Balancing the gradient: Water activity, risk and microbial growth

BY DR TIM SANDLE | 05 January 2024

All microorganisms require some amount of water in order to reproduce (1). Microorganisms take up water by moving it through their cell membranes. This is why, even with all the necessary growth factors, water is proportionately the largest ingredient within culture media (2).  Water is required for metabolic activity and as a biophysical factor. The amount of water and the amount of ‘available’ water to a microorganism are not the same thing. For example, it is possible for a substance to have a high moisture content but have very little water activity. This happens when various components of a substance bind water, making it unavailable to microorganisms.

The water movement process depends on a water gradient (a high water activity outside of the cell to a lower water activity within the cell). If the amount of available water decreases, conditions of osmotic stress would be created. In this situation, the cell would no longer be able to take in water and would therefore be incapable of growth. Different microorganisms have different water activity requirements.

Therefore, microbiologists describe the water requirements of microorganisms in terms of the water activity (aw). It is this ‘availability’ of water that is the critical factor affecting the growth of all cells - not simply how much water there is.