## 1 Introduction  
Mycoplasma is a significant issue in research laboratories as contaminants in cell culture. The bacterial contamination potentially leads to disastrous laboratory results since mycoplasma can alter the infected host cell at a molecular level, providing inaccurate and unreliable experimental data. The bacteria compete with the host cell and deplete their nutrients, which consequently disrupt host cell metabolism, morphology, and function at a molecular level. The molecular alterations reportedly include, e.g., interference of nucleic acid synthesis, disruption of amino acid and lipid metabolism, chromosomal aberrations, cell membrane leakage, and interference with signal transduction [1,2]. Mycoplasmas are small prokaryotic bacteria, in the _Mollicutes_ genus, that lack a cell wall around their cell membranes [3]. The organisms are naturally resistant to many commonly used antibiotic drugs, such as those targeting cell wall synthesis or beta-lactam antibiotics [4]. Mycoplasma contamination rates have been reported around 15%-35% and could be as high as 70% [5]. This data suggests that mycoplasma contamination could simply spread among vulnerable medium. Good aseptic techniques should be, therefore, complied with in order to reduce the infection rate, as well as routine contamination checks. FTIR microspectroscopy has been known as a nondestructive, label-free, sensitive, inexpensive, and potential analytical method with several useful applications in biomedical research [6]. The IR spectra provide the data relating to specific bonding vibrations, which refer to a specific functional group present in a tested sample [7]. The technique has been used to investigate or discriminate cells before and after treatment in several studies since it yields the information on biological component alterations. For example, cell death induction by different compounds against U937 leukemia cells was examined and discriminated against using FTIR microspectroscopy, yielding the information on biomolecule variations among different stages of cell death [8]. FTIR spectra from cancerous kidney and ovarian cells could also be segregated from their normal counterparts, indicating cellular component alteration toward cancer progression [9,10]. These studies have demonstrated the effectiveness of FTIR microspectroscopy to define biomolecular changes during different stages of the cell. Since mycoplasma contamination causes alteration of biochemical composition (i.e., lipid, protein, and nucleic acid) in their host cells [11,12], the detection of overall biomolecular compartments, therefore, seems to be useful for examining the effect of mycoplasma contamination in the host cell, as well as discriminating them from decontaminated cells. To the best of our knowledge, only one publication has applied FTIR spectroscopy for determination of the biocomponent changes of glioblastoma cells contaminated by specific mycoplasma species (_M. bovis_) [13]. The previous study reported that biomolecular components in a human glioblastoma cell line (DBTRG) detected by FTIR microspectroscopy were altered during mycoplasma infection. The result showed that FTIR microspectroscopy can be applied for the detection of mycoplasma contamination in cell culture samples [13]. However, the multitude of mycoplasma effects on different cells could be in a different manner and to different degrees [2]. In this study, we propose the use of FTIR microspectroscopy to investigate the biochemical variation before and after the removal of mycoplasma contamination in HepG2 cells. The cells were collected before and after decontamination by antibiotic BM-Cyclin treatment. Contaminated and decontaminated cells were then identified using the standard mycoplasma detection PCR (polymerase chain reaction) method. Moreover, the universal primer [14] covering most of the mycoplasma strains regularly found in cell culture [15] was used in the study to represent mycoplasmas commonly detected in cell culture labs.