I am doing a research on presence of P. aeruginosa in water vending machine and I am using asparagine broth as the presumptive test and acetemide broth for the confirmed test. Most of the reading I did said that only the tubes that are positive presumptive showing fluorescent in asparagine broth will then be incubated in acetemide broth. My question is do I need to check the negative fluorescent from the asparagine in acetemide also just to confirm that the results are negative? Because I have tried to do so but some of the negative from the presumptive test did change the colour of acetemide broth from red to pink.