Slower Growth and Increased Biofilm Formation of Escherichia coli K-12 Stringent Response Mutant, ΔrelA/spoT, under Isoleucine Starvation

Biofilm formation contributes to bacterial virulence and its formation is known to be affected by environmental condition and nutrient availability. Cells enter
stringent response under nutrient-limiting conditions where guanosine 5’-triphosphate-3’-diphosphate and guanosine 5’-triphosphate-3’-diphosphate, collectively referred to as (p)ppGpp, are continually synthesized by the enzymes RelA and SpoT, and hydrolysis of (p)ppGpp is downregulated. (p)ppGpp regulates growth and other cellular processes including fimbriae expression. To examine the effect of stringent response on biofilm formation of Escherichia coli K-12, we constructed a growth curve and measured biofilm formation for wild type (WT) K-12 and ΔrelA/spoT mutant JKLL12W-2 under nutrient-rich and isoleucine-limiting conditions. While the growth of the two strains were similar in nutrient-rich lysogeny broth, ΔrelA/spoT mutant displayed slower growth under isoleucinelimiting conditions. The ΔrelA/spoT mutant had significantly greater biofilm formation in nutrient-rich media and formed higher level of biofilm under lower level of isoleucine. Since the ΔrelA/spoT mutant showed higher biofilm formation under lower isoleucine levels, stringent response in WT E. coli K-12 downregulates biofilm formation in a nutrient availability dependent manner. Our study of ppGpp0 E. coli mutants behave similarly to Pseudomonas putida KT2440, another member of the gammaproteobacteria family, in biofilm formation and may suggest that the stringent response of these two bacteria follow similar mechanisms.