**3.3.3 PCA results for other metal ions**

The same approach performed for bacteria/Cd+2 interaction was employed to analyse the interaction of the other metal ions with the bacterial surface. The scores and loadings plots obtained after the removal of outliers from PCA, carried out on the spectral data sets corresponding to samples of different concentrations of the three metal ions attached to the bacterial surfaces are shown in Fig. 8. A clear discrimination between objects corresponding to different concentrations of lead and zinc trapped on the bacterial surface was observed along the PC-space [Figure 8(a) & (b)]. The information provided by the loadings explains the distribution of samples in clusters surrounded by ellipses and labeled as I, II and III.

The dashed rectangles depicted in the loadings plots indicate the spectral regions that are statistically influent in the distribution of objects. For lead, region labeled as I corresponds to the superficial polysaccharides (1000-800 cm-1 region), region labeled as II, to the fingerprint region (700-250 cm-1), and region labeled as III, to the amide I region (1650-1500 cm-1). For zinc, region I corresponds to superficial polysaccharides (1000-750 cm-1), region II, to amide I (1700-1500 cm-1), and region III, to the fingerprint region (450-250 cm-1).

It is important to point out that in both bacteria/Pb+2 and bacteria/Zn+2 interactions, the same spectral regions were identified as influent for the objects distribution observed in PCA. This indicates that similar molecular structures may be involved in the bacteria/metal interaction. The scores plot corresponding to the bacteria/Ni+2 samples indicate a poor discrimination among objects [Fig. 8 (c)]. According to the loadings plots of the bacteria/Ni+2 samples, the spectral similarities and differences between samples are mainly in the fingerprint region (region surrounded by a blue dashed line).
