Week 6 – Selective & Differential Media, Streak for Isolation

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Vocabulary flashcards covering key terms related to selective & differential media (EMB, MSA) and the streak-for-isolation technique discussed in Week 6.

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15 Terms

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Selective Medium

A culture medium formulated to inhibit the growth of some microorganisms while allowing others to grow.

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Differential Medium

A medium designed to distinguish between microorganisms, often through visible color changes produced by metabolic differences.

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Eosin Methylene Blue (EMB) Agar

Selective for Gram-negative bacteria and differential for lactose fermentation; strong fermenters produce metallic green, black, or purple colonies.

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Mannitol Salt Agar (MSA)

Selective for Gram-positive staphylococci due to 7.5 % NaCl and differential for mannitol fermentation indicated by a yellow color change.

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Enterobacteriaceae (Enterics)

Gram-negative bacilli commonly found in the gut or soil that can ferment lactose and other sugars.

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Escherichia coli (E. coli)

A lactose-fermenting enteric that forms a metallic green sheen on EMB agar.

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Staphylococcus aureus

Gram-positive staphylococcus that ferments mannitol, turning MSA yellow.

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Staphylococcus epidermidis

Gram-positive staphylococcus that grows on MSA but does not ferment mannitol; medium remains pink/red.

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Pseudomonas aeruginosa

Gram-negative rod that grows on EMB without dark coloration because it does not ferment lactose.

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Phenol Red Indicator

pH indicator in MSA that turns yellow in acidic conditions produced by mannitol fermentation.

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7.5 % NaCl

High salt concentration in MSA that inhibits most bacteria except staphylococci, making the medium selective.

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Streak for Isolation

Technique that dilutes bacterial samples across an agar plate to obtain discrete colonies for pure cultures.

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T Streak

Three-section streak pattern shaped like a ‘T’ used to isolate colonies on an agar plate.

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Quadrant Streak

Four-section streak method that progressively dilutes bacteria to achieve isolated colonies.

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Metallic Green Sheen (on EMB)

Colony appearance indicating high acid production from lactose fermentation, characteristic of E. coli.