**3.3. Combined effects of cinnamic and vanillic acids and pH on cell count of the isolate 13M**

This phase focused on the evaluation of pH, cinnamic and vanillic acids (combined through a 2k experimental design) on the viability of the isolate 13M. Figure 9 shows the evolution of cell count in some selected combinations of the design; when MSM medium was adjusted to pH 7.0 in presence of 2 g/l of the phenols (combination D) the viable count (7 log cfu/ml) was drastically reduced to ca. 4.73 log cfu/ml. On the other hand, an alkaline pH played a protective role, as the viable count was not reduced both in absence and with phenol added (combinations E and H).

**Figure 9.** Viable count of the isolate 13M in some selected combinations of 2k design (see Table 1).

Finally, Figure 10 shows the increase/decrease of cell count after 12 days; a positive value indicates a significant increase in the viable count, while a negative value indicates a death kinetic. These values were used as input data to run a multiple regression procedure and pinpoint the weight of each factor of the design; the results of this statistical analysis are listed in Table 4. The individual effects of phenols were not significant, while their interactive term played a negative role, i.e., it was the leading factor of death kinetic; on the other hand, the statistical analysis pinpointed a positive mathematical effect of pH, thus confirming its protective role toward viable count.


**Table 4.** Standardized effects of vanillic and cinnamic acids and pH on the reduction of the viable count of the isolate 13M after 12 days in MSM. Ns, not significant. R2 ad, determination coefficient corrected for multiple regression.

Selection of Promising Bacterial Strains as Potential Tools for the Bioremediation of Olive Mill Wastewater http://dx.doi.org/10.5772/60896 95

**Figure 10.** Increase/decrease of the viable count of the isolate 13M in MSM with phenols after 12 days. Mean values ± standard deviation (for the combinations see Table 1).
