## 3 Results and Discussion
### Photometric Growth Inhibition Experiments
The extended lag phases in three of the four bacteria in the presence of acetate suggest that an adaptive response was required to increase acid tolerance and exploit the new nutrient source. Energy is required to pump out hydrogen ions from the cell, and acetate is known to trigger the production of several proteins in _E. coli_[27], which means that energy is not available to initiate growth. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Modification of Nutrient Broth**} & \multicolumn{4}{c}{**Ratio of Growth Parameters of _Staphylococcus aureus_ in:**} \\ \cline{2-5}  & **Modified NB with Co-cit/Modified NB without Co-cit & **Modified NB/Standard NB** \\ \hline  & **Max. Growth Rate** & **Lag Phase** & **Max. Growth Rate** & **Lag Phase** \\ \hline None (standard NB) & 0.74 * & 1.5 & - & - \\ Sorbate (10 g/L) & 0.52 *† & 1.4 & 0.72 * & 2.0 \\ Benzoate (2 g/L) & 0.76 * & 1.2 & 0.77 * & 1.6 \\ Propionate (20 g/L) & 0.61 * & 1.2 & 0.80 * & 1.4 \\ HCl (pH 5.5) & 0.75 * & 1.2 & 1.05 & 2.2 \\ Acetic acid (pH 5.5) & 0.65 * & 1.4 & 1.03 & 1.7 \\ NaNO\({}_{2}\) (0.15 g/L) & 0.55 * & 0.9 & 1.05 & 1.5 \\ NaCl (50 g/L) & 0.56 * & 1.0 & 0.65 * & 1.0 \\ \hline  & & & & \\ \end{tabular}
* * Significant (_p_ < 0.05) difference between the mean maximum growth rates in standard or modified broth with and without Co-cit (column 2) or between modified NB and standard NB (column 4). Statistical analysis was not performed on the lag phases. *† Significant (_p_ < 0.05) difference between the ratio of maximum growth rates with and without Co-cit in modified broth compared to that in unmodified nutrient broth (0.61).  
\end{table} 
Table 2: Relative maximum growth rates and lag phases of _Staphylococcus aureus_ in modified and standard nutrient broth (NB) with and without cobalt citrate [Co-cit (0.5 mM)]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Modification of Nutrient Broth**} & \multicolumn{4}{c}{**Ratio of Growth Parameters of _Escherichia coli_ in:**} \\ \cline{2-5}  & **Max. Growth Rate** & **Lag Phase** & **Max. Growth Rate** & **Lag Phase** \\ \hline None (standard NB) & 0.70 * & 1.2 & - & - \\ Sorbate (10 g/L) & 1.04 + & 1.0 & 0.36 * & 3.7 \\ Benzoate (2 g/L) & 0.76 * & 0.9 & 0.68 * & 2.0 \\ Propionate (20 g/L) & 0.55 *† & 1.3 & 1.02 & 1.2 \\ HCl (pH 5.5) & 0.79 * & 1.8 & 0.99 & 1.2 \\ Acetic acid (pH 5.5) & 1.05 + & 1.2 & 0.78 * & 3.8 \\ NaNO\({}_{2}\) (0.15 g/L) & 0.62 * & 0.8 & 0.84 * & 1.0 \\ NaCl (50 g/L) & 0.43 *† & 1.2 & 0.21 * & 2.8 \\ \hline \hline  & & & & \\ \end{tabular}
* * Significant (_p_ < 0.05) difference between the mean maximum growth rates in standard or modified broth with and without Co-cit (column 2) or between modified NB and standard NB (column 4). Statistical analysis was not performed on the lag phases. *† Significant (_p_ < 0.05) difference between the ratio of maximum growth rates with and without Co-cit in modified broth compared to that in unmodified nutrient broth (0.61).  
\end{table} 
Table 3: Relative maximum growth rates and lag phases of _Escherichia coli_ in modified and standard nutrient broth (NB) with and without cobalt citrate [Co-cit (0.5 mM)]. Co-cit in broth with no other additives (0.5 mM) significantly reduced (_p_ < 0.05) the maximum growth rate of all bacteria tested to between 51% (_P. aeruginosa_) and 74% (_S. aureus_) of the rate in the control without Co-cit. Evidence of synergistic action of Co-cit with the different preservatives and acids was observed in several combinations of bacteria with preservatives, but there was no consistent pattern. _P. aeruginosa_ was the most strongly inhibited organism of the four bacteria by (Co-cit)-preservative combinations (Table 4). Growth was completely prevented by Co-cit in combination with propionate, NaCl, and HCl, almost completely inhibited by Co-cit with acetic acid, and strongly inhibited by Co-cit with nitrite. The inhibitory activity of Co-cit against _L. monocytogenes_ was enhanced by acidifying the medium to pH 5.5 with both acetic and hydrochloric acids (Table 1).