7/21/2019 Selective Media Allows the Growth of Certain Type of Organisms

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Selective media allows the growth of certain type of organisms, while inhibiting the

growth of other organisms. These media contain antimicrobials, dyes, or alcohol to

inhibit the growth of the organisms not targeted for study.

Example: Eosin methylene blue agar, Mannitol Salt agar, MacConey agar,

and !henylethyl "lcohol agar.

#i$erential media are widely used for di$erentiating closely related organisms or

groups of organisms. %ecause of the presence of certain dyes or chemicals in the

media, the organisms will produce certain characteristic changes or growth patterns

that are used for identi&cation or di$erentiation of microorganism.

Example: %lood agar, Eosin methylene blue agar, Mannitol Salt agar,

MacConey agar.

E. coli, !suedomonas aeruginosa and 'lebsiella pneumonia produce pin colonies onMacConey agar. These organisms are gram negative lactose(fermenting bacteria,

they ferment lactose producing an acid which lowers the p) of the agar below *.+

and results in the appearance of pin colonies.

Candle ar is a -ar with a lid providing a gas(tight seal in which a small white candle

is placed and lit after the culture plates have been placed inside. The candle ame

will consume most of the oxygen in the ar and will produce an elevated level of

carbon dioxide.

 The candle ar inhibit certain obligate anaerobes, thus it isolate certain pathogenic

bacteria that re/uire anaerobic atmosphere relatively enriched with C0 1.

 The isolation of pathogens from CS2 specimen is much easier that isolating it from

stool specimen, because CS2 does not contain normal ora, whereas stool is rich

with 32.

 Types of hemolysis:

"lpha hemolysis 4incomplete, partial, green hemolysis5: it is caused by hydrogen

peroxide produced by the bacteria, that oxidi6e hemoglobin to green

methemoglobin. So, the agar under the colony is dar and greenish.

7/21/2019 Selective Media Allows the Growth of Certain Type of Organisms

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%eta )emolysis 4complete hemolysis5: it is caused by Streptolysin 4an exotoxin5

produced by the bacteria, this en6yme causes a complete lysis of red blood cells,

producing lightened 4yellow5 and transparent area.

7ama hemolysis: 8f an organism does not induce hemolysis 4does not brea down

9%Cs5, the agar under and around the colony will not changed.

!haryngitis : Streptococcus pyogenes.

rinary tract infection: E. coli.

;ound and burn infection: Staphylococcus aureus.

%lood diarrhea: Salmonella.

"bscess and sin infection : Staphlucoccus epidermidis.

Endocarditis in intravenous drug user : Staphylococcus aureus.

Endocarditis after dental extraction : Staphylococcus variadans.

0ral or vaginal thrush: Candida albicans.