Abstract
SummaryThe application of beneficial, plant‐associated microorganisms is a sustainable approach to improving crop performance in agriculture. However, microbial inoculants are often susceptible to prolonged periods of storage and deleterious environmental factors, which negatively impact their viability and ultimately limit efficacy in the field. This particularly concerns non‐sporulating bacteria. To overcome this challenge, the availability of protective formulations is crucial. Numerous parameters influence the viability of microbial cells, with drying procedures generally being among the most critical ones. Thus, technological advances to attenuate the desiccation stress imposed on living cells are key to successful formulation development. In this review, we discuss the core aspects important to consider when aiming at high cell viability of non‐sporulating bacteria to be applied as microbial inoculants in agriculture. We elaborate the suitability of commonly applied drying methods (freeze‐drying, vacuum‐drying, spray‐drying, fluidized bed‐drying, air‐drying) and potential measures to prevent cell damage from desiccation (externally applied protectants, stress pre‐conditioning, triggering of exopolysaccharide secretion, ‘helper’ strains). Furthermore, we point out methods for assessing bacterial viability, such as colony counting, spectrophotometry, microcalorimetry, flow cytometry and viability qPCR. Choosing appropriate technologies for maintenance of cell viability and evaluation thereof will render formulation development more efficient. This in turn will aid in utilizing the vast potential of promising, plant beneficial bacteria as sustainable alternatives to standard agrochemicals.
Highlights
The application of beneficial, plant-associated microorganisms is a sustainable approach to improving crop performance in agriculture
We discuss the core aspects important to consider when aiming at high cell viability of non-sporulating bacteria to be applied as microbial inoculants in agriculture
Desiccation stress triggers a change in the phospholipid fatty acid profile of the cell membrane, as it was observed for example for Pseudomonas aureofaciens (Kieft et al, 1994), Sinorhizobium meliloti, Bradyrhizobium elkanii, B. japonicum (Boumahdi et al, 1999) and P. putida (Halverson and Firestone, 2000)
Summary
Agricultural plant production is the basis for food, feed and fibre industry and plays a central role in supplying goods for our daily lives. Each drying method is associated with a characteristic stress regime and may be more or less suitable for a given bacterial strain It should be considered, that desiccation occurs on the field under non-controlled conditions after application of the inoculant. Desiccation stress triggers a change in the phospholipid fatty acid profile of the cell membrane, as it was observed for example for Pseudomonas aureofaciens (Kieft et al, 1994), Sinorhizobium meliloti, Bradyrhizobium elkanii, B. japonicum (Boumahdi et al, 1999) and P. putida (Halverson and Firestone, 2000) Based on these natural protection strategies, the bacterial desiccation tolerance may be improved during the formulation process either by (i) the external addition of protectants, (ii) triggering of stress adaptation or (iii) indirect protection by a ‘helper strain’ (Fig. 2). For E. cloacae: lactose + bovine serum albumen, lactose, sucrose, fructose); for P. fluorescens: buffer, spent broth, sucrose, stachyose, raffinose, melezitose, trehalose, lactose
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