Browsing by Author "Sobral, R. G."
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- Living bacteria rheology: Population growth, aggregation patterns, and collective behaviour under diferent shear flowsPublication . Patricio, Pedro; Almeida, Pedro L.; Portela, R.; Sobral, R. G.; Grilo, I. R.; Cidade, T.; R. Leal, CatarinaThe activity of growing living bacteria was investigated using real-time and in situ rheology-in stationary and oscillatory shear. Two different strains of the human pathogen Staphylococcus aureus-strain COL and its isogenic cell wall autolysis mutant, RUSAL9-were considered in this work. For low bacteria density, strain COL forms small clusters, while the mutant, presenting deficient cell separation, forms irregular larger aggregates. In the early stages of growth, when subjected to a stationary shear, the viscosity of the cultures of both strains increases with the population of cells. As the bacteria reach the exponential phase of growth, the viscosity of the cultures of the two strains follows different and rich behaviors, with no counterpart in the optical density or in the population's colony-forming units measurements. While the viscosity of strain COL culture keeps increasing during the exponential phase and returns close to its initial value for the late phase of growth, where the population stabilizes, the viscosity of the mutant strain culture decreases steeply, still in the exponential phase, remains constant for some time, and increases again, reaching a constant plateau at a maximum value for the late phase of growth. These complex viscoelastic behaviors, which were observed to be shear-stress-dependent, are a consequence of two coupled effects: the cell density continuous increase and its changing interacting properties. The viscous and elastic moduli of strain COL culture, obtained with oscillatory shear, exhibit power-law behaviors whose exponents are dependent on the bacteria growth stage. The viscous and elastic moduli of the mutant culture have complex behaviors, emerging from the different relaxation times that are associated with the large molecules of the medium and the self-organized structures of bacteria. Nevertheless, these behaviors reflect the bacteria growth stage.
- Living S. aureus bacteria rheologyPublication . Portela, R.; Franco, J. M.; Patricio, Pedro; Almeida, Pedro L.; Sobral, R. G.; Leal, Catarina R.The rheological characterization of Staphylococcus aureus cultures has shown a complex and rich viscoelastic behavior, during the bacteria population growth, when subject to a shear flow [1,2]. In particular, in stationary shear flow, the viscosity keeps increasing during the exponential phase reaching a maximum value (∼30x the initial value) after which it drops and returns close to its initial value in the stationary phase of growth, where the cell number of the bacterial population stabilizes. These behaviors can be associated with cell density and aggregation patterns that are developed during culture growth, showing a collective behavior. This behavior has no counterpart in the bacterial growth curve obtained by optical density monitorization (OD620nm and cfus/ml measurements). In oscillatory flow, the elastic and viscous moduli exhibit power-law behaviors whose exponents are dependent on the bacteria growth stage. These power-law dependencies of G’ and G’’ are in accordance with the Soft Glassy Material model [3], given by: G’~ ωx and G’’~ ωx To describe the observed behavior, a microscopic model considering the formation of a dynamic web-like structure was hypothesized [1], where percolation phenomena can occur, depending on the growth stage and on cell density. Recently, using real-time image rheology was possible to visualize the aggregation process associated with these dramatic changes in the viscoelastic behavior. In particular, the formation of web-like structures, at a specific time interval during the exponential phase of the bacteria growth and the cell sedimentation and subsequent enlargement of bacterial aggregates in the passage to the stationary phase of growth. These findings were essential to corroborate the microscopic model previously proposed and the main results of this study are compiled and presented in this work, see Fig.1.
- Real-time rheology of actively growing bacteriaPublication . Portela, R.; Almeida, Pedro L.; Patricio, Pedro; Cidade, T.; Sobral, R. G.; R. Leal, CatarinaThe population growth of a Staphylococcus aureus culture, an active colloidal system of spherical cells, was followed by rheological measurements, under steady-state and oscillatory shear flows. We observed a rich viscoelastic behavior as a consequence of the bacteria activity, namely, of their multiplication and density-dependent aggregation properties. In the early stages of growth (lag and exponential phases), the viscosity increases by about a factor of 20, presenting several drops and full recoveries. This allows us to evoke the existence of a percolation phenomenon. Remarkably, as the bacteria reach their late phase of development, in which the population stabilizes, the viscosity returns close to its initial value. Most probably, this is caused by a change in the bacteria physiological activity and in particular, by the decrease of their adhesion properties. The viscous and elastic moduli exhibit power-law behaviors compatible with the "soft glassy materials" model, whose exponents are dependent on the bacteria growth stage. DOI: 10.1103/PhysRevE.87.030701.
- Rotational and translational motion observed in Escherichia coli aggregates during shearPublication . Portela, R.; Franco, J. M.; Patricio, Pedro; Almeida, Pedro L.; Sobral, R. G.; Leal, Catarina R.Recently, the growth of an Escherichia coli culture was studied using real-time and in situ rheology and rheoimaging measurements, allowing to characterize their rheological behavior during time [1]. In the lag phase, bacteria are adapting to the new environmental growth conditions, with a characteristic slow division rate. Accordingly, the viscosity shows a slow and constant increase with time. In the exponential phase the viscosity presents a dramatic increase, but exhibits several drops and recoveries. In the late phase of growth, the viscosity increase slows down, reaching na intermittent plateau of maximum viscosity, with several drops and recoveries. In this phase, the highest bactéria density is attained: bacteria still grow and divide, but at a lower rate; big and irregular bacteria aggregates are observed, which keep moving in suspension and no significant sedimentation is observed; the aggregates present translational motion in the shear flow direction and rotational motion in the vorticity direction; the aggregates become larger along time, due to the incorporation of smaller aggregates; due to the rotational motion, the aggregates become elongated along the rotational axis; apparently, the size of the aggregates does not influence the rotational motion, since almost all aggregates rotate with the same angular velocity, which is related to the applied shear rate. As a first approximation, and because an explicit individual motion of the cells within each aggregate is not observed, this behavior is interpreted in light of a simple rigid-body motion, in which shear rate and angular velocity are dependent, which will be presented as a microscopic model.