Changes in the strength of attachment of micro-organisms to surfaces following treatment with disinfectants and cleansing agents.

Suspensions of Pseudomonas aeruginosa and Staphylococcus epidermidis, and biofilms established (16 h) on submerged glass and stainless steel (216 2B) coupons, were exposed to sodium hypochlorite (0.02% or 0.015% w/v), Dodigen (0.0015% w/v or 0.0006% w/v), sodium dodecylsulphate (6% w/v or 0.1% w/v) and Tween-80 (6% w/v) for 5 min at 20 degrees C. Survival was assessed by viable counts and blot succession. Biofilm bacteria were significantly less susceptible to these biocides than were planktonic cells, but their attachment to the surfaces was loosened by such treatments. Treatment with the non-ionic surfactant, Tween-80, however, strengthened the attachment of Staph. epidermidis to stainless steel. Such effects on attachment strength, which are species and surface dependent, have profound implications on post-treatment cleansing and possible re-contamination of product in clean-in-place (CIP) systems.

Quantification of the ease of removal of bacteria from surfaces.

This paper describes a technique which reproducibly quantifies the ease of removal of microorganisms from surfaces. Tiles (22 mm x 22 mm) of various materials were colonised with Staphylococcus epidermidis NCTC 11047, Escherichia coli K12 HB101 or Pseudomonas aeruginosa PaWH, by submersion, for various times (2 min-48 h), in inoculated Tryptone Soya broth (37 degrees C). Colonised tiles were blotted onto a Tryptone Soya agar plate for 1 min and the process was repeated through a succession of agar plates. The final plate contained tetrazolium salts (0.05% w/v) and was incubated in situ with the tile. Tetrazolium plates indicated that very few organisms remained on the tiles after 15 successive blots. In all instances, the number of recovered colonies per plate decreased exponentially with plate succession number, according to the relationship, CFU = A.10-kN, where CFU is the number of colonies transferred, k is the removal exponent, A is the intercept and N is the plate succession number. Removal exponents differed significantly between organisms (P > 0.95), depended on the nature of the test surface, and decreased as the inital attachment and colonisation time was increased from 2 min-48 h. Intercept values (A) but not the gradients were dependent upon the initial numbers of bacteria in suspension. These data indicate that the gradients derived from counting recoverable viable cells from successive blots of test tiles onto agar is a measure of the strength of attachment of the organisms to the surface.