**3. Co-inoculation of multiple PGPRs**

Inoculation with mixed different strains could be an alternative to inoculation with individual strains, likely reflecting the different mechanisms used by each strain in the consortium [173]. Combined inoculation with N<sup>2</sup> -fixing and phosphate solubilizing bacteria were more effective than a single microorganism for providing a more balanced nutrition for plants [19, 174]. There are numerous examples in wheat whereby synergistic effects of multiple PGPRs are observed [97, 175, 176]. Among those, notable is the combined inoculation of mixtures and biofilmed bio-inoculants (*Anabaena torulosa* + *Pseudomonas striata* and/or *Anabaena torulosa* + *Azotobacter chroococcum*) were superior over single inoculation and chemical fertilizer control in term of plant growth and nutrient uptake [177]. The benefits can be on nutrient uptake, but also in root physiology as exemplified by Manjunath et al. as co-inoculation of wheat with two proteobacterial (*Providencia* sp. and *Alcaligenes* sp.) and two cyanobacterial (*Anabaena oscillarioides* and *Anabaena torulosa*) inoculants, similarly in Ref. [178, 179].

Seed bacterization with both strains, *P. fluorescens* BAM-4 and *B. cepacia* BAM-12 single or combined significantly enhanced growth and yield, but increase in bacterial population, spike length, P content of shoots and grain yield was more in co-inoculation treatment than single. The best among the bioinoculation treatments was *B. cepacia* BAM-12 + TCP and *B. cepacia* BAM-12 + *P. fluorescens* BAM-4 + TCP for P content with free and immobilized cells [180].

Several authors conducted experiments on wheat either under pot and field conditions to examine the effect of co-inoculations of PGPR on the growth and yield of wheat. Kumar et al. found that *B. megaterium, A. chlorophenolicus* and *Enterobacter* significantly increased plant height, grain yield and straw yield [181]; Baris et al. concluded that *Bacillus megaterium* M3 and Mixed (*Bacillus subtilis* 05U142, *B. megaterium* M3, *Azospirillum brasilense* Sp245) inoculation provided greater plant nutrient element concentrations than mineral fertilizer application [182]. Similar outcomes also compared with chemically fertilized soils are reported in Refs. [53, 183–185].

Nowadays, there is a greater awareness to use biological components such as PGPR and mycorrhizal fungi as a component of integrated nutrient management strategies to obtain higher input use efficiency, to maintain the desired productivity through optimization of the benefits from all possible sources, to cope with increasing fertilizer costs and their long-term adverse effects on agricultural ecosystems such as increased nutritional imbalances, declining productivity, adverse conditions prevailing in this ecosystem, and or a combination of these factors, as reported in Refs. [113, 177]. Note that some PGPR inoculants may adversely affect mutualistic associations between plants and indigenous soil microorganisms and suggest a possible reason as to why spring wheat growth was not consistently enhanced by these *pseudomonad* PGPR [186]. Co-inoculation of *Azotobacter* and *Mycorrhiza* increased grain yield and yield components of wheat [187].

Wheat rhizobacterial community structure is highly dynamic and influenced by different factors such as wheat cultivar line ages, plant's age, growth stage, distance from the soil to the root, root exudation pattern, multiple soil properties and agronomic practises [162, 188, 189]. Roesti et al. employed a consortia formed by a PGPR *Pseudomonas* spp. and an indigenous AMF to study their effect on the bacterial community structure and wheat growth [162].

All in all, greater attention should be paid to new combinations of different types and properties organisms such as N<sup>2</sup> -fixing and P-solubilizing bacteria for improvement of biofertilizers efficiency [19].
