I have looked at spore stability in suspensions (and in direct products) in the past. Are you looking at Clostridium spp. or Bacillus spp.? In the past, I have had suspensions held at varying temperatures of interest (4C, room temp, 37C, etc) and performed aerobic plate counts of Bacillus species. 

Thank you David. Actually I plan to work on  Clostridium spp., Bacillus spp., and molds when stored in 4°C. What parameter or acceptance criteria used to prove its stability? 

Sounds like interesting work! For my work, my spore suspensions were of Bacillus species in animal feed products for probiotic applications. I have done previous work determining what inoculation amount (cfu/mL) was required. After 1, 3, 6 months and 1 year stored at these temperatures, I performed aerobic plate counts to determine what their cfu/mL counts were as I mentioned above. For my products, I wanted to compare how stable certain species were compared to one another to ultimately choose for production. For suspensions that had greater than 2 log or more loss, those candidates were deselected. A few of my suspensions proved to be very stable. So in your case, it would depend on your ultimate goal. Hope this helps. 

Thank you very much for the information. The spore suspension of Clostridia stored at 4°C, showed consistent results for at least 6 month (CFU/mL by plate count method). I used them to perform microbial recovery study and media growth promotion. I wanted to know how to evaluate or to determine its shelf life so I can assign an expiration date. In your work, is there any lot that loss more that 2 logs when stored at 4°C? Did you do a heat shock treatment to your Bacillus suspension?

No problem Sinta, I hope everything is going well! If I recall correctly, I think there was one lot that did see a 2 log loss. Yes, I do a standard heat shock for my Bacillus suspensions.