toxicity, inhibitionReading time:
For anaerobic or aerobic fermentation to take place satisfactorily, fairly stringent medium conditions must prevail. Among these conditions, temperature and pH play an important part. However, the medium must also be free of any inhibitor and toxic products that could slow down, or even irreversibly shut down bacterial activity.
Most heavy metals behave as toxic products as far as the bacterial flora is concerned. This is more especially the case of copper, chromium, nickel, zinc, mercury and lead and also of some anions and organic substances (see harmful substances)
Furthermore, not all bacteria display the same sensitivity to different toxic products and, for instance, table 5 in the page 2 of suspended growth activated sludge, lists some of the organics inhibitors that are specific to nitrifying autotrophic bacteria. On the other hand, some strains are even capable of breaking down the organics inhibitors such as cyanides or phenols if they are sufficiently diluted and, in practice, a bacterial culture can frequently be adapted to cope with the presence of toxic or inhibiting products, thereby pushing back their toxicity thresholds.
However, beyond a certain threshold, some metabolites themselves are capable of inhibiting bacterial activity (e.g. NH3 in anaerobic fermentation).