Soumya El Abed

Adhesion of Aspergillus niger and Penicillium expansum spores on Fez cedar wood substrata

In this study, we investigated the ability of Aspergillus niger and Penicillium expansum spores isolated on fez medina cedar wood to adhere to this substrata. Furthermore, the physicochemical properties of both Aspergillus niger and Penicillium expansum spores and cedar wood were also determined to better understand the adhesion phenomenon. We found that spores surface were hydrophilic, strongly electron donating and weakly electron accepting using contact angle measurements. In contrast, wood surface was hydrophobic, exhibits a character relatively more electron-donor than electron-acceptor (γ− = 5.5; γ+ = 0). Acid-basic interactions were found at least to be involved in the adhesion step. Finally, quantitative adhesion study using environmental scanning electron microscopy (ESEM) combined with image analysis showed that Aspergillus niger spores (62%) adhered stronger than those of Penicillium expansum (30%).

Scanning Electron Microscopy (SEM) and Environmental SEM: Suitable Tools for Study of Adhesion Stage and Biofilm Formation

For most of the history of microbiology, microorganisms have primarily been characterized as planktonic, freely suspended cells and described on the basis of their growth characteristics in nutritionally rich culture media. The discovery of microorganisms, 1684, is usually ascribed to Antoni van Leeuwenhoek, who was the first person to publish microscopic observations of bacteria. The direct quantitative recovery techniques showed unequivocally that more than 99.9% of the bacteria grow in biofilms on a wide variety of surfaces. Although the most common mode of growth for microorganisms on earth is in surface associated communities (Stoodley et al., 2002; Sutherland, 2001), the first reported findings of microorganisms “attached in layers” were not made until the 1940s. During the 1960s and 70s the research on “microbial slimes” accelerated but the term “biofilm” was not unanimous formulated until 1984 (Bryers, 2000). Biofilm has three-dimensional (3D) structured, heterogeneous community of microbial cells enclosed in an exopolysaccharide matrix (also called glycocalyx) that are irreversibly attached to an inert or living surface. As establish, biofilm formation has a serious implications in public health and medicine. In the case of human health, a number of microbial infections are associated with surface colonization not only on live surfaces (sinusitis, pulmonary infection in cystic fibrosis patients, periodontitis, etc. (Hall-Stoodley et al., 2004) but also on medical implants (contact lenses, dental implants, intravascular catheters, urinary stents) etc. (Donlan, 2001; Hall-Stoodley et al., 2004). Biofilms affect heat exchangers, filters, etc. because they induce biocorrosion and biofouling, producing damages on metallic surfaces and the efficiency loss in industrial set-up (Dunne, 2002; Garret et al., 2008). However,biofilms have also useful applications in bioremediation (Vidali, 2001) of different environments (microorganisms degrade and convert pollutants into less toxic forms) and biolixiviation (bacteria can efficiently dissolve minerals used in industry, to obtain copper and gold).

Biocontrol potential of a Bacillus subtilis strain against Bactrocera oleae

Within the Mediterranean basin, pest infestation of the olive tree especially by Bactrocera oleae is a serious economic problem. In this study, we have isolated 115 bacterial strains from various ecological niches, and tested their ability to protect the olive fruits against Bactrocera oleae. Among these strains, culture supernatant (CS) of one bacterial strain displayed the highest rate of larval mortality, and was identified as Bacillus subtilis by 16S rRNA molecular analysis. Further characterization of the CS of the Bacillus sp. strain showed that the highest insecticidal activity against third instar larvae occurs at pH 7. Our results indicate that this bacteria strain may be a prospective alternative in pest control programs.

Dairy biofilm: an investigation of the impact on the surface chemistry of two materials: silicone and stainless steel

Biofilms are the most common mode of bacterial growth in nature and the formation will occur on organic or inorganic solid surfaces in contact with a liquid. The aims of this study were, by combining numeration and sessile drop technique, (i) to characterize the structural dynamics of dairy biofilm growth and the physico chemical properties on silicone and stainless steel and (ii) to evaluate the impact of bio-adhesion on chemistry of surfaces at different times of contact (2, 7, 9 and 24 h). Significantly, greater biofilm volumes were observed after 48 h on two materials. Gram-positive bacteria and fungal population exhibited a significantly higher biofilm organization than gram-negative (43–64%). Elsewhere, after 48 h, results showed a slight difference on gram-negative adhered cells on stainless steel than silicone (2.6 × 107 cfu/cm2 and 4.7 × 105 cfu/cm2, respectively). Moreover, the physico chemical properties of the surfaces showed that the silicone and stainless steel have a hydrophobic character (Giwi = −68.28 mJ/m2 and −57.6 mJ/m2, respectively). Also, both the surfaces present a weak electron donor character (γ − = 2.2 mJ/m2 and 4.1 mJ/m2, respectively). The real-time investigation of the impact of dairy biofilm on the physico chemical properties of the materials has shown a decrease of hydrophobicity degree of the silicone surface that becomes hydrophilic (ΔGiwi = 11.47 mJ/m2) after 7 h and the increase of electron donor character (γ − = 75.8 mJ/m2). Elsewhere, bio-adhesion on stainless steel was accompanied with a decrease of hydrophobicity degree of the surface, which becomes hydrophilic after 7 h of contact (ΔGiwi = 6.62 mJ/m2) and the increase of the electron donor character (γ − = 44.8 mJ/m2). While, after 24 h of contact, results showed a decrease of the hydrophilicity degree and surface energy components of silicone and stainless steel that become hydrophobic (ΔGiwi = −21.2 mJ/m2 and ΔGiwi = −56.51 mJ/m2, respectively) and weak electron donor (γ − = 14.0 and 2.3 mJ/m2, respectively).