Tholen A, Brune A
Fakultät für Biologie, LS Mikrobielle Okologie, Universität Konstanz, Germany.
Environ Microbiol. 2000 Aug;2(4):436-49. doi: 10.1046/j.1462-2920.2000.00127.x.
The symbiotic digestion of lignocellulose in the hindgut of the wood-feeding termite Reticulitermes flavipes is characterized by two major metabolic pathways: (i) the oxidation of polysaccharides to acetate by anaerobic hydrogen-producing protozoa; and (ii) the reduction of CO2 by hydrogenotrophic acetogenic bacteria. Both reactions together would render the hindgut largely homoacetogenic. However, the results of this study show that the situation is more complex. By microinjection of radiolabelled metabolites into intact agarose-embedded hindguts, we showed that the in situ rates of reductive acetogenesis (3.3 nmol termite(-1) h(-1)) represent only 10% of the total carbon flux in the living termite, whereas 30% of the carbon flux proceeds via lactate. The rapid turnover of the lactate pool (7.2 nmol termite(-1) h(-1)) consolidates the previously reported presence of lactic acid bacteria in the R. flavipes hindgut and the low lactate concentrations in the hindgut fluid. However, the immediate precursor of lactate remains unknown; the low turnover rates of injected glucose (< 0.5 nmol termite(-1) h(-1)) indicate that free glucose is not an important intermediate under in situ conditions. The influence of the incubation atmosphere on the turnover rate and the product pattern of glucose and lactate confirmed that the influx of oxygen via the gut epithelium and its reduction in the hindgut periphery have a significant impact on carbon and electron flow within the hindgut microbial community. The in situ rates of reductive acetogenesis were not significantly affected by the presence of oxygen or exogenous H2, which is in agreement with a localization of homoacetogens in the anoxic gut lumen rather than in the oxic periphery. This adds strong support to the hypothesis that the co-existence of methanogens and homoacetogens in this termite is based on the spatial arrangement of the different populations of the gut microbiota. A refined model of metabolic fluxes in the hindgut of R. flavipes is presented.
取食木材的黄胸散白蚁（Reticulitermes flavipes）后肠中木质纤维素的共生消化具有两个主要代谢途径：（i）厌氧产氢原生动物将多糖氧化为乙酸盐；（ii）氢营养型产乙酸细菌将二氧化碳还原。这两个反应共同作用会使后肠在很大程度上呈现同型产乙酸状态。然而，本研究结果表明情况更为复杂。通过将放射性标记的代谢物微量注射到完整的琼脂糖包埋后肠中，我们发现原位还原产乙酸速率（3.3 nmol白蚁⁻¹ h⁻¹）仅占活体白蚁总碳通量的10%，而30%的碳通量通过乳酸进行。乳酸池的快速周转（7.2 nmol白蚁⁻¹ h⁻¹）证实了先前报道的黄胸散白蚁后肠中存在乳酸菌以及后肠液中乳酸浓度较低。然而，乳酸的直接前体仍然未知；注射葡萄糖的低周转率（< 0.5 nmol白蚁⁻¹ h⁻¹）表明游离葡萄糖在原位条件下不是重要的中间体。孵育气氛对葡萄糖和乳酸周转率及产物模式的影响证实，通过肠上皮的氧气流入及其在后肠周边的还原对后肠微生物群落内的碳和电子流有显著影响。原位还原产乙酸速率不受氧气或外源氢气存在的显著影响，这与同型产乙酸菌定位于缺氧的肠腔而非有氧的周边区域一致。这为该白蚁中甲烷菌和同型产乙酸菌共存基于肠道微生物群不同种群的空间排列这一假说提供了有力支持。本文提出了黄胸散白蚁后肠代谢通量的精细模型。
