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This formula is satisfiable because if we set to b to true and a to false, then the whole formula is true. All other assignments make the formula false, but it doesn't change that the formula is satisfiable as long as there is at least one assignment makes the formula true.
Before agar, microbiologists had experimented with other foodstuffs as microbial media. They turned to substances rich in the starches, proteins, sugars, fats, and minerals that organisms need for growth, testing with broths, bread, potatoes, polenta, egg whites, coagulated blood serums, and gelatine. However, none worked particularly well: all were easily broken down by heat and microbial enzymes, and their surface, once colonized, became mushy and unsuitable for isolating microbes.