Microbial cells water content

When reporting on dry-heat test data, relative humidity is measured and controlled instead of water activity. This is because of the fact that biological materials more closely parallel vapor pressure than water content, and vapor pressure of the gas atmosphere surrounding dry microbial cells is more easily measured and controlled than the water content inside the cell. ... 

Microbial cells are surrounded by a cell wall, which retains the cell contents, and is the primary barrier between the cell surface and the environment in which it exists. The quality of the cell wall, in terms of selective permeability, maintains the necessary levels of nutrients, trace elements, and cell internal pH. The cell membrane is the site of transfer processes water is able to pass through this membrane, in or out of the cell, depending on the thrust of the osmotic pressure. The chemistry of the cell wall affects its properties in terms of surface electric charge and the availability of binding ions. 

First-order reaction kinetics, with one or two linear stages, have been reported for the sterilization of a range of microbial cells using CO2 (11,53-55). The presence of water is required for efficient sterilization (12,53,56). For example, Kumagai and coworkers (53) examined the sterilization of Saccha-romyces cerevisiae in an aqueous solution at 40°C and a pressure range of 4-15 MPa. At low water content, the sterilization rate increased with increasing content of water. At a sufficiently high water content, free water (not bound to the cells) existed in the system and the sterilization rate was almost constant with water content. The authors conclude the presence of dissolved CO2 in the aqueous-cell environment, and not system depressurization (which occurred at a rate of 8 MPa/h), was responsible for the inactivation of the microbial cells. Cell inactivation kinetics are difficult to compare across laboratories because the aqueous media is frequently not saturated with dissolved CO2 and the actual concentrations are not measured (44,51). 

This intriguing observation seems to suggest the significance of the polyhydric ions which result presumably from the various pretreatment of the cells and which are due to be the bridging factor in the flocculation. This bridging concept in discussing the structure of a microbial floe is in conformity with the previous discussion on water content of the floe. The sophisticated structure of the microbial surface which should not be overlooked makes a consistent and quantitative discussion on the reaction-rate constant, /Cn exceedingly difficult, indeed. 

The colon contains the most species-rich and densely populated human-associated microbial communities, and the most widely studied. The colonic microbiota comprises many hundreds of bacterial species, 1000s of strains and populations of about 10 cells/g contents. The physiology of the colon renders it a particularly suitable bacterial habitat, as the flow of intestinal digesta slows, allowing bacteria ready access to dietary growth substrates. The pH is moderately acidic in the proximal colon, mainly due to bacterial fermentation itself, and increases to a more neutral pH in the transverse and distal colon with secretions by the host and water absorption. Indeed, using in vitro models of the human gut microbiota, pH has been shown to be a critical modulator of both microbiota composition and metabolic activity under the physicochemical conditions of the human colon. ... 

I.M., Velasquez-Orta, S.B., and Scott, K. (2009) A single chamber packed bed microbial fuel cell biosensor for measuring organic content of wastewater. Water Sci. Technol., 60 (11), 2879-2887. 

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