Department of Microbiology, University of Massachusetts, Amherst, Massachusetts, USA, MA 01003.
AMB Express. 2012 Jan 20;2(1):6. doi: 10.1186/2191-0855-2-6.
Sinorhizobium meliloti is a microorganism commercially used in the production of e.g. Medicago sativa seed inocula. Many inocula are powder-based and production includes a drying step. Although S. meliloti survives drying well, the quality of the inocula is reduced during this process. In this study we determined survival during desiccation of the commercial strains 102F84 and 102F85 as well as the model strain USDA1021.The survival of S. meliloti 1021 was estimated during nine weeks at 22% relative humidity. We found that after an initial rapid decline of colony forming units, the decline slowed to a steady 10-fold reduction in colony forming units every 22 days. In spite of the reduction in colony forming units, the fraction of the population identified as viable (42-54%) based on the Baclight live/dead stain did not change significantly over time. This change in the ability of viable cells to form colonies shows (i) an underestimation of the survival of rhizobial cells using plating methods, and that (ii) in a part of the population desiccation induces a Viable But Non Culturable (VBNC)-like state, which has not been reported before. Resuscitation attempts did not lead to a higher recovery of colony forming units indicating the VBNC state is stable under the conditions tested. This observation has important consequences for the use of rhizobia. Finding methods to resuscitate this fraction may increase the quality of powder-based seed inocula.
根瘤菌属是一种商业上常用于生产例如紫花苜蓿种子接种剂的微生物。许多接种剂是粉末状的，生产过程包括干燥步骤。尽管根瘤菌属在干燥过程中能很好地存活，但接种剂的质量在这个过程中会降低。在这项研究中，我们确定了商业菌株 102F84 和 102F85 以及模式菌株 USDA1021 在干燥过程中的存活情况。在相对湿度为 22%的条件下，我们估计了根瘤菌属 1021 的存活情况，为期 9 周。我们发现，在菌落形成单位最初快速下降后，下降速度减缓，每 22 天菌落形成单位减少 10 倍。尽管菌落形成单位减少，但根据 Baclight 活菌/死菌染色，可鉴定为存活的种群比例（42-54%）并没有随时间显著变化。存活细胞形成菌落的能力的这种变化表明：（i）平板计数法对根瘤细胞存活的低估，以及（ii）在一部分种群中，干燥诱导了一种存活但非可培养（VBNC）状态，这是以前从未报道过的。复苏尝试并没有导致更高的菌落形成单位回收，表明在测试条件下 VBNC 状态是稳定的。这一观察结果对根瘤菌的使用有重要影响。寻找使这一部分复苏的方法可能会提高粉末状种子接种剂的质量。
