Microrheology of Bacterial Biofilms In Vitro: Staphylococcus aureus and Pseudomonas aeruginosa

The rheology of bacterial biofilms at the micron scale is an important step to understanding the communal lifecycles of bacteria that adhere to solid surfaces, as it measures how they mutually adhere and desorb. Improvements in particle-tracking software and imaging hardware have allowed us to successfully employ particle-tracking microrheology to measuring single-species bacterial biofilms, based on Staphlococcus aureus and Pseudomonas aeruginosa. By tracking displacements of the cells at a range of timescales, we separate active and thermal contributions to the cell motion. The S. aureus biofilms in particular show power-law rheology, in common with other dense colloidal suspensions. By calculating the mean compliance of S. aureus biofilms, we observe them becoming less compliant during growth, and more compliant during starvation. The biofilms are rheologically inhomogeneous on the micron scale, as a result of the strength of initial adhesion to the flow cell surface, the arrangement of individual bacteria, and larger-scale structures such as flocs of P. aeruginosa. Our S. aureus biofilms became homogeneous as a function of height as they matured: the rheological environment experienced by a bacterium became independent of how far it lived from the flow cell surface. Particle-tracking microrheology provides a quantitative measure of the ��strength�� of a biofilm. It may therefore prove useful in identifying drug targets and characterizing the effect of specific molecular changes on the micron-scale rheology of biofilms.微米尺度下细菌生物膜的流变学研究，是理解附着于固体表面的细菌群落生命周期的重要环节，因其可量化表征细菌间相互附着与解吸附的行为。粒子追踪软件与成像硬件技术的进步，使得我们得以成功将粒子追踪微流变学（Particle-tracking microrheology）应用于以金黄色葡萄球菌（Staphylococcus aureus）和铜绿假单胞菌（Pseudomonas aeruginosa）为研究对象的单菌种细菌生物膜研究。通过追踪不同时间尺度下细菌细胞的位移，我们将细胞运动拆分为主动运动与热运动两种贡献组分。其中，金黄色葡萄球菌生物膜与其他致密胶体悬浮液一样，表现出幂律流变学（power-law rheology）行为。通过计算金黄色葡萄球菌生物膜的平均柔顺性，我们观察到其在生长过程中柔顺性逐渐降低，而在饥饿状态下柔顺性则会升高。受初始附着于流动池表面的强度、单个细菌的排布方式，以及铜绿假单胞菌絮凝体等更大尺度结构的影响，该类生物膜在微米尺度上存在流变学不均匀性。我们的研究发现，金黄色葡萄球菌生物膜随着成熟度提升，其流变学特性沿高度方向趋于均一：细菌所处的流变学环境不再与其距离流动池表面的高度相关。粒子追踪微流变学可定量表征生物膜的“强度”，因此有望在筛选药物靶点、表征特异性分子变化对生物膜微米尺度流变学的影响等方面发挥应用价值。