## 3 Results and Discussion
### Metallic Nanoparticle Synthesis by M. magnetotacticum Using Either Fe, Ag, Au or Zn
(2008) reported the use of ferric citrate, ammonium chloride and sodium lactate in their culture media as iron, nitrogen and carbon sources, respectively, using a chemostat culture technique [36]. A yield of approximately 0.0436 mg/mL was achieved which is ten times less than the yield obtained in the current study using MSGM 6. Interestingly, all studies mentioned above made use of ferric citrate as the iron source when compared to our study that used ferric guinea; thus, the difference in iron sources may have been a contributing factor, i.e., had an effect on the biomineralization process itself, accounting for the significant differences in yield. In addition, it is also quite evident that an optimal nitrogen source is in the form of sodium nitrate at a high concentration of 40 mM. MSGM original, MSGM 6 and MSGM 9 were then evaluated to determine whether _M. magnetotacticum_ was able to use metals such as silver, gold and zinc to synthesize metallic nanoparticles, apart from the inherently biomineralized iron nanoparticles. These metals were selected for this study as they were identified as the most commonly used for downstream application such as antibacterial agents, antioxidant agents, drug delivery and biosensing [21; 22; 37; 38; 39; 40; 41; 42; 43; 44]. According to Table 3 below, _M. magnetotacticum_ was unable to synthesize silver nanoparticles when using MSGM original and MSGM 6 (sodium nitrate) respectively as no nanoparticles were visible during TEM analysis and evaluation of elemental composition using EDX did not indicate the presence of silver. However, when Isogro(tm) was added to the medium resulting in MSGM 9 (sodium nitrate and Isogro(tm), silver nanoparticles were successfully produced. This suggests that the additive Isogro(tm) present in MSGM 9 was indeed beneficial for silver nanoparticle production as it allowed for the successful uptake of the metal silver and its subsequent biomineralization into silver-containing nanoparticles as verified by TEM and EDX analysis. All three media evaluated (i.e., MSGM original, MSGM 6 and MSGM 9) were able to be used to successfully synthesize gold nanoparticles as verified by TEM and EDX analysis. Interestingly, MSGM 9 \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Bacterial Growth Phase** & **Time Point (Hours)** & **Nanoparticle Yield (mg/50 mL)** \\ \hline Lag phase & 24 & 16 \\ \hline Exponential phase & 48 & 22 \\ \hline Stationary/death phase & 72 & 18 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Dry weight yield (mg/50 mL) of iron-containing nanoparticles produced by _M. magnetotacticum_ and isolated at different phases of bacterial growth after incubation in MSGM 6 at 30 \({}^{\circ}\)C.  
produced an excellent amount of gold nanoparticles when compared to the other medium modifications used. However, in this study using all three media (i.e., MSGM original, MSGM 6 and MSGM 9), zinc nanoparticles could not be synthesized, suggesting that the nature of this bacteria possibly does not favour the production of zinc nanoparticles which can be supported by the fact that no other studies to date have been able to synthesize zinc nanoparticles using _M. magnetotacticum_. Overall, when comparing the bacterial growth to the nanoparticle yields obtained, it could be seen that improved bacterial growth does not necessarily mean an improvement in nanoparticle yield which was evident in MSGM 1, 2 and 10 where improved growth was seen having OD values of 1.14, 1.00 and 1.14 after 48 h of incubation, respectively. However, the nanoparticle yields obtained for these MSGM modifications were found to not statistically significantly differ (\(p>0.05\)) from that of MSGM original thereby indicating no improvement in nanoparticle yield. Therefore, according to these results there was no correlation between growth and nanoparticle production in the present study which is in line with the findings of [PERSON] and [PERSON] (2016) [45].