title: Living bacteria rheology: population growth, aggregation patterns, and collective behavior under different shear flows.
abstract: The 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.
authors.affiliation: ISEL, Rua Conselheiro Emídio Navarro 1, P-1959-007 Lisboa, Portugal and CEDOC, Faculdade de Ciências Médicas, Universidade Nova de Lisboa, 1169-056, Lisboa, Portugal.
authors.forename: P
authors.initials: P
authors.lastname: Patrício
authors.affiliation: ISEL, Rua Conselheiro Emídio Navarro 1, P-1959-007 Lisboa, Portugal and CENIMAT/I3N, Faculdade Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
authors.forename: P L
authors.initials: PL
authors.lastname: Almeida
authors.affiliation: Centro de Recursos Microbiológicos, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
authors.forename: R
authors.initials: R
authors.lastname: Portela
authors.affiliation: Centro de Recursos Microbiológicos, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
authors.forename: R G
authors.initials: RG
authors.lastname: Sobral
authors.affiliation: Laboratório de Genética Molecular, ITQB, Universidade Nova de Lisboa, 2780 Oeiras, Portugal.
authors.forename: I R
authors.initials: IR
authors.lastname: Grilo
authors.affiliation: CENIMAT/I3N, Faculdade Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal and Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
authors.forename: T
authors.initials: T
authors.lastname: Cidade
authors.affiliation: ISEL, Rua Conselheiro Emídio Navarro 1, P-1959-007 Lisboa, Portugal and CENIMAT/I3N, Faculdade Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
authors.forename: C R
authors.initials: CR
authors.lastname: Leal
mesh_terms: D055119:Elastic Modulus; D008954:Models, Biological; D009154:Mutation; D061848:Optical Imaging / Q000379:methods; D010507:Periodicity; D012212:Rheology; D013045:Species Specificity; D013211:Staphylococcus aureus / Q000235:genetics / Q000254:growth & development / Q000502:physiology*; D013314:Stress, Mechanical; D014783:Viscosity