Bacteria are a large and ubiquitous group of unicellular microorganisms. They are typically a few micrometers in diameters, and can have a variety of shapes, ranging from spheres to rods and spirals. Because of their ubiquity, size, and fast growth rates, bacteria, along with yeasts and molds, are the most commonly encountered biological contaminants in cell culture.

Detecting Bacterial Contamination

Bacterial contamination is easily detected by visual inspection of the culture within a few days of it becoming infected;

  • Infected cultures usually appear cloudy (i.e., turbid), sometimes with a thin film on the surface.
  • Sudden drops in the pH of the culture medium is also frequently encountered.
  • Under a low-power microscope, the bacteria appear as tiny, moving granules between the cells, and observation under a high-power microscope can resolve the shapes of individual bacteria.
 The simulated images below show an adherent 293 cell culture contaminated with E. coli.

Figure 2.2:  Simulated phase contrast images of adherent 293 cells contaminated with E. coli.  The spaces between the adherent cells show tiny, shimmering granules under low power microscopy, but the individual bacteria are not easily distinguishable (panel A).  Further magnification of the area enclosed by the black square resolves the individual E. coli cells, which are typically rod-shaped and are about 2 µm long and 0.5 µm in diameter.  Each side of the black square in panel A is 100 µm.