green colonies, if present
|-
! ''Enterobacter aerogenes''
| brown-green colonies, if present
|-
! ''Salmonella typhi''
| black colonies with metallic sheen
|-
! ''Salmonella enteritidis''
| black colonies with metallic sheen
|-
! ''Salmonella paratyphi''
| green colonies
|-
! ''Salmonella typhimurium''
| green colonies
|-
! ''Proteus morganii''
| green colonies
|-
! ''Shigella flexnerii''
| brown colonies, if present
|-
|}




Reference: Dehydrated Culture Media and Reagents for Microbiology.  DIFCO Laboratories, Detroit, MI. 1984.
Reference: Dehydrated Culture Media and Reagents for Microbiology.  DIFCO Laboratories, Detroit, MI. 1984.


==Sugar Fermentation Media==
==Links to Labs==
'''General Recipe Carbohydrate fermentation''': Purple broth base (1% peptone, 0.1% beef extract, 0.5% NaCl, 20 µg/ml bromcresol purple) plus 1% of the desired carbohydrate. 
[[BISC209:BootCamp | Lab 1 ]]<br>
 
[[BISC209: Lab2 | Lab 2 ]]<br>
'''Commonly Used Fermentation Media'''<BR>
[[BISC209: Lab3 | Lab 3 ]]<br>
Inoculate one tube for each organism: adonitol, dulcitol, glucose, inositol, lactose, mannitol, and sucrose. Incubate for 24 to 72 hours at 35C.  The fermentation of carbohydrates by Gram negative organisms gives rise to the production of pyruvic acid causing a change in color of the media from purple to yellow due to the reduction in pH.<BR>
[[BISC209: Lab4 | Lab 4 ]]<br>
 
[[BISC209: Lab5 | Lab 5 ]]<br>
A color change to yellow in the fermentation media indicates a positive test (+).<BR>
[[BISC209: Lab6 | Lab 6 ]]<br>
 
[[BISC209: Lab7 | Lab 7 ]]<br>
In positive tests only, gas production is evidenced by gas bubbles trapped in the smaller inner tube (Durham tube).  If gas is present the result is recorded as +/gas.<BR>
[[BISC209: Lab8 | Lab 8 ]]<br>
 
[[BISC209: Lab9 | Lab 9 ]]<br>
A negative test (-) is evidenced by a purple to gray color and obvious growth* in the tube.<Br>
[[BISC209: Lab10 | Lab 10 ]]<br>
Example, the lactose fermentation test contains 1% lactose dissolved in the purple broth base.<BR><BR>
[[BISC209: Lab11 | Lab11 ]]<br>
 
[[BISC209: Lab12 | Lab 12 ]]<br>
'''Phenol Red Agar with sugars mannitol or sucrose''': 1% mannitol(or sucrose) is added to phenol red agar base (1% peptone,  0.1% beef extract, 0.5% NaCl, 1.5% agar  and 25µl/ml phenol red) to test for the fermentation of mannitol by the bacteria. 
 
A positive test is a yellow color due to the formation of acidic products of mannitol fermentation.  The indicator, phenol red, turns yellow when the pH falls below 6.8.  ''Staphylococcus aureus'' usually ferments mannitol within 24 to 48 hours.
 
An inoculum should be spread on the slant and then stabbed to the bottom of the butt,  a region that remains fairly anaerobic.  The inoculated slants should then be incubated at 37°C for 24-48 hours.  Yellow color indicates a positive (+) test.
 
<BR><BR>

Latest revision as of 12:26, 11 March 2010

Wellesley College-BISC 209 Microbiology -Spring 2010

Selective / Differential / Enrichment media
Selective media helps select for growth of certain organisms in a mixed population by using a ingredient that inhibits the growth of other microorganisms, but not the desired species or group. Enrichment media selects for certain microorganisms by including a nutrient that the desired microorganism or group can use and its competitors can not. (Sometimes enrichment media also limits alternate sources of nutrition). Differential media does not select for any particular group by inhibiting or enhancing their growth over competitors, but it does show a visible difference between or among groups of microorganisms. Some media can be both selective and differential.
For more information on the formulations and types of media available in microbiology see: BD diagnostice Systems Difco catalog of media http://www.bd.com/ds/technicalCenter/inserts/difcoBblManual.asp

Selective for Gram positive Organisms

Phenylethyl Alcohol Agar (PEA) PEA selects for the growth of Gram positive organisms by inhibiting the growth of Gram negative bacilli. The phenylethyl alcohol interfers with DNA synthesis in Gram negative organisms. This medium is particularly useful at inhibiting the overgrowth of Gram negative Proteus species that tend to swarm (they are highly motile) and, thus, make isolation of Gram positive organisms difficult in a mixed population .
Recipe: 10g tryptose, 3g Beef extract, 5g Sodium Chloride, 2.5g Phenylethyl alcohol, 15g Agar to 1 liter distilled or deionized water pH 7.1-7.5.

Mannitol Salt Agar (an alternative to PEA, not used in 2010)

Postive control organism: Staphylococcus epidermidis

Selective for Gram negative Organisms

Eosin–Methylene Blue (EMB) Agar is a differential medium for the detection of Gram negative enteric bacteria. The medium contains peptone, lactose, sucrose, dipotassium phosphate, eosin and methylene blue dyes. Eosin and methylene blue act as indicators to differentiate between Gram negative organisms that ferment lactose and those that do not ferment lactose. Most bacteria that ferment lactose form colonies on EMB agar that are dark blue to black with a metallic sheen due to precipitation of the dyes by the acid by-products of fermentation. Colonies produced by lactose non-fermentors are not dark blue or black. The growth of Gram positive bacteria is generally inhibited on EMB agar because of the toxicity of methlyene blue dye. In low concentration, the protective lipid outer membrane of Gram negative bacteria prevents entry of the toxic water soluble dye while the more porous cell wall of Gram positive bacteria without the protective outer membrane makes them more sensitive to the toxicity of methyene blue.

Recipe:10g peptone, 10g Lactose, 2g dipotassium phosphate, 0.4g eosin Y, 0.065 g methylene blue 15 g Agar. final pH 6.9-7.3

Table 2. Colonial appearance on EMB Agar after 18-24 hours at 35°C. A differential medium can be used to differentiate Gram negative enteric organisms based on the colony color.

Organism Colonial Appearance
Escherichia coli purple with black center/ green metallic sheen
Klebsiella pneumoniae dark centered colonies/ green metallic sheen
Enterobacter aeorogenes pink colonies/ no metallic sheen
Proteus mirabilis colorless colonies
Salmonella typhimurium colorless colonies




Reference: Dehydrated Culture Media and Reagents for Microbiology. DIFCO Laboratories, Detroit, MI. 1984.

Links to Labs

Lab 1
Lab 2
Lab 3
Lab 4
Lab 5
Lab 6
Lab 7
Lab 8
Lab 9
Lab 10
Lab11
Lab 12

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