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Social Network in Bacteria

Bacterial biofilms form complex networks dictated by signaling patterns known as quorum sensing. Bacteria use quorum sensing to alter genetic expression in local populations in response to external factors. The first step in biofilm formation is the attachment to a surface suitable for biofilm growth. As bacteria begin to attach to the surface, they excrete molecules which signal for the attachment of more bacteria. Next bacteria excrete various polymeric substances forming an extracellular matrix containing large aggregates of bacteria. Depending on the surface conditions, bacteria are capable of forming complex biofilm structures and patterns. It is important to understand how and under what conditions biofilms form as they pose many problems. For instance, biofilm formation on surgical implants can have fatal consequences on patients. Dacheng Ren’s study on biofilm formation on microtopographic patterns explores how different surface features affect communication between different populations of bacteria.

In this study, E. coli was grown on PDMS surfaces with varying topologies. The PDMS sufaces contained rectangular structures with variable features. The height and length of each box differed on the various surfaces, as did the distance between the boxes. The distances between boxes were 5, 10, 15, and 20 um while E. coli cells in the experiment were between 1 and 5 um. Bacteria were genetically engineered to glow red under fluorescent microscopy. After depositing bacteria on PDMS surfaces, it was shown that bacterial clusters preferred to grow in the valleys between the box-like features.  The bacteria were shown to form complex networks within clusters. Within clusters, bacteria are closely linked to one another as they exist in an aggregate enclosed by the excreted extracellular matrix. Signaling between these bacteria, determines their growth patterns. Interestingly, bacteria only grew on the protruding surfaces that had dimensions significantly larger than that of the individual bacteria. This suggests that the bacterial growth on surfaces is more efficient when cell-to-cell interactions are present. Thus the connectivity of the bacteria was crucial to successful biofilm formation.

While communication between neighboring bacteria is important to form individual aggregates, larger biofilms require more extensive connectivity. Amongst the aggregates formed in the experiment, there were some observed connections between populations. Various bridges were observed connecting local populations to each other. This mimics local bridges linking highly connected groups in nodal networks. These local bacterial bridges help connect different bacterial populations allowing communication through quorum sensing to occur. This way, the populations can behave similarly given different external inputs. Bacteria use this sort of network connectivity to coordinate activities such as the release of toxins.

Understanding the surface features which affect bacterial growth and connectivity, may help improve the design of surgical implants. Using implants with microscale features that are unfavorable to coordinated bacterial growth may eliminate biofilm related complications after surgery.

source: http://pubs.acs.org/doi/abs/10.1021/la1046194

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