Assessment of aflatoxin B1 content and aflatoxigenic molds in imported food commodities in Muscat, Oman

Aflatoxins, mainly produced by Aspergillus flavus and A. parasiticus are considered as serious food safety and human health issues due to their hepatotoxic effects. In the present study, the occurrence of aflatoxin B1 (AFB1), the most potent human liver carcinogen, and prevalence of toxigenic isolates of Aspergillus spp. were assessed in 140 food commodities in Muscat markets, Oman, and the 95 quarantined imported food commodities. These samples consisted of rice, corn, peanut, red chilli powder, soybean, dried dates and tree nuts. AFB1 was analyzed using competitive ELISA/LC-MS and the aflatoxigenic fungi were detected using plating technique followed by molecular identification. No AFB1 was detected in 89 (63.6%) samples collected from local markets, while 44 (31.4%) samples contained 1-5 ppb and the remaining 7 (5%) samples (red chili powder) contained 6-10 ppb. None of the samples exceeded the maximum permissible limit of 10 ppb set for foods by Oman legislation. Of the 95 quarantined samples, only 17 (17.9%) samples were positive and contained AFB1 at concentrations ranging from 1-3.4 ppb. Four isolates of Aspergillus pp. were isolated from the collected samples and were identified as Aspergillus flavus (A14, A16 and A23) and A. chevalieri (A46) on the basis of internal transcribed spacer (ITS) sequences of ribosomal DNA. Among them, A. flavus strain A14 alone produced AFB1 (7.6 ppb), while A16, A23, and A46 were non-toxigenic. This is the first detailed report on the occurrence of AFB1 in food commodities imported into Oman

Antifungal Activity of Shirazi Thyme (Zataria multiflora Boiss.) Essential Oil against Hypomyces perniciosus, a causal agent of wet bubble disease of Agaricus bisporus

Wet bubble disease (WBD) caused by Hypomyces perniciosus is a major constraint of button mushroom (Agaricus bisporus) cultivated worldwide. A few synthetic chemical fungicides are used to control WBD. In our study, the potential of essential oil (EO) from Zataria multiflora in inhibition of H. perniciosus was evaluated as an alternative to chemical fungicides. An isolate of H. perniciosus was isolated from wet bubble diseased A. bisporus and pathogenicity of the mycoparasite was determined under artificially inoculated conditions. The mycoparasitic fungus was identified using sequences of the internal transcribed spacer (ITS) region of ribosomal DNA. The EO was extracted from the aerial parts of Z. multiflora by microwave extraction method and evaluated in vitro for its antifungal activity against H. perniciosus. The EO of Z. multiflora (ZEO) at the tested concentrations (50% and 100%) inhibited the growth of H. perniciosus in the agar diffusion test. The minimum inhibitory concentration (MIC) of ZEO was 0.04% as assessed by the poisoned food technique. The chemical composition of ZEO was determined by gas chromatography-mass spectrometry analysis. A total of 23 compounds were identified. Among them, the most abundant compounds were Linalool (20.3%) and Bornyl acetate (15.5%). Linalool at the tested concentrations of 0.25% and 0.125% completely inhibited the mycelial growth of H. perniciosus in an in vitro assay. These results suggest that ZEO can be exploited for control of WBD

The effect of salt-tolerant antagonistic bacteria from tomato rhizosphere on plant growth promotion and damping-off disease suppression under salt-stress conditions

A total of 24 morphologically distinct salt-tolerant bacteria were isolated from the tomato rhizosphere soil and tested in vitro against Pythium aphanidermatum, causing damping-off disease in tomato. Among them, five bacterial isolates viz., BTR1.0, BTR1.1, BTR4.4, BTR7.0 and BTR8.6 were found to inhibit the mycelial growth of P. aphanidermatum. The maximum antagonistic effect was shown by BTR1.0 followed by BTR7.0 and BTR1.1. Based on the MALDI Biotyper analysis, these bacterial isolates were identified as Serratia marcescens (BTR1.0 and BTR1.1), Pseudomonas aeruginosa (BTR7.0 and BTR8.6) and Alcaligenes faecalis (BTR4.4). Analysis of plant growth stimulating activity of bacterial strains revealed that A. faecalis BTR4.4 recorded the highest vigour index followed by S. marcescens BTR1.0. The biocontrol activity of bacterial antagonists against tomato damping-off was tested under salt-water irrigation. Among the bacterial strains tested, S. marcescens BTR1.1 was found to be highly effective under non-saline water irrigation, which suppressed the disease by 68% compared to infected control. However, at 50 mM NaCl concentration, S. marcescens strain BTR1.0, P. aeruginosa strain BTR7.0 and A. faecalis BTR4.4 provided a significant level of control of the disease. At 100 mM NaCl concentration, no significant effect of the bacterial strains on the disease incidence was observed

Potential of indigenous antagonistic rhizobacteria in the biological control of Monosporascus root rot and vine decline disease of muskmelon

Monosporascus root rot and vine decline (MRVD) of muskmelon, caused by Monosporascus cannonballus, is an economically important disease worldwide. The objectives of this study were to isolate native rhizobacterial strains and to evaluate their ability to promote plant growth and to provide protection against M. cannonballus. Thirty eight native bacterial isolates from the rhizosphere soil of muskmelon and cucumber were screened for their antagonism against M. cannonballus in a dual culture assay. Among them, five isolates viz., Bacillus amyloliquefaciens (B4), Pseudomonas mendocina (B7), Bacillus endophyticus (B10), Pseudomonas resinovorans B11 and P. aeruginosa AT3, identified based on the 16S rRNA gene sequence analysis, showed a significant level of antagonism and the inhibition zone ranged from 5.6 mm to 25.9 mm. Scanning electron microscopic observation of the hyphae of M. cannonballus at the inhibition zone revealed morphological abnormalities including shrinkage, loss of turgidity, pit formation and deformation. These bacterial isolates showed compatibility with one another and with Trichoderma viride. Greenhouse experiments revealed that P. resinovorans B11 was the most effective among the bacterial antagonists in controlling MRVD in melon. When applied as seed treatment and soil application, P. resinovorans B11 reduced the incidence of MRVD by 93.1%, relative to the infected control

In vitro antagonistic potential, plant growth-promoting activity and indole-3-acetic acid producing trait of bacterial isolates from spent mushroom substrate of Agaricus bisporus

Spent mushroom substrate (SMS) is widely used as fertilizer and to control plant diseases. The microorganisms surviving in SMS play a crucial role in plant growth promotion and biocontrol properties of SMS. In this study, an effort was made to isolate and characterize the bacterial species present in the SMS of Agaricus bisporus and to study their antagonistic potential, plant growth-promoting ability and indole-3-acetic acid (IAA) producing trait. Six different bacterial isolates exhibiting morphological variabilities were obtained from the SMS by serial dilution technique. On the basis of 16S rRNA gene sequences, these isolates were identified as Staphylococcus epidermidis (Sh1 and Sh3), S. aureus (Sh2), Bacillus albus (Sh4), Delftia lacustris (Sh6) and Comamonas aquatica (Sh7). These bacterial strains were assayed for their antagonism against Pythium aphanidermatum, a phytopathogenic oomycete. The results of in vitro dual culture assay revealed that all the 6 bacterial isolates showed low levels of suppression of P. aphanidermatum and recorded less than 5 mm inhibition zone. Among the bacterial isolates, S. epidermidis Sh3 recorded the maximum inhibition zone of 4.2 mm. Plant growth promotion test using roll paper towel method revealed that C. aquatica Sh7, B. albus Sh4, D. lacustris Sh6 and S. epidermidis Sh3 caused a significant increase in seedling vigour of cucumber compared to control. The seeds treated with the bacterial isolate C. aquatica Sh7 showed the maximum seedling vigor. Assessment of in vitro production of IAA by the bacterial isolates revealed that the bacterial isolates highly varied (ranging from 0.28 to 9.25 mg L-1) in their potential for production of IAA. The maximum amount of IAA was produced by C. aquatica Sh7 (9.25 mg L-1), while the minimum was produced by S. epidermidis Sh1 (0.28 mg L-1)

In vitro detoxification of aflatoxin B1 by aqueous extracts of medicinal herbs

Aflatoxin contamination in food commodities is a major food safety concern all over the world. These aflatoxins are a group of carcinogenic secondary metabolites produced by some fungi belonging to Aspergillus section Flavi. To minimize the level of aflatoxins in food commodities, a wide range of pre- and post-harvest procedures have been employed. In this study, aqueous extracts of 13 medicinal herbs were assessed for their ability to detoxify aflatoxin B1 (AFB1), the most potent and commonly occurring carcinogen in foods. Among them, herbal extracts of Hybanthus enneaspermus, Eclipta prostrata and Centella asiatica exhibited over 70% detoxification of AFB1. The degradation of AFB1 upon treatment with these herbal extracts was confirmed by liquid chromatography–mass spectrometry analysis. Two fractions (Rf 0.75 and 0.87) purified from H. enneaspermus by using thin-layer chromatography displayed in vitro AFB1-detoxifying properties. Gas chromatography–mass spectrometry analysis of the active fractions revealed the presence of linalool and bornyl acetate as the major components suggesting possible involvement of volatile compounds of H. enneaspermus in the detoxification of AFB1. To our knowledge, this is the first report on the detoxification of AFB1 by H. enneaspermus, E. prostrata and C. asiatica aqueous extracts