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Pseudomonas putida cells were grown in confined volumes in dual-membrane immobilized cell reactors constructed from microporous polyethylene hollow fibers and silicone rubber tubules as a model system for the study of mass transport in microbial aggregates. Local cell concentrations in the reactors reached 300 g dry mass/L. Pulse-chase radioisotope labeling with (35)SO(4) (2-) was used to estimate the rates of cell mass synthesis and degradation. Sulfur incorporation consistently exceeded sulfur release, implying that the cell mass concentration continually increases. The location and size of the cell growth region was determined using liquid emulsion autoradiography of thin sections prepared from labeled reactors. Cell growth occurs in a region less than 25 mum in depth adjacent to the oxygen supply, and the expansion of the cells caused by cell growth promotes convection of the cell mass into regions of the reactor where starving cells accumulate. The combination of mass-balance and spatial distribution measurements that can be made using radioisotope tracers provides a versatile method for determining metabolic rates and limitations caused by mass transfer in immobilized cell reactors.

A model to describe the response of the blood pool to an oral dose of nitrate in humans has been developed. The permeability-area product of the small intestine to nitrate was estimated by comparing simulations from a three-compartment model with published data for blood nitrate concentration following nitrate ingestion. The transport of nitrate from the bloodstream to the lumen of the large intestine and the metabolism of nitrate by enteric bacteria were examined by including an additional compartment representing the large bowel. The simulations indicate that the bacteria of the large intestine may be responsible for about half of the extrarenal removal of nitrate from the body. This prediction was tested experimentally by comparing the urinary recoveries of 15NO3- in conventional and germfree rats following an i.p. dose of Na15NO3. The mean urinary recovery in germfree rats (71% of dose) substantially exceeded that in rats with conventional bacterial flora (54%). This suggests that of the 40-45% of a nitrate dose that is metabolized in the body rather than excreted in urine as nitrate, approximately half is metabolized by mammalian processes and approximately half by enteric bacteria. This conclusion is consistent with that obtained from our pharmacokinetic model of nitrate in humans.

The effects of diffusion on a reaction taking place in a permeable catalyst are examined theoretically for the case where the reaction has two substrates supplied from opposite sides of a catalytic slab. The solutions of the reaction-diffusion equation for combinations of zeroth-and first-order kinetics are given in terms of an effectiveness factor and a parameter describing the position in the layer where the reaction occurs. In these terms, the results vary only weakly with reaction order. The use of the exact solutions for a reaction that is zeroth order in both sub strates is proposed as a general rule for estimating the reaction rate and the reaction position.