Short-term effect of reclaimed wastewater quality gradient on soil microbiome during irrigation.

To investigate the effect of wastewater (WW) treatment on soil bacterial communities, water of different quality was used to irrigate eight lettuces per tank: raw municipal wastewater (RWW), WW treated with an aerated constructed wetland (CWW) and WW treated with a membrane bioreactor (MBW), and tap water (TW). The physicochemical and microbiological characteristics (quality indicators) of these water types were characterized, and the water and soil bacterial communities were monitored by quantitative PCR (qPCR) and 16S rRNA gene sequencing. Despite marked differences in microbial load and diversity of waters, soil communities remained remarkably stable after irrigation. Microbial biomass was increased only in soils irrigated with RWW. At the end of the irrigation period (day 84), soil and water shared a large fraction of their bacterial communities, from 43 % to 70 %, depending on the water quality, indicating a transfer of bacterial communities from water to soil. Overall, the relative abundance of Proteobacteria and Acidobacteria was increased and that of Actinobacteria was decreased in soils irrigated with MBW, CWW and even more with RWW. Multivariate ordination clearly separated soils in three groups: soils irrigated with the cleanest water (TW), with treated WW (MBW and CWW), and with untreated WW (RWW). Nitrifying, denitrifying, and nitrogen-fixing bacteria were quantified by qPCR targeting amoA, narG, and nifH, respectively. Nitrifying bacteria were the most affected by the water quality, as indicated by amoA copy number increase in RWW-irrigated soil and decrease in CWW-irrigated soil. Overall, the abundance of all three genes was positively influenced by RWW treatment. In conclusion, the 84 days of irrigation influenced the soil microbial communities, and the impact depended on the quality of the used water.

Reclaimed wastewater reuse in irrigation: Role of biofilms in the fate of antibiotics and spread of antimicrobial resistance.

Reclaimed wastewater associated biofilms are made up from diverse class of microbial communities that are continuously exposed to antibiotic residues. The presence of antibiotic resistance bacteria (ARB) and their associated antibiotic resistance genes (ARGs) ensures also a continuous selection pressure on biofilms that could be seen as hotspots for antibiotic resistance dissemination but can also play a role in antibiotic degradation. In this study, the antibiotic degradation and the abundance of four ARGs (qnrS, sul1, blaTEM, ermB), and two mobile genetic elements (MGEs) including IS613 and intl1, were followed in reclaimed wastewater and biofilm samples collected at the beginning and after 2 weeks of six antibiotics exposure (10 µg L-1). Antibiotics were partially degraded and remained above lowest minimum inhibitory concentration (MIC) for environmental samples described in the literature. The most abundant genes detected both in biofilms and reclaimed wastewater were sul1, ermB, and intl1. The relative abundance of these genes in biofilms increased during the 2 weeks of exposure but the highest values were found in control samples (without antibiotics pressure), suggesting that bacterial community composition and diversity are the driven forces for resistance selection and propagation in biofilms, rather than exposure to antibiotics. Planktonic and biofilm bacterial communities were characterized. Planktonic cells are classically defined "as free flowing bacteria in suspension" as opposed to the sessile state (the so-called biofilm): "a structured community of bacterial cells enclosed in a self-produced polymeric matrix and adherent to an inert or living. surface" as stated by Costerton et al. (1999). The abundance of some genera known to harbor ARG such as Streptococcus, Exiguobacterium, Acholeplasma, Methylophylaceae and Porphyromonadaceae increased in reclaimed wastewater containing antibiotics. The presence of biofilm lowered the level of these genera in wastewater but, at the opposite, could also serve as a reservoir of these bacteria to re-colonize low-diversity wastewater. It seems that maintaining a high diversity is important to limit the dissemination of antimicrobial resistance among planktonic bacteria. Antibiotics had no influence on the biofilm development monitored with optical coherence tomography (OCT). Further research is needed in order to clarify the role of inter-species communication in biofilm on antibiotic degradation and resistance development and spreading.

The microbial signature of aerosols produced during the thermophilic phase of composting.

AIMS: The microbial diversity of bioaerosols released during operational activities at composting plants is poorly understood. Identification of bacteria and fungi present in such aerosols is the prerequisite for the definition of microbial indicators that could be used in dispersal and exposure studies. METHODS AND RESULTS: A culture-independent analysis of composting bioaerosols collected at five different industrial open sites during the turning of composting piles in fermentation was performed by building 16S rDNA and 18S rDNA libraries. More than 800 sequences were analysed. Although differences in the phylotypes distribution were observed from one composting site to another, similarities in the structure of microbial diversity were remarkable. The same phyla dominated in the five bioaerosols: Ascomycota among fungi, Firmicutes and Actinobacteria among bacteria. For each phylum, some dominant phylotypes were common to at least four bioaerosols. These common phylotypes belonged to Thermomyces, Aspergillus, Penicillium, Geobacillus, Planifilum, Thermoactinomyces, Saccharopolyspora, Thermobifida and Saccharomonospora. CONCLUSIONS: The microbial signature of aerosols produced during the thermophilic phase of composting was determined. The similarities observed may be explained by the selection of thermophilic and sporulating species. SIGNIFICANCE AND IMPACT OF THE STUDY: Several bacteria and fungi identified in this study may represent potential indicators of composting bioaerosols in air.

Structural divergence of bacterial communities from functionally similar laboratory-scale vermicomposts assessed by PCR-CE-SSCP.

AIMS: To evaluate bacterial community structure and dynamics in triplicate vermicomposts made from the same start-up material, along with certain physico-chemical changes. METHODS AND RESULTS: The physico-chemical parameters (pH, temperature, carbon, nitrogen, soluble substances and cellulose) evolved similarly in the triplicate vermicomposts, indicating a steady function. The 16S bacterial gene abundance remained constant over time. To monitor changes in the bacterial community structure, fingerprinting based on capillary electrophoresis single-strand conformation polymorphism was employed. A rise in bacterial diversity occurred after precomposting and it remained stable during the maturation phase. However, a rapid shift in the structure of the bacterial community in the vermicompost replicates was noted at the beginning that stabilized with the process maturation. Multivariate analyses showed different patterns of bacterial community evolution in each vermicompost that did not correlate with the physico-chemical changes. CONCLUSIONS: The broad-scale functions remained similar in the triplicates, with stable bacterial abundance and diversity despite fluctuation in the community structure. SIGNIFICANCE AND IMPACT OF THE STUDY: This study has demonstrated that microbial fingerprinting with multivariate analysis can provide significant understanding of community structure and also clearly suggests that an ecosystem's efficacy could be the outcome of functional redundancy whereby a number of species carry out the same function.