Research paper Parameters influencing adsorption of Paraburkholderia phytofirmans PsJN onto bentonite, silica and talc for microbial inoculants Ana Bejarano, Ursula Sauer, Birgit Mitter, Claudia Preininger ⁎ Center for Health & Bioresources, Bioresources Unit, AIT Austrian Institute of Technology GmbH, Konrad-Lorenz-Strasse 24, 3430 Tulln, Austria abstract article info Article history: Received 18 December 2016 Received in revised form 20 February 2017 Accepted 21 February 2017 Available online 2 March 2017 The aim of this study was to evaluate the mineral carriers bentonite, silica and talc as potential supports for im- mobilization of the plant growth promoting bacterium Paraburkholderia phytofirmans PsJN, and determine the factors influencing bacterial adsorption to provide stable and efficient microbial inoculants for use in the field. Re- sults reveal that adsorption of PsJN depends on pH, the number of immobilized cells decreasing from pH 5.5 to 9. Zeta potential measurements indicated that the surface charge of the carrier had certain, but not major influence on bacteria immobilization. The amount of Mg 2+ contained in the carrier was a key feature, determining the ex- tent of immobilization of PsJN in buffer (talc N bentonite N silica). Moreover, we evaluated the hydrophobicity and its influence on adsorption of PsJN by measuring the contact angle and the number of adsorbed bacterial cells. Highest number of bacterial cells was found on talc, the most hydrophobic material of the three tested ones (ben- tonite: 3.8 × 10 9 CFU g −1 ; silica: 3.0 × 10 9 CFU g −1 ; talc: 1.4 × 10 10 CFU g −1 ). By contrast, similar immobilization capacity was observed on the three materials, when bacteria culturing and bacteria adsorption were performed in a single step. This might be related to the fact that during culturing biofilm is formed as a result of clonal growth of initially attached bacteria, rather than the recruitment of planktonic cells. Altogether, the important factors for adsorption in buffer (pH 5.5) appeared to be mainly the electrostatic and hy- drophobic interactions. © 2017 Elsevier B.V. All rights reserved. Keywords: Paraburkholderia phytofirmans PsJN PGPB Adsorption Bentonite Silica Talc 1. Introduction Microbial biofertilizers and biocontrol agents are promising alterna- tives to agrochemicals in sustainable agriculture; however the lack of ef- fective formulations is a major limitation for their application in fields. To maximize the chances of inoculation success, the formulation of an inoculant should combine at least three fundamental and essential char- acteristics: supporting the growth of the intended microorganisms, pro- viding viable microbial cells in good physiological condition for an acceptable period of time and deliver enough microorganisms at the time of inoculation to reach a threshold number of bacteria that is usu- ally required to obtain a plant response. In addition, bacteria must sur- vive in soil, compete with adapted microflora and withstand predation by soil microfauna (Bashan et al., 2014). In this regard, it is necessary to develop novel conveyance systems which provide suitable microen- vironments and physical protection against harsh biological conditions to prevent rapid decline of introduced bacteria. Recently, some advanced technologies have been developed for the effective storage, transportation and enhanced efficiency of formula- tions by encapsulating cells in biocompatible polymers like alginate and acacia gum. The principle of this technique lies in the entrapment of cells within a shell or capsule that protects, isolates and releases grad- ually the microorganism of interest, though many of the encapsulation technologies require special equipment, long preparation times and high production cost (John et al., 2011). Alternatively, cell adsorption on solid carriers, mostly mineral parti- cles, is applied to bring plant-growth promoting bacteria (PGPB) to the field. For example, Albareda et al. (2008) used perlite, attapulgite, sepi- olite and amorphous silica for immobilization of Sinhorhizobium fredii and Bradyrhizobium japonicum achieving 10 9 –10 10 CFU g −1 and showed that those materials can be used as carriers for rhizobia. Especially per- lite gave good results in terms of long survival and seed yield. Likewise, Jiang et al. (2007) reported the immobilization of Pseudomonas putida,a bioremediation and biocontrol agent, on montmorillonite, kaolinite and goethite yielding 10 10 CFU g −1 (Albareda et al., 2008; Jiang et al., 2007). This method is simple, inexpensive and has minor influence on physio- logical activities (Li et al., 2014). The process of immobilization involves the transport of cells from the bulk phase to the surface of the support, followed by adhesion of cells, and subsequent settlement at the support surface. The initial at- tachment in general can be evoked by either unspecific or specific inter- actions. The latter ones involve proteins that bind at the interacting surfaces. Among the non-covalent unspecific interactions such as Applied Clay Science 141 (2017) 138–145 ⁎ Corresponding author. E-mail address: claudia.preininger@ait.ac.at (C. Preininger). http://dx.doi.org/10.1016/j.clay.2017.02.022 0169-1317/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay