Pesticides are extensively used in agriculture as a part of pest control strategies. Owing to their xenobiotics characteristics, pesticides may adversely affect the proliferation of beneficial soil microorganisms and their associated biotransformation in the soil. Inactivation of nitrogen‐fixing and phosphorus‐solubilizing microorganisms is observed in pesticide‐contaminated soils. Recent studies show that some pesticides disturb molecular interactions between plants and N‐fixing rhizobacteria and consequently inhibit the vital process of biological nitrogen fixation. Similarly, many studies show that pesticides reduce activities of soil enzymes that are key indicators of soil health. The applied pesticides may also influence many biochemical reactions such as mineralization of organic matter, nitrification, denitrification, ammonification, redox reactions, methanogenesis, etc. However, a few reports reveal some positive effects of applied pesticides on soil health. In this chapter, we attempt to analyze the impacts of pesticides on soil microbial communities, soil biochemical reactions, and soil enzymes.

Chapter 5: Impact of pesticides on soil microbial diversity, enzymes, and biochemical reactions.

Background
The release of heavy metal-containing nanoparticles (NP) into the environment may be harmful to the efficacy of beneficial microbes that function in element cycling, pollutant degradation and plant growth. Nanoparticles of Ag, CuO and ZnO are of interest as antimicrobials against pathogenic bacteria. We demonstrate here their antimicrobial activity against the beneficial soil microbe, Pseudomonas putida KT2440.

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Antimicrobial activities of commercial nanoparticles against an environmental soil microbe, Pseudomonas putida KT2440.

Characterization and influence of biochars on nitrous oxide emission from agricultural soil.

Limitations in endodontic disinfection are collectively due to the biofilm mode of bacteria in root canal systems, anatomical complexities, dentin structure/composition, and limitations associated with chemical disinfectants. Consequently, advanced disinfection strategies are developed and tested in Endodontics. The primary aim of these advanced anti-biofilm strategies is to eliminate biofilm bacteria from the uninstrumented portions and anatomical complexities of the root canal system without inducing untoward effects on healthy tissues. This article outlines various challenging factors in root canal disinfection, and describes in detail different advanced therapeutic strategies for endodontic biofilms such as antibacterial nanoparticles, antimicrobial photodynamic therapy, laser-assisted root canal disinfection, ozone, and herbal/enzyme alternatives.

Advanced therapeutic options for endodontic biofilms

Removal of heavy metals from contaminated domestic-industrial effluent using eight resistant indigenous bacteria isolated from acclimatized activated sludge was investigated. Molecular identification using 16S rDNA amplification revealed that all strains were Gram-negative among which two were resistant to each of copper, cadmium and cobalt while one was resistant to each of chromium and the heavy metal mixture. They were identified as Enterobacter sp. (Cu1), Enterobacter sp. (Cu2), Stenotrophomonas sp. (Cd1), Providencia sp. (Cd2), Chryseobacterium sp. (Co1), Comamonas sp. (Co2), Ochrobactrum sp. (Cr) and Delftia sp. (M1) according to their resistance pattern. Strains Cu1, Cd1, Co2 and Cr were able to resist 275 mg Cu/l, 320 mg Cd/l, 140 mg Co/l and 29 mg Cr/l respectively. The four resistant strains were used as a mixture to remove heavy metals (elevated concentrations) and reduce the organic load of wastewater effluent. Results revealed that using the proposed activated sludge with the resistant bacterial mixture was more efficient for heavy metal removal compared to the activated sludge alone. It is therefore recommended that the proposed activated sludge system augmented with the acclimatized strains is the best choice to ensure high treatment efficiency and performance under metal stresses especially when industrial effluents are involved.

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Bioremediation of heavy metal-contaminated effluent using optimized activated sludge bacteria

Tolerance to Zn 2+ by pure cultures of Bacillus, Salmonella and Arthrobacter species isolated from New Calabar River sediment was assessed through dehydrogenase assay. The cultures were exposed to Zn 2+ concentrations of 0.2 to 2.0 mM in a nutrient broth-glucose-TTC medium. The responses of the bacterial strains varied with Zn 2+ concentration. In Salmonella sp. SED2, Zn 2+ stimulated dehydrogenase activity at 0.2 mM. In Bacillus sp. SED1 and Arthrobacter sp. SED4, dehydrogenase activity was progressively inhibited with increasing Zn 2+ concentration. The IC50 ranges from 0.206 ± 0.030 to 0.807 ± 0.066 mM. Total inhibition of dehydrogenase activity was observed at concentrations ranging from 1.199 ± 0.042 to 1.442 ± 0.062. The order of zinc tolerance is: Salmonella sp. SED2 > Arthrobacter sp. SED4 > Bacillus sp. SED1. The result of the in vitro study indicated that Zinc is potentially toxic to sediment bacteria and could pose serious threat to their metabolism in natural environments.

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Toxicity of zinc to heterotrophic bacteria from a tropical river sediment

Toxicity of Zn 2+ on four planktonic bacteria isolated from New Calabar River water was assessed via dehydrogenase assay. Pure cultures of the bacterial strains were exposed to various Zn 2+  concentrations (0.2 - 2.0 mM) in a nutrient broth amended with glucose and TTC. The responses of the bacterial strains to Zn 2+ is concentration-dependent. At 0.2 mM, Zn 2+ stimulated dehydrogenase activity in Proteus sp. PLK2 and Micrococcus sp. PLK4. In all strains, dehydrogenase activity was progressively inhibited at concentrations greater than 0.2 mM. The IC 50 ranges from 0.236 ± 0.044 to 0.864 ± 0.138 mM. Total inhibition occurred at concentrations ranging from 1.283 ± 0.068 to 2.469 ± 0.045 mM. The order of zinc tolerance is: Micrococcus sp. PLK4 > Proteus sp. PLK2 > Pseudomonas sp. PLK5 > Escherichia sp. PLK1. The result of the in vitro study indicated that the bacterial strains are sensitive to Zn 2+ stress. Therefore, Zn 2+ contamination would pose serious threat to their metabolism in natural environments. Key words: New Calabar River, zinc toxicity, planktonic bacteria, dehydrogenase.

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Response of planktonic bacteria of New Calabar River to zinc stress

Effects of heavy metals and pesticides on cowpea (Vigna unquiculata) rhizoplane microbial community were assessed in vitro via dehydrogenase activity. The microbial community was exposed to various concentrations of heavy metals and pesticides in a nutrient broth-glucose-2,3,5-triphenyl chloride (TTC) medium. At 0.2 mM, iron and cadmium stimulated the dehydrogenase activity of the microbial community. For all the metal ions, there was progressive inhibition with each successive increase in the concentration of metal ion, reaching near 100% at 0.6, 0.8, 1.2, 0.12 and 12 mM for cobalt, cadmium, iron, mercury and nickel, respectively. Between 0.2 and 0.4 mM, zinc sharply inhibited dehydrogenase activity and at concentration above 0.4 mm, inhibition of dehydrogenase activity became less pronounced. The order of toxicity is Hg2+ > Co2+ > Cd2+ > Zn2+ > Fe2+ > Ni2+. The herbicides Cotrazine (Atrazine 80W) and Northrin®10EC stimulated dehydrogenase activity of the microbial community at 0.2% and inhibited it at higher concentrations. The median inhibitory concentrations (IC50s) of Cotrazine (Atrazine 80W) and Northrin®10EC were 0.552 ± 0.028 and 0.593 ±0.051%, respectively. The dehydrogenase activity varied significantly (p < 0.05) with the type and concentrations of metals or pesticides. The result indicates that the heavy metals and pesticides are potentially toxic to V. unquiculata root surface microorganisms. In soil, this toxicity may affect nitrogen fixation processes and by extrapolation affect crop yield.

	 

	Key words: Dehydrogenase activity, rhizosplane bacteria, atrazine, cypermethrin, heavy metals, cowpea.

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In vitro effects of metals and pesticides on dehydrogenase activity in microbial community of cowpea (Vigna unguiculata) rhizoplane

The effects of 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol and 4-bromophenol on the activities of periplasmic nitrate reductase and dehydrogenase enzymes of Acinetobacter sp. isolated from a tropical soil were assessed via reduction of nitrate and 2,3,5-triphenyltetrazoluim chloride, respectively. There were significant differences in the response of dehydrogenase enzyme but not periplasmic nitrate reductase to the various compounds. However, at different concentrations, the phenolic compounds had significant effects on periplasmic nitrate reductase but not dehydrogenase activity. Estimations of the degree of inhibition/stimulation of both enzyme activities showed a significant dose-dependent response. 2,4-Dinitrophenol showed decreasing progressive stimulation of dehydrogenase enzyme with increasing concentration (40 ‐ 200 mg/l) while 2-nitrophenol, 4-nitrophenol and 4-bromophenol showed increasing progressive inhibition of both enzyme activities. The IC50 of the phenolic compounds to the periplasmic nitrate reductase activity were comparable (33.46 ‐ 44.75 mg/l) but lower than those for the dehydrogenase activity (>200 mg/l). This suggested greater sensitivity of periplasmic nitrate reductase than dehydrogenase enzyme to the phenolic compounds. This study suggested that the toxic effects of chemicals on specific microbiological activities are better studied using the appropriate enzymes involved rather than the dehydrogenase activity.

Toxicity of phenolic compounds to oxidoreductases of Acinetobacter species isolated from a tropical soil

Tomato remains one of the most nutritive edible berries but challenged by incessant attack and spoilage by fungi among others. The negative effects of synthetic preservatives have shifted attention to bio-preservatives. This study investigated the shelf-life of post-harvest tomato fruits treated with the two medicinal plants: Azadirachta indica (neem leaf) and Vernonia amygdalina (bitter leaf) extracts. Fresh tomato fruits and leaves of both plants were sourced from Lokoja. The leaves were air-dried, pulverized and extracted with distilled water and absolute ethanol. The extracts were analyzed phytochemically and graded concentrations (2.5 g/mL - 10.0 g/mL) were applied to the tomato samples in five replications each. Weight loss, appearance of fungal mycelia and deteriorations on the tomato samples were monitored for 30 days. Fungal isolates from the deteriorated samples were recovered and subjected to in vitro inhibitory activities. Alkaloids, glycosides, saponins, flavonoids and tannins were present in both extracts, except for A. indica, where saponins was not detected. Both extracts significantly (p<0.05) reduce the weight loss (63.4 %) and extended the shelf life of the tomato fruits to 24 days at 10.0 g/mL. Aspergillus niger, Fusarium oxysporum, Rhizopus stolonifer and Alternaria alternata were recovered from the spoilt tomatoes. The most and least susceptible isolates were R. stolonifera (84.56 %) and A. niger (71.45 %), respectively. The bioactivities of both extracts were not significantly different (p>0.05) from each other. These findings suggest that relatively higher concentrations of both plant extracts could be potential bio-preservatives to extend the shelf life of post-harvest tomatoes.

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J. C. Okolo

Papers

Toxicity of zinc to heterotrophic bacteria from a tropical river sediment

Response of planktonic bacteria of New Calabar River to zinc stress

In vitro effects of metals and pesticides on dehydrogenase activity in microbial community of cowpea (Vigna unguiculata) rhizoplane

Toxicity of phenolic compounds to oxidoreductases of Acinetobacter species isolated from a tropical soil

The Shelf Life of Tomato Fruits (Solanum lycopersicum L.) Treated with Extracts of Two Medicinal Plants: Azadirachta indica and Vernonia amygdalina