S. aureus and E. coli Co-culture Growth Under Shear

Growing monocultures of two different species of human commensal/pathogenic bacteria, Staphylococcus aureus - a non-motile grampositive coccus and Escherichia coli - a motile gram-negative rod, were characterized using a real-time in situ rheology and rheo-imaging strategy. Subjecting bacterial populations to a shear flow is a closer approximation to bacterial thriving in the host, where they experience mechanical forces such as arterial or venous pressure. For both cultures, as the cell density of the population increases, cells rearrange themselves in different aggregates, capable of strongly influencing their environment, and leading to very different physical rheological responses, where motility appears to be determinant. One of the most striking observations is the behavior of the viscosity growth curve, showing dramatic value variations, with no counterpart in traditional biological measurements, as well as the coupling between translational and rotational motion of the E. coli aggregates along the growth curve [1], while S. aureus cells tend to sediment [2], over long periods of time. In the present study, a similar approach was applied to a co-culture of these two bacteria, S. aureus and E. coli, to evaluate the effect of possible interspecies interactions on the viscosity curve of the culture, during growth, when subject to a shear flow. Surprisingly, the observed behavior of the viscosity growth curve was enhanced in comparison to each individual curve and reveals a combination of details specific of each monoculture, suggesting synergy between these two bacterial species. After the rheological analysis, the final co-culture was recovered and inoculated on different solid media that allow to distinguish the development of S. aureus or E. coli colonies. Unexpectedly, S. aureus showed the capacity to accelerate its growth rate relatively to E. coli, when the two-species community is subjected to a shear flow. This behavior may reflect the occurrence of specific growth adaptations during co-culture upon shear flow, getting one step closer to physiological conditions.