Changes in the cell size of Brevundimonas diminuta using different growth agitation rates

Brevundimonas diminuta (ATCC 19146) is a standard organism for validation of sterilizing-grade membrane filters. Cell size is critical for the determination of retention characteristics of 0.2 micron rated membrane filters. In this study, cell size changes of B. diminuta cultured under different physiologic states and variable agitations at 50, 100 and 200 rpm were measured by a particle size analyzer and scanning electron microscope (SEM). The smallest cells were obtained at initial stationary phase in saline lactose broth (SLB) as a shaking culture at 50 rpm. Cells grown under agitation at 50, 100 and 200 rpm showed an increase of specific growth rate (mu), about 2.9, 3.6 and 3.6 fold, respectively, compared to the non-agitated cells in SLB media. These results suggested that the cell size decreased proportionally with increase of the specific growth rate (mu) in SLB. These size changes were associated with penetration through a 0.2 micron rated cellulose acetate filter. A scale-down filtration system was developed and performed bacterial challenge test and bubble point test with cells cultured in SLB. Cells grown under agitation conditions in SLB were not retained by 0.2 micron rated membrane filter.