Abstract
The use of inoculants carrying diazotrophic and other plant growth–promoting bacteria plays an essential role in the Brazilian agriculture, with a growing use of microorganism-based bioproducts. However, in the last few years, some farmers have multiplied microorganisms in the farm, known as “on farm” production, including inoculants of Bradyrhizobium spp. for soybean (Glycine max L. Merrill.) and Azospirillum brasilense for corn (Zea mays L.) or co-inoculation in soybean. The objective was to assess the microbiological quality of such inoculants concerning the target microorganisms and contaminants. In the laboratory, 18 samples taken in five states were serial diluted and spread on culture media for obtaining pure and morphologically distinct colonies of bacteria, totaling 85 isolates. Molecular analysis based on partial sequencing of the 16S rRNA gene revealed 25 genera of which 44% harbor species potentially pathogenic to humans; only one of the isolates was identified as Azospirillum brasilense, whereas no isolate was identified as Bradyrhizobium. Among 34 isolates belonging to genera harboring species potentially pathogenic to humans, 12 had no resistance to antibiotics, six presented intrinsic resistance, and 18 presented non-intrinsic resistance to at least one antibiotic. One of the samples analyzed with a shotgun-based metagenomics approach to check for the microbial diversity showed several genera of microorganisms, mainly Acetobacter (~ 32% of sequences) but not the target microorganism. The samples of inoculants produced on farm were highly contaminated with non-target microorganisms, some of them carrying multiple resistances to antibiotics.
Highlights
The objective of this study was to assess the microbiological quality of inoculants based on Bradyrhizobium spp. and A. brasilense produced on farm in Brazil, concerning the intended microorganisms, presence, and characterization of probable contaminants
That was a case in which the sample was taken only 4 h after the tank had been inoculated with a commercial inoculant
The isolate probably originated from the commercial inoculant used as inoculum, not from the multiplication, since the short time between the addition of inoculum and the sampling may still have allowed the microorganism to survive
Summary
Merr.) and corn (Zea mays L.) are the main Brazilian grain crops [1], with a production ~ 125 million tons in ~ 37 million hectares of soybean, Responsible Editor: Luc F.M. Rouws. The industrial production of inoculants is a complex process, but improvements in the last two decades have resulted in high-quality products in terms of cell concentrations, no contaminants, and very low cost, probably the cheapest inoculant in the world [11]. The production system is rudimentary and varies in terms of installations, equipment, microbiological standards, and technical capacity. Very often the bioproducts are produced in fermenters, open tanks, or even water tanks, without appropriate control of contaminations, which may result in highly contaminated, non-effective products [12, 13]
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