2 ml of stock antibiotic (64 mg/ml) was diluted into a test tube containing 2 ml of broth (final volume is 4). A series of 2-fold SERIAL DILUTIONS were performed by transferring 2 ml to three other tubes, consecutively (final volumes now 2 ml in each tube). The final concentrations of antibiotic in the four tubes are: 32, 16, 8, and 4.

Next, 2 ml of water were added to each each tube individually (not a serial dilution). What is the final concentration of antibiotics in the tubes?

I think this is another 2-fold dilution where the answers are 16, 8, 4, and 2, but I'm not sure.

To calculate the final concentration of antibiotics in each tube after adding 2 ml of water to each tube individually, we need to consider the dilutions that have already been performed.

Let's break down the calculations step by step:

1. Initial dilution:
- 2 ml of stock antibiotic (64 mg/ml) was diluted into a test tube containing 2 ml of broth.
- The total volume after dilution is 4 ml (2 ml stock + 2 ml broth).

The initial concentration of the antibiotic in the test tube can be calculated using the formula:
Initial Concentration = (Volume of stock antibiotic x Concentration of stock antibiotic) / Total volume after dilution

Initial Concentration = (2 ml x 64 mg/ml) / 4 ml = 32 mg/ml

2. Serial dilutions:
- 2 ml of the initial dilution were transferred to three other tubes, with a final volume of 2 ml in each tube.

The concentration of the antibiotic in each subsequent tube after dilution can be calculated by dividing the previous concentration by 2 (two-fold dilution).

Tube 1: 32 mg/ml / 2 = 16 mg/ml
Tube 2: 16 mg/ml / 2 = 8 mg/ml
Tube 3: 8 mg/ml / 2 = 4 mg/ml

3. Adding water to each tube:
- Finally, 2 ml of water were added to each tube individually.

When water is added, it dilutes the concentration of the antibiotic. The total volume after adding water becomes 4 ml (2 ml initial dilution + 2 ml water).

The final concentration of the antibiotic in each tube after adding water can be calculated using the formula:
Final Concentration = (Volume of initial dilution x Initial Concentration) / Total volume after adding water

Tube 1: (2 ml x 16 mg/ml) / 4 ml = 8 mg/ml
Tube 2: (2 ml x 8 mg/ml) / 4 ml = 4 mg/ml
Tube 3: (2 ml x 4 mg/ml) / 4 ml = 2 mg/ml

Therefore, the final concentrations of antibiotic in the tubes, after adding 2 ml of water to each tube individually, are 8 mg/ml, 4 mg/ml, and 2 mg/ml for tubes 1, 2, and 3 respectively.