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Spatial Distribution and Stability of the Eight Microbial Species of the Altered Schaedler Flora in the Mouse Gastrointestinal Tract. Ramahi B. Sarma-Rupavtarm, 2004.The overall complexity of the microbial communities in the gastrointestinal (GI) tracts of mammals has hindered observations of dynamics and interactions of individual bacterial populations . However, such information is crucial for understanding the diverse disease-causing and protective roles that gut microbiota play in their hosts . Here, we determine the spatial distribution, interanimal variation, and persistence of bacteria in the most complex defined-flora (gnotobiotic) model system to date, viz., mice colonized with the eight strains of the altered Schaedler flora (ASF) . Quantitative PCR protocols based on the 16S rRNA sequence of each ASF strain were developed and optimized to specifically detect as few as 10 copies of each target . Total numbers of the ASF strains were determined in the different regions of the GI tracts of three C.B-17 SCID mice . Individual strain abundance was dependent on oxygen sensitivity, with microaerotolerant Lactobacillus murinus ASF361 present at 105 to 107 cells/g of tissue in the upper GI tract and obligate anaerobic ASF strains being predominant in the cecal and colonic flora at 108 to 1010 cells/g of tissue . The variation between the three mice was small for most ASF strains, except for Clostridium sp . strain ASF502 and Bacteroides sp . strain ASF519 in the cecum . A comparison of the relative distribution of the ASF strains in feces and the colon indicated large differences, suggesting that fecal bacterial levels may provide a poor approximation of colonic bacterial levels . All ASF strains were detected by PCR in the feces of C57BL/6 restricted flora mice, which had been maintained in an isolator without sterile food, water, or bedding for several generations, providing evidence for the stability of these strains in the face of potential competition by bacteria introduced into the gut . Methanogenic Inhibition by Arsenic Compounds. Reyes Sierra-Alvarez, 2004. Regulation of nap Gene Expression and Periplasmic Nitrate Reductase Activity in the Phototrophic Bacterium Rhodobacter sphaeroides DSM158. Mónica Gavira, 2002.Bacterial periplasmic nitrate reductases (Nap) can play different physiological roles and are expressed under different conditions depending on the organism . Rhodobacter sphaeroides DSM158 has a Nap system, encoded by the napKEFDABC gene cluster, but nitrite formed is not further reduced because this strain lacks nitrite reductase . Nap activity increases in the presence of nitrate and oxygen but is unaffected by ammonium . Reverse transcription-PCR and Northern blots demonstrated that the napKEFDABC genes constitute an operon transcribed as a single 5.5-kb product . Northern blots and nap-lacZ fusions revealed that nap expression is threefold higher under aerobic conditions but is regulated by neither nitrate nor ammonium, although it is weakly induced by nitrite . On the other hand, nitrate but not nitrite causes a rapid enzyme activation, explaining the higher Nap activity found in nitrate-grown cells . Translational nap'-'lacZ fusions reveal that the napK and napD genes are not efficiently translated, probably due to mRNA secondary structures occluding the translation initiation sites of these genes . Neither butyrate nor caproate increases nap expression, although cells growing phototrophically on these reduced substrates show a very high Nap activity in vivo (nitrite accumulation is sevenfold higher than in medium with malate) . Phototrophic growth on butyrate or caproate medium is severely reduced in the NapA- mutants . Taken together, these results indicate that nitrate reduction in R . sphaeroides is mainly regulated at the level of enzyme activity by both nitrate and electron supply and confirm that the Nap system is involved in redox balancing using nitrate as an ancillary oxidant to dissipate excess reductant .
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