Desiccation induces viable but Non-Culturable cells in Sinorhizobium meliloti 1021

Abstract

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.

Ecological patterns of seed microbiome diversity, transmission, and assembly

“…of the seed microbiota may be inactive or dormant [65,66], it seems unlikely that priority effects play a major role in a seed ultimate community structure, though this remains to be tested specifically. Still, we can hypothesize the general drivers that may be niche or neutral in seed microbiota assembly.…”

Identification and Characterization of a NaCl-Responsive Genetic Locus Involved in Survival during Desiccation in Sinorhizobium meliloti

“…These conditions include the intrageneric differences to cope with desiccation stress which affect survival (5,6,9,10,13,18,21,22). These data suggest that no single gene affects the ability of rhizobia to survive desiccation but that several mechanisms are likely responsible.…”

“…All strains were maintained on tryptone-yeast extract (TY) plates with the appropriate antibiotics. Media used were TY (43), GTS (41), LB (42), and YMB and PMM (5,13,44). Alfalfa seeds were obtained from the seed company Outsidepride (BS-ALFALFA-5; lot no.…”

“…It is estimated that over 40% of arable land will be affected by desiccation and salinity by 2025 (2). Furthermore, production of seed inocula often includes a drying phase negatively affecting CFU of added rhizobia (3,4), potentially resulting in desiccation-induced viable but nonculturable cells (5).…”

Stress Physiology in Azospirillum and Other PGPRs

“…Rhizobia are distinguished by their existence as both free-living soil bacteria and nitrogen-fi xing root endosymbionts. Among the rhizobia, the osmoadaptive mechanisms of the relatively salt-tolerant species Sinorhizobium meliloti have been examined in great detail (Miller and Wood 1996 ;Vriezen et al 2012 ). Although Azospirillum is also a nitrogen-fi xing Gram-negative bacterium found in the rhizosphere, it does not induce differentiated structures on the roots of its plant hosts.…”