Cellular responses of Staphylococcus aureus as related to NMR detected water and system mobility, water activity and media formulation
The effect of water on growth and survival of Staphylococcus aureus was investigated using liquid (17O) and solid-state (1H) Nuclear Magnetic Resonance (NMR) spectroscopy. Different growth media, solute types, and methods of water activity (aw) adjustment (i.e. moisture versus solute variations) were studied.
For the growth studies (>0.75 aw), mostly all water present was detected by the NMR and was highly mobile. Dependence of S. aureus growth was only partly dependent on the NMR signal intensity (amount of mobile or detected water) and partly dependent on the solute types.
When aw was adjusted by changing moisture content, the use of brain heart infusion (BHI) or chicken meat media (CMM) did not affect the general conclusion. However, CMM resulted in an increased viscosity particularly at lower moisture content and partly influenced the NMR water mobility results. In general, it is postulated from this work that there is a critical amount of mobile water (based on 17O NMR intensity) of ∼40 g water detected/100 g sample below which S. aureus is significantly inhibited.
For the survival study (<0.75 aw), the mobile proton signal was primarily due to the amount of water protons. Upon hydration, the onset NMR mobility increase also correlated with the monolayer value of water. A substantial increase in proton mobility (T2) was observed upon further increase in moisture content. Survival of S. aureus in a freeze-dried gum mixture was dependent on proton mobility, amount of mobile protons, and aw. Added mannitol and raffinose both protected the cells from osmotic-related death. The critical aw's at which cell death dramatically increased were in a similar aw range when proton mobility also increased. This suggested that molecular mobility facilitated the cell damage brought about by osmotic stress and in this case may serve as an indicator of water availability. Thus, molecular mobility plays a critical role in controlling cell survivability at a low moisture condition.