Accumulation of dead cells from contact killing facilitates coexistence in bacterial biofilms

Abstract

Bacterial communities govern their composition using a wide variety of social interactions, some of which are antagonistic. Many antagonistic mechanisms, such as the Type VI Secretion System (T6SS), require killer cells to directly contact target cells. The T6SS is hypothesized to be a highly potent weapon, capable of facilitating the invasion and defense of bacterial populations. However, we find that the efficacy of the T6SS is severely limited by the material consequences of cell death. Through experiments with Vibrio cholerae strains that kill via the T6SS, we show that dead cell debris quickly accumulates at the interface that forms between competing strains, preventing contact and thus preventing killing. While previous experiments have shown that T6SS killing can reduce a population of target cells by as much as one-million-fold, we find that as a result of the formation of dead cell debris barriers, the impact of T6SS killing depends sensitively on the initial concentrations of killer and target cells. Therefore, while the T6SS provides defense against contacting competitors on a single cell level, it is incapable of facilitating invasion or the elimination of competitors on a community level.

Vertical growth dynamics of biofilms

“…Conversely, less is known about the vertical growth of biofilms, despite its importance for determining access to nutrients and oxygen (De Beer et al, 1994;Stewart et al, 2016). The initial steps of this process (Yan et al, 2016;Beroz et al, 2018), as well as some aspects of competitive growth (Oliveira et al, 2015;Rendueles and Ghigo, 2015;Coyte et al, 2017;Steinbach et al, 2020), have been studied. However, our knowledge of how vertical growth proceeds over short and long 1 of 21 (which was not certified by peer review) is the author/funder.…”

“…Conversely, less is known about the vertical growth of biofilms, despite its importance for determining access to nutrients and oxygen( De Beer et al, 1994 ; Stewart et al, 2016 ). The initial steps of this process ( Yan et al, 2016 ; Beroz et al, 2018 ), as well as some aspects of competitive growth( Oliveira et al, 2015 ; Rendueles and Ghigo, 2015 ; Coyte et al, 2017 ; Steinbach et al, 2020 ), have been studied. However, our knowledge of how vertical growth proceeds over short and long time scales is limited.…”

Trade-offs constrain adaptive pathways to T6 survival

“…Physical processes can also offer protection. Extracellular polysaccharide can protect cells from T6 attack, as does the accumulation of cellular material from lysed cells and physical separation, which are both consequences of T6 antagonism [20][21][22][23] . External signaling can play a role in this protection, with recent reports that the presence of glucose enhances survival of E. coli cells to T6 attack, mediated through cyclic AMP and its cognate target, the CRP regulator 24 .…”