- 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
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.