Behavior of Escherichia coli cells and Bacillus cereus spores on poplar wood crates by impedance measurements

Dry poplar wood crates and glass surfaces (bottoms of Pyrex flasks) were contaminated with aqueous suspensions of Escherichia coli Collection de l'Institut Pasteur (CIP) 54.8 cells or Bacillus cereus CIP 78.3 spores. After different periods of storage at 25 degrees C, the number of cells still present on both surfaces was determined by impedance microbiology. The physical and chemical properties of the wood greatly and rapidly decreased the number of cells. After 4 h of contact time, B. cereus residual metabolic activity corresponded to less than 10% of the initial inoculum level. With a low inoculum level of 2.4 x 10(2) E. coli cells, sterility of the wood was obtained after a contact time of 24 h. With a higher inoculum level of 10(6) E. coli cells, only a few viable bacteria, corresponding to the metabolic activity of four cells, could be recovered after a prolonged contact time of 145 h. Conversely, under the same conditions of storage, when bacteria were deposited on inert and nonporous materials such as glass surfaces, growth occurred. After 24 h of contact time at 25 degrees C, bacterial populations were about 10(9) cells for E. coli and 10(7) cells for B. cereus. Under our experimental conditions after a prolonged contact time, wood exhibited growth-inhibiting properties and cells were no longer metabolically active. These results indicate that the potential for cross-contamination of foods stored directly in contact with previously contaminated poplar wood crates is low under our experimental conditions.