The influence of growth phase on the resistance of Staphylococcus aureus to the surface-active agents benzalkonium chloride and dodecylbenzyl sulfonic acid and the oxidizing agents sodium hypochlorite and hydrogen peroxide was studied. The resistances of cells in different growth phases were compared to those of solid medium cells grown according to the European phase 1 suspension test. Using cells from different growth phases (±3 × 107 CFU ml−1), we found that decline-phase cells were the most resistant cells. However, the decline-phase cell suspension contained more than 90% dead cells. A 10-fold–diluted suspension with a total concentration of cells equal to that of the other cell suspensions still revealed decline-phase cells to be generally the most resistant cell type. However, the resistance was drastically reduced, indicating that the large proportion of dead cells provided significant protection to the viable decline-phase cells. Hydrogen peroxide resistance could be partly explained by the high catalase activity in the dead-cell fraction. Exponential-phase cells were less resistant than decline-phase cells, and, surprisingly, stationary-phase cells were the least resistant of the three. Cells grown according to the European phase 1 suspension test were never the most resistant cells. Their survival was 1 to 3 log units lower than that of the most resistant cells. These findings show that the solid-medium cells currently used in disinfectant tests are not the most resistant cells that can be used.

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