1. What causes the difference between nutrient media broth, nutrient media semi-
ID: 195138 • Letter: 1
Question
1. What causes the difference between nutrient media broth, nutrient media semi-solid and nutrient media solid? 2. Luria Bertani Agar (LBA) is used in lab for most experiments. The recipe is: 10g Tryptone, 5g NaCI, 15g agar, IL distilled water. Is this complex or chemically defined? How do you know? 3. If you wanted to sterilize 3L of LBA how would you do it? 4. If you wanted to sterilize a liquid sample of heat labile antibiotic, how would you do it? 5. Your instructor allowed the agar media to cool to about 50° C (after autoclaving) before it was poured into petri plates. Why Introductory Microbiology 27Explanation / Answer
1.
LBA media contains tryptone, NaCl, yeast extract and agar. Here, yeast extract is used as a source of amino acids. These yeast extracts are degraded to form amino acids and small peptides, but the exact amount of these cannot be determined. Thus, though the main ingredients of LBA is known, the exact composition of two of them (yeast extract and agar) remain unknown, which define the LBA medium as a complex medium.
3 L LB media can be sterilized by doing autoclaving.
Antibiotic solution must be prepared separately by filter sterilization. After autoclaving, the media must be cooled down at 50ºC, then the filter sterilized heat labile antibiotic solution is added in the laminar air flow cabinet to maintain the sterilization.
After autoclaving, the agar media is allowed to cool to about 50 ºC to avoid excessive condensation on the lids of the petriplates.
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