Toxicity To Microbes

Bacteria tend to be less sensitive than eukaryotic organisms to toxic substances. This may be due to the relative simplicity of their metabolic processes and the lower degree of specialization. For example, organophosphate pesticides act on acetylcholinesterase in the nervous system, which has no counterpart among bacteria. Thus, bacteria are fairly insensitive to these substances. General metabolic inhibitors, however, can affect bacteria in a manner similar to that of higher organisms. Bacterial membranes, cell walls, and slime layers offer greater resistance to many toxins than animal structures such as gills, lungs, and intestinal surfaces. Furthermore, bacteria can become dormant when conditions are not favorable for growth or survival. Cyst formation can be another barrier to a toxicant entering the cell.

Besides reduced sensitivity, three factors relating to microbial ecology serve to mitigate the impact of toxins on microbial communities:

1. Bacteria grow rapidly and can quickly regain their numbers if only part of the population is eliminated.

2. Bacterial groups have redundant capabilities. That is, although bacteria may be metabolically specialized individually, many different types share similar specialization. Consequently, when one type is poisoned, there will often be others ready to take their place. There are some exceptions, such as the nitrifying bacteria. These grow slowly, and are replaced slowly, yet are important to the ecosystem. Thus, toxicity assessments should consider possible effects on this group.

3. Bacteria are ubiquitous in natural systems. Even in laboratory or industrial systems it is very difficult to sterilize an open system chemically and at the same time to isolate it from external resupply. In the natural environment, it is all but impossible.

Nevertheless, inhibition of bacteria has been measured by various means, including plate counts, biochemical measurements, respirometry, and biomass growth rates. Table 21.12

TABLE 21.12 Effect of Herbicides on Microbial Respiration

Erwinia carotovora Pseudomonas fluorescens Bacillus sp.

TABLE 21.12 Effect of Herbicides on Microbial Respiration

Erwinia carotovora Pseudomonas fluorescens Bacillus sp.

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