The Adaptive Morphology of Bacillus subtilis Biofilms: A Defense Mechanism against Bacterial Starvation

Abstract

Biofilms are commonly defined as accumulations of microbes, embedded in a self-secreted, polysaccharide-rich extra-cellular matrix. This study aimed to characterize specific morphological changes that occur in Bacillus subtilis biofilms under nutrient-limiting growth conditions. Under varying levels of nutrient depletion, colony-type biofilms were found to exhibit different rates of spatial expansion and green fluorescent protein production. Specifically, colony-type biofilms grown on media with decreased lysogeny broth content exhibited increased spatial expansion and more stable GFP production over the entire growth period. By modeling the surface morphology of colony-type biofilms using confocal and multiphoton microscopy, we analyzed the appearance of distinctive folds or "wrinkles" that form as a result of lysogeny broth content reduction in the solid agar growth media. When subjected to varying nutritional conditions, the channel-like folds were shown to alter their morphology; growth on nutrient-depleted media was found to trigger the formation of large and straight wrinkles connecting the colony core to its periphery. To test a possible functional role of the formed channels, a fluorescent analogue of glucose was used to demonstrate preferential native uptake of the molecules into the channels' interiors which supports their possible role in the transport of molecules throughout biofilm structures.

Topography and Expansion Patterns at the Biofilm-Agar Interface in Bacillus subtilis Biofilms

“…For example, Wilking et al [ 29 ] suggested that the abovementioned network of channels facilitates liquid transport throughout the macrocolonies with evaporative flux acting as the driving force. A complementary force that allows the macrocolonies to uptake nutrients from the growth substrate was also suggested by Gingichashvili et al [ 12 ], while a similar channel-driven nutrient uptake and distribution effect was previously reported in E. coli biofilms [ 30 ]. Analysis of macrocolonies that were co-cultured and grown in a mixture with fluorescent microspheres reveals two additional forces present in B. subtilis macrocolonies.…”

“…At the same time, the intermediate region where filaments are tightly packed provides the strongest attachment while the leading zone is again more loosely attached at the leading edge of the biofilm. Under limited nutrient supply, the topographical changes that occur at the contact surface may drive the accelerated expansion of the macrocolonies—their wider and less tightly packed filaments might help explain the enhanced spreading ability of such colony type biofilms [ 12 ] as they are likely to result in looser adhesion at the contact surface, which is further supported by profilometry data results.…”

“…Similarly, changes in colony growth dynamics such as biofilm expansion rate and peak expansion velocity were also observed [ 11 ]. Specific morphological changes such as the appearance of wrinkles that connect colony core to periphery under limited nutrient supply were described by Gingichashvili et al [ 12 ]. Surface topography in B. subtilis colony type biofilms was also shown to be dependent on environmental conditions and determinant of several key properties such as biofilm wettability [ 13 ] as well as erosion sensitivity and antibiotic efficiency [ 14 ].…”

Collective colony growth is optimized by branching pattern formation in Pseudomonas aeruginosa

“…Our findings suggest that evolution may select for the optimal colony morphology, which is a macroscopic, population-level property resulting from cellular-level interactions. The formation of multicellular and macroscopic structures by microbial communities that benefit the population as a whole is widely observed; other examples include the formation of patches (Ratzke & Gore, 2016), filaments (Pfeffer et al, 2012), spore-filled fruiting bodies (Munoz-Dorado et al, 2016), and biofilms with intricate structures (Epstein et al, 2011;Wilking et al, 2013;Kempes et al, 2014;Gingichashvili et al, 2019). These social traits of microbes reflect intercellular cooperation and coordination, which are the hallmarks of multicellularity (Shapiro, 1988;Lyons & Kolter, 2015).…”

Water‐soluble polymers in agriculture: xanthan gum as eco‐friendly alternative to synthetics

“…Furthermore, adhesion of particles to each other or to a target site is a central aim in different applications. ii. Protection from environment: Biofilms help microorganisms to protect themselves in extreme environments (Yin et al ., 2019), against ultraviolet radiation (UV) (de Carvalho, 2017), extreme temperatures (Smith et al ., 2016; Kent et al ., 2018), pH variations (Narayanan et al ., 2016), salinity or drought stress (Alaa, 2018; Wang et al ., 2019) and limitation of nutrients (Gingichashvili et al ., 2020). In general, a biofilm can be described as a protective matrix in which both the polysaccharide‐producing microorganisms and co‐living cells are encapsulated.…”