drops of a 0.04% bromothymol-blue solution are applied to colony sites; the individual colonies should first be removed with a platinum loop.
gelatinolysis is an indicator of toxicity and it is shown by the appearance of clear zones around the colonies about 10 minutes after applying drops of a saturated ammonium sulfate solution or a 20% sulfosalicylic acid solution.
Further tests should be performed to confirm the results.