If you want to take extra care that you have clean yeast, there is an acid washing procedure in the yeast section of probrewer that you could try. They say that it helps "reduce" bacterial contamination.
1. Distilled water
2. 75% phosphoric acid, food grade
3. Pitching yeast
4. Sealable sterile container
5. Sterile mixing paddle
6. Rubber gloves
7. Eye goggles
8. pH meter or pH paper
1. Dilute the phosphoric acid to 7.5% by adding 0.4 cup of 75% phosphoric acid to 3.8 cups of distilled water. Use goggles and wear gloves! Store in a sealed and well labelled plastic container at room temperature.
2. Measure the volume of yeast to wash and add 0.25 to 0.5 cup of the diluted phosphoric acid solution to 4 gallons of yeast in a sterile sealable container. Mix and measure the pH, ideally the pH should be between 2.2 to 2.4. If the pH is too low, add more yeast; if the pH is too high, keep adding more diluted phosphoric acid solution.
3. Keep the yeast-acid solution for a maximum of 2 hours between 32-40°F with regular mixing then pitch directly. Do not store acid-washed yeast!
4. It is important to follow the guidelines above to avoid damaging the yeast. The slurry may appear more fluidized after the wash.
5. Acid washing can be performed after every brew for preventive measures or every so many brew when contamination occurs.