Evaluating Bacteria Binding Mechanisms to Soil Minerals with FTIR Spectroscopy.

Reactions occurring at the bacteria-mineral interface play a critical role in a number of important soil processes involving nutrient cycling, contaminant biodegradation, redox transformations, and pathogen transport. Our research has utilized attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy to probe the molecular-level interactions of bacteria (Pseudomonas putida, P. aeruginosa, Escherichia coli) with pure mineral phases (hematite [α-Fe₂O₃], goethite [α-FeOOH])  and current efforts are underway to examine interactions with kaolinite and natural soils dominated by these mineral phases. In addition to bacteria, the interactions of mixed amino acids, polypeptide extracts, deoxyribonucleic acid (DNA), and a suite of model compounds have also been evaluate. These compounds represent carboxyl, catecholate, amide, and phosphate groups present in siderophores, amino acids, polysaccharides, phospholipids, and DNA IR peaks corresponding to outer-sphere or unbound (1400 cm^-1) and inner-sphere (1310-1320 cm^-1) coordinated carboxyl groups are noted following reaction of bacteria and biomolecules with α-Fe₂O₃ and α-FeOOH have been observed. However, the IR spectra also show that low-levels (i.e., 0.45-0.79%) of biomolecular phosphorous groups result in strong IR bands at ~1043 cm^-1_, corresponding to inner-sphere Fe-O-P bonds. This data highlights the importance of bacteria associated P-containing groups in biomolecule and cell adhesion. In addition, spectral comparisons show somewhat greater P-O-Fe contributions for bacteria (Pseudomonad, E. coli) deposited on α-FeOOH, as compared to α-Fe₂O₃. This data demonstrates that slight differences in bacterial adhesion to Fe oxides can be attributed to bacterial species and Fe-oxide minerals. However, of more importance, the strong binding affinity of phosphate in bacteria and model compounds to both Fe-oxides results in the formation of inner-sphere Fe-O-P bonds, underscoring the critical role of biomolecular P in the initiation of bacterial adhesion.  Similar analyses are underway for bacteria and model compound interactions with Fe-oxide and kaolinite dominated soils.