Dodatko Tetyana, Akoachere Monique, Muehlbauer Stefan M, Helfrich Forrest, Howerton Amber, Ross Christian, Wysocki Vicki, Brojatsch Jürgen, Abel-Santos Ernesto
Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York, United States of America.
PLoS One. 2009 Jul 28;4(7):e6398. doi: 10.1371/journal.pone.0006398.
The first step of the bacterial lifecycle is the germination of bacterial spores into their vegetative form, which requires the presence of specific nutrients. In contrast to closely related Bacillus anthracis spores, Bacillus cereus spores germinate in the presence of a single germinant, inosine, yet with a significant lag period.
We found that the initial lag period of inosine-treated germination of B. cereus spores disappeared in the presence of supernatants derived from already germinated spores. The lag period also dissipated when inosine was supplemented with the co-germinator alanine. In fact, HPLC-based analysis revealed the presence of amino acids in the supernatant of germinated B. cereus spores. The released amino acids included alanine in concentrations sufficient to promote rapid germination of inosine-treated spores. The alanine racemase inhibitor D-cycloserine enhanced germination of B. cereus spores, presumably by increasing the L-alanine concentration in the supernatant. Moreover, we found that B. cereus spores lacking the germination receptors gerI and gerQ did not germinate and release amino acids in the presence of inosine. These mutant spores, however, germinated efficiently when inosine was supplemented with alanine. Finally, removal of released amino acids in a washout experiment abrogated inosine-mediated germination of B. cereus spores.
We found that the single germinant inosine is able to trigger a two-tier mechanism for inosine-mediated germination of B. cereus spores: Inosine mediates the release of alanine, an essential step to complete the germination process. Therefore, B. cereus spores appear to have developed a unique quorum-sensing feedback mechanism to monitor spore density and to coordinate germination.
细菌生命周期的第一步是细菌孢子萌发为营养体形式，这需要特定营养物质的存在。与密切相关的炭疽芽孢杆菌孢子不同，蜡样芽孢杆菌孢子在单一萌发剂肌苷存在下即可萌发，但存在明显的延迟期。
我们发现，在已萌发孢子的上清液存在的情况下，肌苷处理的蜡样芽孢杆菌孢子萌发的初始延迟期消失。当肌苷与共萌发剂丙氨酸一起添加时，延迟期也会消失。事实上，基于高效液相色谱的分析表明，蜡样芽孢杆菌已萌发孢子的上清液中存在氨基酸。释放出的氨基酸包括丙氨酸，其浓度足以促进经肌苷处理的孢子快速萌发。丙氨酸消旋酶抑制剂D-环丝氨酸可增强蜡样芽孢杆菌孢子的萌发，可能是通过增加上清液中L-丙氨酸的浓度来实现的。此外，我们发现缺乏萌发受体gerI和gerQ的蜡样芽孢杆菌孢子在肌苷存在下不会萌发和释放氨基酸。然而，当肌苷与丙氨酸一起添加时，这些突变孢子能够高效萌发。最后，在洗脱实验中去除释放出的氨基酸消除了肌苷介导的蜡样芽孢杆菌孢子萌发。
我们发现单一萌发剂肌苷能够触发蜡样芽孢杆菌孢子肌苷介导萌发的两层机制：肌苷介导丙氨酸的释放，这是完成萌发过程的关键步骤。因此，蜡样芽孢杆菌孢子似乎已经形成了一种独特的群体感应反馈机制来监测孢子密度并协调萌发。
