Abstract
Bacteria face diverse stresses in the environment and, sometimes, respond by forming multi-cellular structures, e.g., biofilms. Here, we report a novel macroscopic and multi-cellular structure formed by Salmonella Typhimurium, which resembles small strings. These string-like structures, ∼1 cm long, are induced under some stress conditions: iron deprivation by 2,2-Bipyridyl or low amounts of antibiotics or ethanol in minimal media. However, cells in strings revert back to planktonic growth upon return to nutrient rich media. Compared to planktonic cells, strings are more resistant to antibiotics and oxidative stress. Also, strains lacking csgD or rpoS, which are defective in the classical rdar biofilm formation, form strings. Furthermore, some biofilm inducing conditions do not result in strings and vice-versa, demonstrating that strings are not related to classical CsgD-dependent biofilms. Cells in a string are held together by cellulose and a strain lacking bcsA, which is defective in cellulose production, does not form strings. In addition, reductive stress conditions such as dithiothreitol (DTT) or mutations in the Disulfide bonding system (DSB) also give rise to strings. The amounts of c-di-GMP are increased upon string formation and studies with single and double deletion strains of the diguanylate cyclases, yedQ (STM1987) primarily and yfiN (STM2672) partly, revealed their importance for string formation. This is the first study showcasing the ability of Salmonella to produce high amounts of cellulose in liquid culture, instead of an interface, in a CsgD-independent manner. The relevance and possible applications of strings in the production of bacterial cellulose and bioremediation are discussed.
Highlights
Bacteria constantly assess their environment for nutrient availability, presence of chemical messengers, stress-inducing chemicals etc
While studying some stress responses, we observed macroscopic string-like structures. We report this novel stringlike multi-cellular structure dependent on cellulose in Salmonella Typhimurium that is induced under certain stress conditions, e.g., iron deprivation by 2,2 –Bipyridyl (Bipd), Dithiothreitol (DTT), sub-Minimum Inhibitory Concentration (MIC) amounts of antibiotics in minimal media
Extensive cell-cell contact due to the rotating motion of the vessel containing the culture was required for string formation and a larger proportion of cells entered the string with increasing RPM as shown by the decreasing normalized OD with increasing RPM (Supplementary Figures 2C,D)
Summary
Bacteria constantly assess their environment for nutrient availability, presence of chemical messengers, stress-inducing chemicals etc. These cues dictate bacterial responses, resulting in changes in morphology at a single cell level or forming multi-cellular structures. An array of bacterial multi-cellular structures has been described, including those made by filamentous bacteria e.g., Candidatus, Mycobacterial cording in detergent free media, aggregates of Macroscopic String Formation by Salmonella. Multi-cellular structures, the most well studied being biofilms, can be induced by a myriad of stresses including differences in temperature, osmolarity, nutrient availability etc. Biofilms are known to be more resistant to antibiotics (up to a 1000fold) and other stresses over planktonic cells by forming a barrier, reducing cellular activity, increasing expression of multiple drug resistance pumps, expression of glycans that trap antibiotics etc. Biofilms are known to be more resistant to antibiotics (up to a 1000fold) and other stresses over planktonic cells by forming a barrier, reducing cellular activity, increasing expression of multiple drug resistance pumps, expression of glycans that trap antibiotics etc. (Mah and O’Toole, 2001; Mah et al, 2003; Høiby et al, 2010; Kaplan, 2011)
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