Fluid-driven Interfacial instabilities and turbulence in bacterial biofilms

Biofilms are thin layers of bacteria embedded within a slime matrix that live on surfaces. They are ubiquitous in nature and responsible for many medical and dental infections, industrial fouling and are also evident in ancient fossils. A biofilm structure is shaped by growth, detachment and response to mechanical forces acting on them. The main contribution to biofilm versatility in response to physical forces is the matrix that provides a platform for the bacteria to grow. The interaction between biofilm structure and hydrodynamics remains a fundamental question concerning biofilm dynamics. Here we document the appearance of ripples and wrinkles in biofilms grown from three species of bacteria when subjected to rapid high-velocity fluid flows. Theoretical treatment of the process as a Kelvin-Helmholtz instability indicates that the rippling process was primarily due to physics rather than chemistry or biology. The analysis also predicted a strong dependence of the instability formation on biofilm viscosity explaining the different surface corrugations observed. Turbulence through Kelvin-Helmholtz instabilities occurring at the interface demonstrated that the biofilm flows like a viscous liquid under high flow velocities applied within milliseconds. Biofilm fluid-like behavior may have important implications for our understanding of how fluid flow influences biofilm biology since turbulence will likely disrupt metabolite and signal gradients as well as community stratification.

生物膜（Biofilms）是一类嵌入黏液基质、附着于各类表面的薄层细菌聚集体。它们在自然界中广泛分布，可引发多种医疗、牙科感染及工业污损，同时也可在古生物化石中被观测到。生物膜的结构由其生长、脱落以及对所受机械力的响应共同塑造。生物膜在应对物理作用力时展现出的多样适应性，主要依托于为细菌生长提供支撑的基质。生物膜结构与流体动力学之间的相互作用，仍是生物膜动力学研究领域的核心基础问题之一。本研究记录了三种细菌形成的生物膜在经受高速急流冲刷时所出现的波纹与褶皱结构。将该过程以开尔文-亥姆霍兹不稳定性（Kelvin-Helmholtz instability）进行理论建模分析后表明，波纹形成过程主要源于物理机制，而非化学或生物学因素。该分析还预测，不稳定性的形成与生物膜黏度存在极强的相关性，这一结论可解释实验中观测到的不同表面褶皱形态。通过界面处的开尔文-亥姆霍兹不稳定性引发的湍流现象，研究证实生物膜在毫秒级施加的高速流体作用下，表现出黏性液体的流动特性。生物膜的类流体行为，对于我们理解流体流动如何影响生物膜生物学特性具有重要意义——湍流大概率会干扰代谢物与信号分子的浓度梯度，同时破坏群落的分层结构。