CH4ts is a previously isolated recombinant mutant of Streptococcus mutans NG8 which produces a thermolabile L-(؉)-lactate dehydrogenase (LDH) activity. It does not grow at 42؇C under a variety of cultivation conditions. In this study, we show that a batch culture of CH4ts shifted from 30 to 42؇C underwent rapid cessation of growth and accelerated cell death. The mutant grew at 42؇C in continuous culture under glucoselimiting conditions. Under these conditions, lactate production was replaced by production of ethanol and, to a smaller extent, acetoin. The cloned Zymomonas mobilis gene for alcohol dehydrogenase II, placed under the control of the S. mutans spaP regulatory signals, complemented LDH deficiency. The alcohol dehydrogenasecomplemented mutant grew as well or better than NG8 on a variety of carbon sources at 42؇C and produced significant amounts of ethanol in place of lactic acid. These results are in accord with other approaches indicating that S. mutans has other enzymatic activities, including pyruvate formate-lyase and pyruvate dehydrogenase, for pyruvate metabolism. However, at high glucose concentrations, the levels of activity of these enzymes are apparently insufficient to compensate for the absence of LDH.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

The relationship between the cathode microstructure and electrochemical performance of lanthanum strontium manganite (LSM) /yttria stabilized zirconia (YSZ) SOFCs was studied for the isochronal sintering at temperatures between 950°C and 1400°C for 1 hour. Previous studies have shown that the Triple-Phase-Boundary (TPB) length at the cathode/electrolyte interface contributes to charge transfer resistance. However, it is unclear the role of the tertiary phase formation. This study investigated the effects of 1) TPB length and 2) tertiary phase formation on the charge transfer resistance. Techniques were developed using a dual-beam FIB system to automatically slice serial-sections of the cathode. These images were assembled to create a 3-D image of the microstructure. Also, cross-section TEM samples were made using the Omniprobe manipulator. The homogeneity of the sample was analyzed using a disector probe. High-resolution TEM-EDS was used to characterize tertiary phase formation at the cathode/electrolyte interface.It was found that TPB length was reduced from 1.41±0.07 µm/µm 2 to 0.21±0.01 µm/µm 2 as the sintering temperature increased from 950°C to 1400°C. Formation of a low conductivity tertiary phase was observed above 1200°C, which appears to contribute to the increase in charge transfer resistance.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005