Hadda Ouzari

UV disinfection of treated wastewater in a large-scale pilot plant and inactivation of selected bacteria in a laboratory UV device

Abstract Efficiency of UV disinfection of unfiltered and filtered secondary wastewater effluent, using a large-scale pilot system, and the inactivation of six bacterial species in a laboratory UV-device have been studied. Pilot plant studies revealed low levels of coliforms and streptococci (3 logarithmic units reduction) when a wastewater UV transmission of 45% and an average effective UV dose of 100 mW s cm−2 were used. By contrast, removal of Pseudomonas aeruginosa appeared insufficient (

Bacillus anthracis Diverges from Related Clades of the Bacillus cereus Group in 16S-23S Ribosomal DNA Intergenic Transcribed Spacers Containing tRNA Genes

ABSTRACT Mung bean nuclease treatment of 16S-23S ribosomal DNA intergenic transcribed spacers (ITS) amplified from several strains of the six species of the Bacillus cereus group showed that B. anthracis Davis TE702 and B. mycoides G2 have other intermediate fragments in addition to the 220- and 550-bp homoduplex fragments typical of the B. cereus group. Long and intermediate homoduplex ITS fragments from strains Davis TE702 and G2 and from another 19 strains of the six species were sequenced. Two main types of ITS were found, either with two tRNA genes (tRNAIle and tRNAAla) or without any at all. Strain Davis TE702 harbors an additional ITS with a single tRNA gene, a hybrid between the tRNAIle and tRNAAla genes, suggesting that a recombination event rather than a deletion generated the single tDNA-containing ITS. Strain G2 showed an additional ITS of intermediate length with no tDNA and no similarity to other known sequences. Neighbor-joining analysis of tDNA-containing long ITS indicated that B. cereus and B. thuringiensis represent a single clade. Three signature sequences discriminated B. anthracis from B. cereus and B. thuringiensis, indicating that the anthrax agent started evolving separately from the related clades of the B. cereus group. B. mycoides and B. weienstephanensis were very closely related, while B. pseudomycoides appeared the most distant species.

Are drought-resistance promoting bacteria cross-compatible with different plant models?

The association between plant and plant growth promoting bacteria (PGPB) contributes to the successful thriving of plants in extreme environments featured by water shortage. We have recently shown that, with respect to the non-cultivated desert soil, the rhizosphere of pepper plants cultivated under desert farming hosts PGPB communities that are endowed with a large portfolio of PGP traits. Pepper plants exposed to bacterial isolates from plants cultivated under desert farming exhibited a higher tolerance to water shortage, compared with untreated control. This promotion was mediated by a larger root system (up to 40%), stimulated by the bacteria, that enhanced plant ability to uptake water from dry soil. We provide initial evidence that the nature of the interaction can have a limited level of specificity and that PGPB isolates may determine resistance to water stress in plants others than the one of the original isolation. It is apparent that, in relation to plant resistance to water stress, a feature of primary evolutionary importance for all plants, a cross-compatibility between PGPB and different plant models exists at least on a short-term.

Diversity and antimicrobial properties of lactic acid bacteria isolated from rhizosphere of olive trees and desert truffles of Tunisia.

A total of 119 lactic acid bacteria (LAB) were isolated, by culture-dependant method, from rhizosphere samples of olive trees and desert truffles and evaluated for different biotechnological properties. Using the variability of the intergenic spacer 16S-23S and 16S rRNA gene sequences, the isolates were identified as the genera Lactococcus, Pediococcus, Lactobacillus, Weissella, and Enterococcus. All the strains showed proteolytic activity with variable rates 42% were EPS producers, while only 10% showed the ability to grow in 9% NaCl. In addition, a low rate of antibiotic resistance was detected among rhizospheric enterococci. Furthermore, a strong antibacterial activity against plant and/or pathogenic bacteria of Stenotrophomonas maltophilia, Pantoea agglomerans, Pseudomonas savastanoi, the food-borne Staphylococcus aureus, and Listeria monocytogenes was recorded. Antifungal activity evaluation showed that Botrytis cinerea was the most inhibited fungus followed by Penicillium expansum, Verticillium dahliae, and Aspergillus niger. Most of the active strains belonged to the genera Enterococcus and Weissella. This study led to suggest that environmental-derived LAB strains could be selected for technological application to control pathogenic bacteria and to protect food safety from postharvest deleterious microbiota.

Diversity of auxin-producing bacteria associated to Pseudomonas savastanoi-induced olive knots

Forty three strains were isolated from knots induced by Pseudomonas savastanoi in different olive cultivars. All the selected bacteria were shown to produce variable amounts of the plant growth hormone indole‐3‐acetic acid (IAA). Amplification of the intergenic transcribed spacers (ITS) between 16S and 23S rDNA genes, allowed the clustering of the isolates into seven distinct groups. All isolates from ITS group 1 were positive to the Pseudomonas savastanoi pv. savastanoi specific iaa L gene as shown by PCR. Partial sequencing of 16S rDNA gene confirmed the identity of these isolates to Pseudomonas savastanoi strains and allowed to tentatively assign the other isolates from the remaining ITS groups to Pantoea oleae/agglomerans, Burkholderia cepacia, Pseudomonas putida, Stenotrophomonas maltophilia and Hafnia alvei. Identification of endophytic knot‐derived isolates revealed association of various saprophytic and putative human pathogenic bacteria with P. savastanoi pv. savastanoi in knot environment of olive infected trees. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microbiological parameters and maturity degree during composting of Posidonia oceanica residues mixed with vegetable wastes in semi-arid pedo-climatic condition

The aim of this study was to characterize the biological stability and maturity degree of compost during a controlled pile-composting trial of mixed vegetable residues (VR) collected from markets of Tunis City with residues of Posidonia oceanica (PoR), collected from Tunis beaches. The accumulation in beaches (as well as their removal) constitutes a serious environmental problem in all Mediterranean countries particularly in Tunisia. Aerobic-thermophilic composting is the most reasonable way to profit highly-valuable content of organic matter in these wastes for agricultural purposes. The physical, chemical, and biological parameters were monitored during composting over 150 d. The most appropriate parameters were selected to establish the maturity degree. The main result of this research was the deduction of the following maturity criterion: (a) C/N ratio < 15; (b) NH4+-N < 400 mg/kg; (c) CO2-C < 2000 mg CO2-C/kg; (d) dehydrogenase activity < 1 mg TPF/g dry matter; (e) germination index (GI) > 80%. These five parameters, considered jointly are indicative of a high maturity degree and thus of a high-quality organic amendment which employed in a rational way, may improve soil fertility and soil quality. The mature compost was relatively rich in N (13.0 g/kg), P (4.74 g/kg) and MgO (15.80 g/kg). Thus composting definitively constitutes the most optimal option to exploit these wastes.

Oasis desert farming selects environment-specific date palm root endophytic communities and cultivable bacteria that promote resistance to drought

Oases are desert-farming agro-ecosystems, where date palm (Phoenix dactylifera L.) plays a keystone role in offsetting the effects of drought and maintaining a suitable microclimate for agriculture. At present, abundance, diversity and plant growth promotion (PGP) of date palm root-associated bacteria remain unknown. Considering the environmental pressure determined by the water scarcity in the desert environments, we hypothesized that bacteria associated with date palm roots improve plant resistance to drought. Here, the ecology of date palm root endophytes from oases in the Tunisian Sahara was studied with emphasis on their capacity to promote growth under drought. Endophytic communities segregated along a north-south gradient in correlation with geo-climatic parameters. Screening of 120 endophytes indicated that date palm roots select for bacteria with multiple PGP traits. Bacteria rapidly cross-colonized the root tissues of different species of plants, including the original Tunisian date palm cultivar, Saudi Arabian cultivars and Arabidopsis. Selected endophytes significantly increased the biomass of date palms exposed to repeated drought stress periods during a 9-month greenhouse experiment. Overall, results indicate that date palm roots shape endophytic communities that are capable to promote plant growth under drought conditions, thereby contributing an essential ecological service to the entire oasis ecosystem.

Plant-associated microbiomes in arid lands: diversity, ecology and biotechnological potential

BackgroundAridification is a worldwide serious threat directly affecting agriculture and crop production. In arid and desert areas, it has been found that microbial diversity is huge, built of microorganisms able to cope with the environmental harsh conditions by developing adaptation strategies. Plants growing in arid lands or regions facing prolonged abiotic stresses such as water limitation and salt accumulation have also developed specific physiological and molecular stress responses allowing them to thrive under normally unfavorable conditions.ScopeUnder such extreme selection pressures, special root-associated bacterial assemblages, endowed with capabilities of plant growth promotion (PGP) and extremophile traits, are selected by the plants. In this review, we provide a general overview on the microbial diversity in arid lands and deserts versus specific microbial assemblages associated with plants. The ecological drivers that shape this diversity, how plant-associated microbiomes are selected, and their biotechnological potential are discussed.ConclusionsSelection and recruitment of the plant associated bacterial assemblages is mediated by the combination of the bio-pedo-agroclimatic conditions and the plant species or varieties. Diversity and functional redundancy of these associated PGPR makes them very active in supporting plant improvement, health and resistance to drought, salt and related stresses. Implementing proper biotechnological applications of the arid and desert-adapted PGPR constitute the challenge to be raised.

Bacillus thuringiensis beyond insect biocontrol: plant growth promotion and biosafety of polyvalent strains

The entomopathogenic bacteriumBacillus thuringiensis is widely used for the control of many agricultural insect pests and vectors of human diseases. Several studies reported also on its antibacterial and antifungal activities. However, to our knowledge there were no studies dealing with its capacity to act as a plant growth promoting bacterium. This review surveys the potential ofB. thuringiensis as a polyvalent biocontrol agent, a biostimulator and biofertiliser bacterium that could promote the plant growth. Also, discussed is the safety ofB. thuringiensis as a bacterium phylogenetically related toBacillus cereus the opportunistic human pathogen andBacillus anthracis, the etiological agent of anthrax.

Characterization and selection of Bacillus sp. strains, effective biocontrol agents against Fusarium oxysporum f. sp. radicis-lycopersici, the causal agent of fusarium crown and root rot in tomato.

The antagonistic activities of 20Bacillus isolates were tested with dual culture and greenhouse conditions againstFusarium oxysporum f. sp.Pseudomonas (FORL) race 0, the causal agent of Fusarium crown and root rot of tomato. Under dual culture, 10 isolates inhibited mycelial growth >38% and the most effective inhibited fungal growth >50%. The 20Bacillus isolates were tested for production of volatiles, cyanide, antibiotics, and phosphorus solubilisation; 15 isolates produced volatiles that inhibited growth of pathogens, 9 isolates produced cyanide, 10 produced antibiotics, and five solubilised phosphorus. Greenhouse experiments with the same 20 isolates revealed the effectiveness of 12 strains, which increased the percentage of healthy plants in the tested cultivar from 66 to 96%. The best disease control was achieved by isolates B11, B5, B17, and B18. However, B11 and B17 were the only isolates that produced cyanide, antibiotics, solubilised phosphate and showed 44% inhibition of fungal growth. The selected strains could be considered in plant growth promotion and biological disease control.