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African Journal of Biotechnology

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Volume 1 Number, 1 2002
Abstracts

Episodic bioavailability of environmental mercury: implications for biotechnological control of mercury pollution


O. A. Ogunseitan


Laboratory for Molecular Ecology, Department of Environmental Analysis and Design, University of California, Irvine, CA 92697-7070. Tel: 949-824-6350, Fax: 949-824-2056 E-mail:
[email protected]

Accepted 14 October 2002

Abstract

Perennial wildfires in Africa and other continents contribute an estimated 8 x 105 kg of mercury to the global atmosphere with a residence time of approximately one year. This phenomenon changes the flux of biologically available mercury in natural microbial communities where enzymatic actions, including mercuric reductase and organomercurial lyase activities, underpin the biogeochemical cycling of mercury with repercussions for human exposure to toxic forms of the element. To elucidate the impact of episodic mercury bioavailability on the response of microbial communities, the expression of microbial proteins and nucleic acids in environmental strains of Pseudomonas species were evaluated under various concentrations of mercury ranging from 0 to 500 µM. Routine cultivation of Pseudomonas aeruginosa PU21 containing the 142.5 kb plasmid Rip64 in medium containing 100 µg of Hg++/ml (500 µM) exhibited a prolonged lag phase survived by hyper-resistant cells able to grow in medium containing 200 µg of Hg++/ml. Nucleic acid analyses showed a distinct mutation in the merA gene encoding for mercuric reductase activity in cells able to grow at elevated mercury concentrations. A similar mutation was detected in the merR locus which serves as the regulator of the mer operon. Mutations were not detected in merC which encodes for a hydrophobic membrane-associated protein implicated in active mercury transport. Protein profiles of cells grown with elevated mercury concentrations were associated with a stable increase in the production of specific polypeptides. In addition, the survival and genetic response of naturally-occurring mercury resistant bacteria inoculated into contaminated environmental samples were monitored in microcosm experiments over a 30 day period. The results suggest that sudden exposure to high concentrations of mercury either decimates the bacterial population or selects for hyper-resistant strains with high level of constitutive expression of active proteins, including mercuric reductase. Methyl mercury was observed to cause a higher level of induction for mercuric reductase than the specific substrate, inorganic mercury. The selection of hyper-resistant strains is potentially useful for biotechnological strategies to control the bioavailability of mercury, and thereby potentially reducing the re-uptake of mercury into vegetation in regions frequently subjected to wildfires.

Key words: Mercury pollution, wildfires, mercuric reductase, organomercurial lyase, proteomics, microorganisms, detoxification.


Swarming modulatory effects of some amino acids on Proteus strains from Lagos, Nigeria


Bamidele A. Iwalokun* and Babatunde O. Akinwumi


Biochemistry Department, Lagos State University. P.M.B. 1087. Apapa — Lagos, Nigeria

* Corresponding author; e-mail:
[email protected]; tel: 234-1-2601089

Accepted 9 September 2002

Abstract

Swarming motility, a multicellular behaviour characterized by periodic concentric growth on solid media has severally been reported as a constraint in the clinical investigation of mixed-culture infections involving Proteus and as a requirement for virulence. While media are being formulated to restrain swarming in this organism, the roles played by amino acids in the biogenesis of swarming have not been fully clarified. The effects of 20 amino acids on swarming, extracellular protease activity, cellular RNA level and total protein concentration in 20 clinical Proteus strains from Lagos, Nigeria were investigated. At 0.5mM concentration, glutamine, serine, and methionine were the only amino acids found to enhance swarming motility on M9 minimal medium (7.1—11.3 mm), cause significant increases in protease activity (33.7—48.3 units/ml), total protein concentration (22.5—28.6 mg/ml) and cellular RNA concentration (192.8—264.7 mg/ml) when compared to values obtained for other 17 amino acids (P < 0.05). Furthermore, the amino acid induced swarming in these strains was found to vary directly and significantly (P < 0.05) with protease activity and cellular RNA concentration. The exclusion of methionine, serine and glutamine when supplementing antiswarming media for clinical investigation of Proteus - associated polymicrobial infections in Nigeria is suggested.

Key words: Proteus strains, swarming motility, amino acids, Lagos.

 

Biological nitrogen fixation in Crotalaria species estimated using the 15N isotope dilution method


R.T. Samba1, S.N. Sylla2, M. Neyra1, M. Gueye3, B. Dreyfus4 and I. Ndoye2*

1Laboratoire de Microbiologie des Sols IRD/ISRA/UCAD, B.P. 1386, Dakar-Sénégal

2Département de Biologie Végétale, Université Cheikh Anta Diop, B.P. 5005 Dakar, Sénégal / Laboratoire de Microbiologie des Sols IRD/ISRA/UCAD, B.P. 1386, Dakar-Sénégal

3MIRCEN-Centre IRD/ISRA/UCAD Laboratoire de Microbiologie des Sols, B.P. 1386, Dakar-Sénégal

4IRD/ Laboratoire des Symbioses Tropicales et Méditerranéennes, IRD, INRA, AGRO-M, CIRAD, TA10/J, Campus International de Baillarguet, 34398 Montpellier cedex 5, France

*Corresponding author; e-mail:
[email protected]

Accepted 16 October 2002

Abstract

A greenhouse experiment was conducted to measure nitrogen fixation in three Crotalaria species : C. ochroleuca, C. perrottetii and C. retusa growing in Senegal by using 15N direct isotope dilution technique. Two non-fixing plants, Senna obtusifolia and Senna occidentalis served as reference plants. The amount of nitrogen fixed two months after planting was obtained using the average of the two reference plants. The atom % 15N excess in the Crotalaria species was significantly lower than that of the reference plants, indicating that significant nitrogen fixation occurred in the three plants. Significant differences were observed between the Crotalaria species; C. ochroleuca yielded more dry matter weight and total nitrogen than did C. perrottetti and C. retusa. The % nitrogen derived from atmosphere (%Ndfa) in leaves and stems was also higher in C. ochroleuca. There was no significant difference in %Ndfa in the whole plant between the three Crotalaria species (47% to 53%). In contrast, interspecific variability was observed based on the %Ndfa. C. ochroleuca significantly exhibited the higher amount of total nitrogen fixed, equivalent to 83 kg of nitrogen fixed per hectare. Based on these data, it was concluded that C. ochroleuca could be used in multiple cropping systems in Senegal for making more nitrogen available to other plants.

Key words: Crotalaria spp, isotope dilution, 15N, nitrogen fixation, reference plant.

 

Persistence of Rhizobium inoculants originating from Leucaena leucocephala fallowed plots in Southwest Nigeria


O.A. Ojo 1* and O.E. Fagade2

1Department of Botany/Microbiology, Lagos State University, P M B 1087, Apapa, Lagos, Nigeria.

2Department of Botany/Microbiology, University of Ibadan, Ibadan, Nigeria.

* Corresponding author; e-mail:
[email protected]

Accepted 2 October 2002

Abstract

Ten core soil samples were collected from experimental plots at IITA, SW Nigeria that were previously inoculated with Rhizobium strains (IRC1045 and IRC 1050) specific for Leucaena leucocephala at two depths; 0—15 cm and 15—30 cm. The control soil samples were collected at similar depths from an adjacent field with no previous history of legume cultivation. Six weeks after planting of L. leucocephala in the soil samples in the greenhouse shoots, roots and nodules were harvested aseptically. Typing of the nodules as well as the identification of the persisting population of the introduced strains were based on the intrinsic resistance of IRC 1045 and IRC 1050 to streptomycin at 500 mg/ml and nodules were found to be made up of 100% of previously introduced strains. The potency and competitive ability of the recovered IRC 1045 and IRC 1050 were thus confirmed via the pot experiment and plant reinfection experiment in the greenhouse. At 0—15 cm and 15—30 cm depths 8.0 x 104 and 9.0 x 104 rhizobia/g of soil were recovered respectively in spite of the 10-year fallow period. Biomass production with the three woody legumes revealed Root and Shoot dry weights of the following order of magnitude Senna siamea > L. leucocephala > Senna spectabilis. This report showed the great potential of application of Rhizobium technology in low input sustainable agricultural practice and environmental pollution abatement for non-use of chemical nitrogen fertilizers.

Key words: Agriculture, Leucaena leucocephala, nodulation, persistence, rhizobium.