The use of direct epifluorescent microscopy (DEM) and the direct epifluorescent filter technique (DEFT) to assess microbial populations on food contact surfaces.

Abstract

Two rapid methods, direct epifluorescent microscopy (DEM) and the direct epifluorescent filter technique (DEFT) on swab resuspension fluids, were compared with the traditional total viable count (TVC) on swab resuspension fluids for their ability to enumerate surface populations of attached bacteria. The degree of error in estimating surface populations was shown to be significantly less with DEM than DEFT followed by TVC. DEM estimated populations in the range 3 x 10(3) to 5 x 10(7) colonies/cm2 whilst DEFT enumerated populations above 3 x 10(4) colonies/cm2 and TVC above 3 x 10(5) colonies/cm2 (as measured by DEM). Swabbing was shown to remove a constant proportion of organisms from the surface populations tested, although below 3 x 10(5) colonies/cm2 most of the organisms remained in the cotton matrix and were difficult to resuspend. DEFT was more able to enumerate swab resuspension fluids obtained from surface populations below 3 x 10(5) colonies/cm2 than was TVC.