## 3 Results and Discussion
### Metallic Nanoparticle Synthesis by M. magnetotacticum Using Either Fe, Ag, Au or Zn
#### 3.2.1 TEM and EDX Analysis  
TEM was used to characterize the morphology of nanoparticles in terms of their shape and size. TEM and EDX analyses were performed for all MSGM modifications (MSGM original and MSGM 1-12) supplemented with iron. The results obtained for MSGM original (Figure 3) indicated that the iron nanoparticles were spherical in shape and well dispersed having narrow size distribution with a size range of 6-7 nm. MSGM 1 which used glucose as a sole carbon source resulted in a cluster of cuboidal-shaped nanoparticles that are characteristic of the _Magnetospirillum_ species (Figure S1 in Supplementary Material). However, these nanoparticles were unable to be sized due to aggregation. The nanoparticles synthesized using MSGM 2 (sodium pyruvate) were spherical in shape and larger in size with size ranges of 13-81 nm although most regions were highly aggregated (Figure S3 in Supplementary Material). The use of lactic acid as the sole carbon source (MSGM 3) did not produce well defined nanoparticles but rather a large cluster of undefined particles (Figure S4 in Supplementary Material), and thus could not be sized. \begin{table}
\begin{tabular}{l l l} \hline \hline \multirow{2}{*}{**Metallic Nanoparticle**} & \multirow{2}{*}{**Medium Modification**} & \multicolumn{2}{c}{**Nanoparticle Yield**} \\  & & **(mg/50 mL)** \\ \hline \multirow{3}{*}{Iron (Fe)} & MSGM original & 16 \\ \cline{2-3}  & MSGM 6 (sodium nitrate) & 22 \\ \cline{2-3}  & MSGM 9 (sodium nitrate and IsogroTM) & 17 \\ \hline \multirow{3}{*}{Silver (Ag)} & MSGM original & - \\ \cline{2-3}  & MSGM 6 (sodium nitrate) & - \\ \cline{2-3}  & MSGM 9 (sodium nitrate and IsogroTM) & 15 \\ \hline \multirow{3}{*}{Gold (Au)} & MSGM original & 17 \\ \cline{2-3}  & MSGM 6 (sodium nitrate) & 22 \\ \cline{2-3}  & MSGM 9 (sodium nitrate and IsogroTM) & 33 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Dry weight yield (mg/50 mL) of metallic (Fe, Ag, Au) nanoparticles produced by _M. magnetotacticum_ after 48 h of incubation at 30 \({}^{\circ}\)C in either MSGM original, MSGM 6 or MSGM 9. Medium modification MSGM 4 (ammonium sulphate) also resulted in iron-containing nanoparticles. Although well-dispersed nanoparticles were observed (Figure S5A in Supplementary Material), it should be noted that highly agglomerated regions were also present (Figure S5B in Supplementary Material). According to Figures S5-S12 in the Supplementary Material, it could be noted that particles were highly agglomerated for medium modifications MSGM 4 to MSGM 8 and MSGM 10 to MSGM 12. Medium modification MSGM 9 (sodium nitrate and Isogro(tm)) showed partial agglomeration; however, well dispersed iron nanoparticles were observed for most regions with a size range of 4-6 nm (Figure 4). Figure 3: **(A,B)** TEM micrographs showing iron-containing magnetosomes formed by _M. magnetotacticum_ after incubation at 30\({}^{\circ}\) for 48 h in MSGM original containing iron (10 mM). Size range of nanoparticles is 6–7 nm. **(C)** EDX profile of the observed nanoparticles presented in **(A)**. Iron peaks are present confirming nanoparticle composition. Samples were analysed using carbon-coated copper grids and therefore high peaks denoting carbon and copper are also present. The other peaks present are due to medium components and elements present as part of normal bacterial composition. MSGM original and medium modifications MSGM 6 and MSGM 9 which were supplemented with the other metals were also analysed via TEM and EDX. Figure 5 shows the TEM and EDX images obtained for silver using MSGM 9 (sodium nitrate and Isogro(tm)), and it could be seen that the majority of the nanoparticles were rod-shaped with a width range of 2-42 nm and a length range of 32-59 nm whilst some were spherical in shape; however, these nanoparticles were also clustered. This size range is similar to the findings of [PERSON] et al. (2021) who was also able to synthesize nanoparticles with a size range of 3-35 nm using the same MTB strain [41].