Sewage sludges generation and their disposal have become one of the greatest challenges of the 21st century. They have great microbial diversity that may impact wastewater treatment plant (WWTP) efficiency and soil quality whether used as fertilizers. Therefore, this research aimed to characterize microbial community diversity and structure of 19 sewage sludges from São Paulo, Brazil, as well as to draw their relations to sludge sources [domestic and mixed (domestic+industrial)], biological treatments (redox conditions and liming), and chemical attributes, using molecular biology as a tool. All sludges revealed high bacterial diversity, but their sources and redox operating conditions as well as liming did not consistently affect bacterial community structures. Proteobacteria was the dominant phylum followed by Bacteroidetes and Firmicutes; whereas Clostridium was the dominant genus followed by Treponema, Propionibacterium, Syntrophus, and Desulfobulbus. The sludge samples could be clustered into six groups (C1 to C6) according their microbial structure similarities. Very high pH (≥11.9) was the main sludge attribute segregating C6, that presented very distinct microbial structure from the others. Its most dominant genera were Propionibacterium > > Comamonas > Brevundimonas > Methylobacterium ∼Stenotrophomonas ∼Cloacibacterium. The other clusters’ dominant genera were Clostridium > > Treponema > Desulfobulbus ∼Syntrophus. Moreover, high Fe and S were important modulators of microbial structure in certain sludges undertaking anaerobic treatment and having relatively low N-Kj, B, and P contents (C5). However, high N-Kj, B, P, and low Fe and Al contents were typical of domestic, unlimed, and aerobically treated sludges (C1). In general, heavy metals had little impact on microbial community structure of the sludges. However, our sludges shared a common core of 77 bacteria, mostly Clostridium, Treponema, Syntrophus, and Comamonas. They should dictate microbial functioning within WWTPs, except by SS12 and SS13.

Burkholderia sp. is a bacterial genus extremely versatile in the environment and has been reported for a great potential to promote plant growth via different mechanisms. Here we evaluate the plant growth-promoting mechanisms in twenty-six Burkholderia strains. Strains were evaluated for their ability to promote plant growth by means of: indole-3-acetic acid (IAA) production under different conditions of pH, salt stress and the presence or absence of L-tryptophan; exopolysaccharides (EPS) production and quorum sensing (ALH). The strains were also characterized in terms of their genetic variability and species identification through Sanger sequencing. Then, the bacteria most responsive in the greatest number of plant-growth promotion mechanisms were selected for a corn seed germination test. All bacteria synthesized IAA in medium with 0.0 or 5.0 mM of L-tryptophan in combination with either 1 or 5% of NaCl, and pH values of either 4.5 or 7.2. The EPS production was confirmed for 61.54% of the bacterial strains. Quorum sensing also occurred in 92.3% of the selected bacteria. The Jaccard similarity coefficient revealed 16 clusters with high genetic variability between bacterial strains. Bacterial strains were assigned to seven species: B. anthina, B. cepacia, B. gladioli, B. ambifaria, B. graminis, B. heleia, and Burkholderia spp. The corn seed bacterization did not affect the germination velocity index (GSI), as well as the first count of germinated seeds (FC). However, inoculations formulated with B. heleia strain G28, B. gladioli strain UAGC723, and B. graminis strain UAGC348 promoted significant increases in root length, seedling height and fresh and dry seedling phytomass, respectively. These results indicate the high biotechnological potential of several strains in the genus Burkholderia sp. as seed inoculants, favoring germination and seedling initial development.