Loading...
Person
Marques Mendes Almeida da Rosa Leal, Catarina
5 results
Search Results
Now showing 1 - 5 of 5
- 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.
- 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.
- Antibiotic activity screened by the rheology of S. aureus culturesPublication . Portela, Raquel; Valcovo, Filipe; Almeida, Pedro L.; Sobral, Rita; Leal, Catarina R.Multidrug resistant bacteria are one of the most serious public health threats nowadays. How bacteria, as a population, react to the presence of antibiotics is of major importance to the outcome of the chosen treatment. In this study we addressed the impact of oxacillin, a β-lactam, the most clinically relevant class of antibiotics, in the viscosity profile of the methicillin resistant Staphylococcus aureus (MRSA) strain COL. In the first approach, the antibiotic was added, at concentrations under the minimum inhibitory concentration (sub-MIC), to the culture of S. aureus and steady-state shear flow curves were obtained for discrete time points during the bacterial growth, with and without the presence of the antibiotic, showing distinct viscosity progress over time. The different behaviors obtained led us to test the impact of the sub-inhibitory concentration and a concentration that inhibited growth. In the second approach, the viscosity growth curves were measured at a constant shear rate of 10 s−1, over time. The obtained rheological behaviors revealed distinctive characteristics associated to the presence of each concentration of the tested antibiotic. These results bring new insights to the bacteria response to a well-known bacteriolytic antibiotic.
- The first normal stress difference and viscosity in shear of liquid crystalline solutions of hydroxypropylcellulose: new experimental data and theoryPublication . Fried, F.; Leal, Catarina R.; Godinho, M. H.; Martins, A. F.The constitutive equations for liquid crystalline polymers recently proposed by one of us [1] are applied here to interpret the behaviour of the shear viscosity η equation image and the first normal stress difference N1($ \dot \gamma $equation image) measured for liquid crystalline (LC) solutions of hydroxypropylcellulose in acetic acid. N1(equation image) is observed to change from positive to negative and again to positive, as the shear rate $ \dot \gamma $equation image increases, at lower concentrations, in the LC phase. The $ \dot \gamma $equation image-values at which N1 changes sign depend on the molecular mass (degree of polymerization) and on the concentration. η $ \dot \gamma $equation image shows a small Newtonian plateau at low shear rates and a strong shear-thinning at higher values of $ \dot \gamma $equation image. The rate of decrease of η $ \dot \gamma $equation image in this region shows an “hesitation” similar to one previously observed in LC solutions of poly-γ-benzyl-L-glutamate PBLG. All these observations can be rationalized within the frame-work of Martins' theory. The expressions for N1($ \dot \gamma $equation image) and η $ \dot \gamma $equation image derived from this theory fit very well (quantitatively) to the experimental data and some fundamental viscoelastic parameters of the system under study are thereby obtained for the first time.
- Motility and cell shape roles in the rheology of growing bacteria culturesPublication . Portela, Raquel; Almeida, Pedro L.; Sobral, Rita; R. Leal, CatarinaCell shape, size and self-motility appear as determinant intrinsic cell factors in the rheological behavior of living bacterial cultures during the growth process. In this work three different species were considered due to their differences on these intrinsic characteristics: two different strains of Staphylococcus aureus – strain COL and its isogenic cell wall autolysis mutant, RUSAL9 – both non-motile and Escherichia coli and Bacillus subtilis – both presenting intrinsic motility. In situ real-time rheology, was used to characterize the activity of growing bacteria, under steady-shear conditions, in particular the viscosity growth curve was measured, for a constant shear flow rate, presenting for all studied cultures, different and rich flow curves. These complex rheological behaviors are a consequence of two coupled effects: the cell density continuous increase and its changing interacting properties, where cell size and shape and intrinsic motility are major players.