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- Rheology of living cellsPublication . Portela, R.; Franco, Jose M.; Almeida, Pedro L.; Patricio, Pedro; Sobral, Rita; R. Leal, CatarinaThe mechanical behavior of living cells, during planktonic growth, has been thoroughly explored combining common biological techniques with rheology and rheo-imaging measurements. Under a shear flow, bacterial cultures of Staphylococcus aureus revealed a complex and rich rheological behavior not usually accessed in biological studies. In particular, in stationary shear flow, the viscosity increased during the exponential phase and returned close to its initial value at the late phase of growth, accompanied by the stabilization of the bacterial population. In oscillatory flow, the elastic and viscous moduli exhibited power-law behaviors whose exponents are dependent on the bacteria growth stage, and can be associated to a Soft Glassy Material behavior. These behaviors were framed in a microscopic model that suggests the formation of a dynamic web-like structure, where specific aggregation phenomena may occur, depending on growth stage and cell density. Furthermore, systematic measurements combining optical density and dry weight techniques presented new evidences, which confirmed that the observed cell aggregation patterns developed during growth, under shear, can not only be cell density dependent.
- Bacterial growth screened by rheologyPublication . Portela, Raquel; Pereira, M.; Sobral, Rita; Almeida, Pedro L.; R. Leal, CatarinaThe study of bacterial growth is a challenging field since it aims to describe the behaviour of microorganisms under different physical or chemical conditions. Bacterial growth as a biofilm is of particular interest as these gel-like structures are detrimental for all applications where “clean” surfaces are most important, and are related to failure of infection treatment, food spoilage and oil pipeline contamination, amongst others. In the literature one can find several works concerning the characterization of the mechanical behaviour of bacterial biofilms, although mostly are implemented over solid biofilms, as they appear in real situations [1-3], to study the adhesion properties in surfaces. In this work we applied a different technique to monitor the growth rate of a coccoid shaped bacterial species, the human pathogen Staphylococcus aureus.
- S. aureus and E. coli Co-culture Growth Under ShearPublication . Portela, Raquel; Almeida, Pedro L.; Sobral, Rita; R. Leal, CatarinaGrowing 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.
- Staphylococcus aureus strains rheology during growthPublication . Portela, Raquel; Almeida, Pedro L.; Patricio, Pedro; M.T., Cidade; Sobral, Rita; R. Leal, CatarinaThe study of the mechanical properties of living bacteria in a liquid rich medium, environment commonly used in laboratorial settings, opens a new perspective on the bacterial physiology and behaviour during population growth. In this work, the human pathogen Staphylococcus aureus was used as a study model due to its coccoid shape and regular morphology: MRSA strain COL [1] and its mutant strain RUSAL9 [2], which presents a deficient daughter-cell separation mechanism. Cultures were grown under a mechanical stress solicitation in parallel with optical density monitorization. Complex viscoelastic behaviour was revealed by these bacterial systems [3]. In particular the shear viscosity measurement during growth time, for a constant shear rate, showed an unexpected behaviour that cannot be observed by common optical experimental techniques.