The contamination could be due to the presence of yeast in the culture. Cultures contaminated with yeasts become turbid with no pH change. Under microscope, yeast appear as small spherical particles. It could not be bacterial contamination because bacterial contanmination is accompanied with sudden drop in the pH of the culture medium. It could not be virus or mycoplasma contamination because the media does not change turbid in the case of mycoplasma contamination. They are very difficult to detect and they do not be appear as small round dots under microscope. It could not be mold contamination because the mycelia usually appear as thin, wisp-like filaments under microscope. Solution The contamination could be due to the presence of yeast in the culture. Cultures contaminated with yeasts become turbid with no pH change. Under microscope, yeast appear as small spherical particles. It could not be bacterial contamination because bacterial contanmination is accompanied with sudden drop in the pH of the culture medium. It could not be virus or mycoplasma contamination because the media does not change turbid in the case of mycoplasma contamination. They are very difficult to detect and they do not be appear as small round dots under microscope. It could not be mold contamination because the mycelia usually appear as thin, wisp-like filaments under microscope..