## 3 Results and Discussion
### Spectral Marker Inspection  
Mycoplasma contamination can induce cytopathic changes to the host cells by competing with host cells for biosynthesis precursors and nutrients as well as altering DNA/RNA synthesis. The mycoplasmas significantly affect cellular metabolism, morphology, and function, which consequently promote cellular transformation [12]. Our FTIR result confirms that there are alterations in the cellular biomolecules in the mycoplasma-contaminated cells. The major change was observed in the lipid region between 3000-2800 cm\({}^{-1}\). The reduction of lipid content in the contaminated cells could be explained by the release of a cytolytic phospholipase by mycoplasmas that hydrolyzes the lipid membrane during host-cell invasion [33]. The phospholipid head group (1743 cm\({}^{-1}\)) was also decreased with its nonpolar part by mycoplasma infection. The protein content was increased in the infected cells (1700-1450 cm\({}^{-1}\)). The expression of (1) mycoplasma genes to facilitate their survival and reproduction, as well as (2) the host gene counteracting the infection [1], possibly resulted in the enhancement of intracellular proteins as translation products. Housed in the host cells, mycoplasma activates many of its genes and produces various peptide products that facilitate its invasion [34]. On the other hand, the host cell responds by overexpressing several genes, including cell-cycle and growth regulators, and the immune response leads to specific protein expression to counteract the infection [35]. A study in the leukemic cell line using reverse-phase protein array reported that 19% of proteins expressed in host cells were affected by mycoplasma contamination, and the most affected are apoptotic-related proteins, reflecting cellular stress [36]. The previous experiments using FTIR to detect cell death revealed that the secondary protein \(\beta\)-sheet was increased upon apoptotic induction [8; 20; 37]. This could explain the increase of \(\beta\)-sheet observed in our study as a host-cell response to mycoplasma infection by expressing apoptotic-related proteins. Figure 6: A percentage of the integral area of the secondary structure proteins corresponds to a curve-fitting analysis. Inset graphic illustrates FTIR absorbance of amide I band contour with best-fit 50% Gaussian/Lorentzian individual component bands. RMS error \(<\) 0.005 in all treatment groups. Data are presented as mean \(\pm\) SD (n = 3). *, \(p\)\(<\) 0.05, paired \(t\)-test. The subtle decrease was detected in the nucleic acid region (1280-900 cm\({}^{-1}\)). The alteration upon mycoplasma contamination on nucleic acids could be contributed by both host cells and the mycoplasma itself. (1) Mycoplasma contamination was previously reported to disrupt the cell-cycle transition of the host cell. The cells infected with mycoplasma were arrested in G\({}_{1}\), leading to a reduction of the cell population in the S phase, and, subsequently, less cell population undergoing proliferation [38]. During the S phase, the IR nucleic acid peaks markedly increase, owing to the DNA deconvolution (for DNA replication), which makes the nucleic acid visible to IR light. On the contrary, the DNA is tightly packed in the form of chromatin and chromatid in the G\({}_{1}\) and G\({}_{2}\)/M phases, which appear as opaque subjects to IR light [39]. Therefore, the decrease of nucleic acid content in the study could be due to the reduction of cells in the S phase during mycoplasma contamination. (2) Upon entry into the host cell, mycoplasmas multiply and the number of mycoplasmas far exceeds the number of infected host-cells, often over 1000-fold [11]. Therefore, the nucleic acid region could increase due to the multiplication of the mycoplasma genome, outnumbering host cells. Taken together, the final results from the present study show the decrease of nucleic acid. However, without any detailed information, it is hard to conclude whether it is the host cell or the mycoplasma genome that affects the observed FTIR spectra.