Negative control is the experimental setup in which no response is expected or the control in which the result is bound to be negative.
Positive control is the experimental setup in which a response is expected or the control in which the result may be positive.
So our,
Positive controls are: Currently used antibiotic
Negative control is: Water
Currently used antibiotic affects the bacteria whereas the water does not. Antibiotic X,Y,Z treatments are being checked for their activity and effect on the bacteria.