The influence of calcium on the initial adhesion of S-thermophilus to stainless steel under flow studied by metallurgical microscopy

Observation of microorganisms adhering to metal surfaces is more difficult than of those adhering to glass or translucent polymers and usually requires special techniques as e.g. epifluorescent microscopy, scanning electron microscopy or removal of adhering microorganisms followed by plating. In this paper, the use of metallurgical microscopy to observe microorganisms adhering to metal surfaces is described. The method is simple, attachment of the vertical illumination set up required to an ordinary light microscope is relatively cheap, and no additional manipulations are necessary. The image quality is high and individual adhering microorganisms can be easily observed, even without employing additional image analysis techniques. As an example of the application of this technique, the influence of Ca2+ ions on the adhesion of three strains of dairy Streptococcus thermophilus to 316-L stainless steel was studied. Using a parallel plate flow chamber with a stainless steel bottom plate and a glass top plate, adhesion was measured in situ on the bottom plate under moderate flow (15 s(-1)) in an adhesion buffer with various amounts of CaCl2 added. Neither the initial deposition rates nor the numbers of organisms adhering after 200 min showed systematic variation with the amounts of CaCl2 added. However, for S. thermophilus G1 initial the deposition rates with 1 mM CaCl2 were much higher (2170 cm(-2) s(-1)) than was the case without CaCl2, or with 5 mM CaCl2 added (approximately 1300 cm(-2) s(-1)). For S. thermophilus B and N-12 the initial deposition rates were only slightly higher with 5 mM CaCl2 than without CaCl2 or without 1 mM CaCl2 added. Thus in order to study adhesion of thermophilic, dairy streptococci under different conditions, such as in a pasteurizer, calcium must be added as its presence can have a major effect.