You have two different samples of bacteria (both growing on separate agar plates) - one is Escherichia coli, the other is Mycoplasma fermentans.  You tried a Gram stain on each bacterium: E. coli was clearly Gram-negative; M. fermentans also stained pink (Gram-negative).  You are somewhat confused because although you expected E. coli to be Gram-negative, you know that Mycoplasma spp. have evolved from Gram-positive bacteria, therefore, you were expecting a Gram-positive result. 

You approach your supervisor who suggests you add a colony of each type of bacterium to a test tube of water and see what happens (as shown below).  You add each type of bacteria to a sample of water, wait 10 minutes and then remove a sample from each tube and view it using phase-contrast microscopy.  You can visualise the E. coli, but you cannot visualise the M. fermentans. 

What happened when you added each bacterium to the water and how does this relate to the Gram staining result you had at the beginning?   (4 marks)