We find that both Brevundimonas diminuta and Serratia marcescens can grow through sterilizing grade filter membranes of different membrane polymer compositions. Although this passage does not occur on a consistent basis, generation of "grow-through positive" results indicate that grow-through can occur stochastically at basal levels. This observation argues that the following risk mitigation strategies during pharmaceutical aseptic processing are warranted: minimization of processing times, and monitoring, minimizing and characterizing pre-filter bioburden.