Interaction of caffeine with the SOS response pathway in Escherichia coli

Abstract

Background: Previous studies have highlighted the antimicrobial activity of caffeine, both individually and in combination with other compounds. A proposed mechanism for caffeine's antimicrobial effects is inhibition of bacterial DNA repair pathways. The current study examines the influence of sub-lethal caffeine levels on the growth and morphology of SOS response pathway mutants of Escherichia coli.

Methods: Growth inhibition after treatment with caffeine and methyl methane sulfonate (MMS), a mutagenic agent, was determined for E. coli mutants lacking key genes in the SOS response pathway. The persistence of caffeine's effects was explored by examining growth and morphology of caffeine and MMS-treated bacterial isolates in the absence of selective pressure.

Results: Caffeine significantly reduced growth of E. coli recA- and uvrA-mutants treated with MMS. However, there was no significant difference in growth between umuC-isolates treated with MMS alone and MMS in combination with caffeine after 48 h of incubation. When recA-isolates from each treatment group were grown in untreated medium, bacterial isolates that had been exposed to MMS or MMS with caffeine showed increased growth relative to controls and caffeine-treated isolates. Morphologically, recA-isolates that had been treated with caffeine and both caffeine and MMS together had begun to display filamentous growth.

Conclusions: Caffeine treatment further reduced growth of recA- and uvrA-mutants treated with MMS, despite a non-functional SOS response pathway. However, addition of caffeine had very little effect on MMS inhibition of umuC-mutants. Thus, growth inhibition of E. coli with caffeine treatment may be driven by caffeine interaction with UmuC, but also appears to induce damage by additional mechanisms as evidenced by the additive effects of caffeine in recA- and uvrA-mutants.

Keywords: Caffeine; DNA repair; Escherichia coli; SOS response pathway.

Fig. 1

E. coli SOS response pathway. SOS response pathway in E. coli cells involving recombination repair (recA), trans-lesion synthesis (umuC), and nucleotide excision repair (uvrA).

Fig. 2

Caffeine effects on E. coli Cells. a Comparison of final cell density between untreated and caffeine treated (0.625 mg/mL) isolates of E. coli K12 wild-type and recA-, uvrA-, and umuC-mutants. b Represents the growth rate of recA-bacterial cells treated with caffeine alone, as compared to an untreated control. c Growth curve of uvrA-bacterial cells treated with 0.625 mg/mL caffeine alone, as compared to untreated control. d Growth curve of untreated umuC-mutants compared with cells treated with 0.625 mg/mL caffeine.

Fig. 3

Growth inhibition of E. coli recA-cells. Change in growth of E. coli SOS pathway mutants when treated with 0.625 mg/mL of caffeine and varying concentrations of MMS, or MMS alone. a Represents density of recA-cells under both treatments at 24 h and b 48 h. c Growth inhibition of uvrA-cells under both treatments at 24 h and d 48 h. e Growth inhibition of umuC-cells treated with MMS with or without caffeine at 24 h and f 48 h. Error bars represent SE.

Fig. 4

Persistence of caffeine effects on recA-cells. a Growth of E. coli JW2669-1 (recA-) when treated with 1.25 mg/mL MMS, 0.625 mg/mL caffeine, and MMS and caffeine co-treatment, in comparison with the growth of untreated cells. Morphology of E. coli recA-cells following inhibition assay. Treatment groups are no treatment (b), caffeine alone (c), MMS alone (d), and MMS and caffeine combined treatment (e). Arrows indicate cells displaying filamentous growth. Cells were observed at ×1,000 magnification. Pictures scales are 5 µm.