## 4 Discussion
### Qualitative Comparison of the Extraction Methods  
The first finding of the qualitative comparison of the extraction methods was the difference in yield when extracting the bacterial cultures. The yield of the resin extract was 325.7 mg, and this was about 6 times higher compared with the EtOAc extract, yielding 53.6 mg. Investigation of the chromatograms of the UHPLC-MS analysis revealed that the resin extract contained a higher share of polar compounds that originated from bacteria and media, as comparison with a media reference for DVR1 media confirmed. For the targeted natural products, the two extraction methods were quite comparable, as we detected the same spiked natural products in the resin and the EtOAc extracts (Table 2). Interestingly, ampicillin and gentamycin were not extracted by any protocols or at least they were not extracted in sufficient quantity for detection. When comparing the retention times in Table 2, it appears that ampicillin and gentamycin had rather short retention times (1.51 and 0.46 min, respectively) and vancomycin eluted between them (0.97 min). We speculate that both extraction methods are rather unsuitable for very polar and hydrophilic compounds. When comparing the structures of the analytes, it appears that vancomycin (**5** in Figure 1) is a relatively large molecule (1449.2 u) and consists of five benzylic rings, while ampicillin (2, 349.4 u) and gentamycin (**4**, 482.5 u) have substantially lower molecular weights. For the quantitative comparison of the two extraction protocols, water was used as matrix to dissolve the natural products for reproducibility reasons since the media com-ponents and bacterial metabolites in a fermentation could affect the extraction (as we observed above that the solid-phase extract contained significantly more of those compounds). We intended to compare the extraction of the natural products in a simple way, from purified water, including the effect of the pH levels of the aqueous solutions on the extraction yield. Some bacteria prefer to grow or grow exclusively at acidic (acidophil bacteria) [21] or alkale pH levels (alkaliphile bacteria) [22], and we also wanted to investigate if a post-fermentative change in pH by adding acid or alkali could be a strategy to increase yields. For the quantitative study, we selected four natural products (tetracycline, cyclosporine, colchicine and gentamycincin), trying to cover a broad range of polarities and molecular weights. We investigated pH levels of 4.0, 5.0, 6.0, 7.0 and 8.5. In Figure 3, the yields of the respective natural products are given as percentage recovery or yield of the initially spiked compound quantity (1.0 mg for tetracycline, cyclosporine and gentamycin and 0.5 mg for colchicine). As expected, after the observations from the preceding qualitative comparison, gentamicin was not detected in the extracts of either extraction technique or at any pH-level. Generally, as visible in Figure 3, there were few variations among the pH levels within the same extraction. Generally, EtOAc extraction had a better recovery of the spiked compounds (except for gentamycin), tetracycline was recovered at about 10% using resin and 15% using EtOAc, colchicine was recovered at levels between 50% and 70%, depending on the condition, for colchicine; the resin extraction performed better. The recovery of cyclosporine was, depending on the condition, at least around 2 times higher in the EtOAc extract samples compared with the resin extracts. We conclude that the two extraction techniques are qualitatively comparable and that both are less suitable for polar compounds, which was expected given the chemical composition of the poly-benzyl resin. Quantitatively the EtOAc extraction showed higher yields for cyclosporine, while the other two compounds were comparable. One of the prominent advantages of EtOAc extracts is that the extracts contain fewer media-components and polar metabolites, which eases downstream purification of compounds from the extracts.