The bacteria were stored frozen until now. Living cells can be stored in the freezer for long periods of time; however, eventually they will lose viability as the cells continually die. Ice crystals forming inside the cell will rupture the cell membranes. Also, freezing ice can concentrate dissolved salts in parts of the cell and denature essential proteins. Nonetheless, slowing down chemical reactions in the cell will extend their lifespan. Keeping cells in the freezer is a delicate balance between preservation and minimizing damage.
Proper buffers added to the cells can help reduce the amount of cell death due to ice formation. Cryoprotectants, such as glycerol, can lower the freezing point so that ice crystals form less often. Crypresrevatives, such as polysaccharides or proteins, can form a protective layer around the cell membranes. These can extend the preservation time of frozen cells by upwards of 10 years.
When we need to use the bacteria, we can take a small swab from the frozen stock and add it to a warm nutrient broth. Any bacteria still living will quickly take advantage of nutritious media and start growing rapidly. Within a couple days we will have fresh healthy vials of the local bacteria to use for our experiments.