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Herbicide effects on Bdellovibrio bacteriovorus Public Deposited

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  • Bdellovibrio bacteriovorus is a small parasitic bacterium present in soil, mud, and aquatic habitats. Due to its lysis of Gramnegative bacteria, an important role in soil ecology has been suggested for this organism. A study of effects of herbicides upon Bdellovibrio might consequently be of considerable value. In this study a Bdellovibrio bacteriovorus strain parasitic on a Pseudomonas species indigenous to local soil was isolated and characterized. Seventeen herbicides were screened for activity against these and ten additional cultures. General effects of linuron [3-(3, 4-dichlorophenyl)-1-methoxy-1-methyluxrea], diuron [3-(3, 4-dichlorophenyl)-1, 1-dimethylurea] and benefin (N-butyl-N-ethyl-α, α, α-trifluoro-2, 6-dinitro-p-toluidine) on the Bdellovibrio isolate and three saprophytic bacteria were evaluated in liquid culture. The effect of linuron on Bdellovibrio activities was studied in depth. Herbicide antimicrobial activity appeared to show a positive correlation with presence of an aromatic ring with a polar side chain linked by an electronegative atom. However, DNBP (4, 6-dinitro-o-sec-butyiphenol) inhibited all cultures regardless of its lack of some postulated structural requirements. Picloram (4-amino-3, 5, 6-trichloropicolinic acid); 2, 4-D amine salt (2, 4-dichlorophenoxyacetic acid, dimethylamine salt); 2, 4, 5- T (2, 4, 5-trichlorophenoxyacetic acid); diuron; linuron; IPC (isopropyl N-phenylcarbamate); and RoNeet (3-ethylcyclohexylethylthiolcarbamate), listed in order of decreasing activity, inhibited a majority of the cultures. EPTC (ethyl N, N-dipropylthiolcarbamate), two substituted-uracil herbicides, and MCPA ester (2-methyl-4-chlorophenoxyacetic acid, isooctyl ester) were less active. Benefin, two substituted-triazine herbicides, and the sodium salts of two chlorinated aliphatic acids produced no observable inhibition. Bdellovibrio exhibited sensitivity to 11 of the 17 herbicides tested. The host, two known Pseudomonas species, and Serratia marcescens were the most resistant of the cultures tested on solid media. In liquid cultures the host species and Pseudomonas fluorescens were relatively resistant to effects of linuron, diuron, and benefin. Bacillus cereus growth, determined by optical density measurements, was inhibited 50% by 50 μg/ml benefin and totally inhibited by the two urea herbicides at the same concentration. The net multiplication of Bdellovibrio in liquid cultures was decreased by linuron; diuron and benefin appeared less active. Motility, attachment rate, and penetration rate of the parasite appeared to be unaffected by linuron at 50 μg/ml. Rapid death ensued in linuron-treated non-multiplying Bdellovibrio cultures. Linuron's principal site of action against Bdellovibrio thus appears to be at the level of maintenance of cellular integrity. Chelation apparently is not involved. It is postulated that linuron disturbs semipermeability of the Bdellovibrio cell membrane due to adsorption phenomena, resulting in an increased death rate. Application of conclusions derived from in vitro studies to the soil environment must be made with caution. In soil, competition between linuron-binding sites on soil particles and on bacteria might decrease the herbicide's activity against Bdellovibrio. However results of this in vitro study should be considered in future investigations of herbicide effects on soil microbial ecology.
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