Transcriptional response of Enterococcus faecalis to sunlight

Microarrays were used to investigate the transcriptional response of Enterococcus faecalis to photostress. E. faecalis are Gram-positive bacteria used as indicators of water quality and have been shown to vary diurnally in response to sunlight. E. faecalis in filtered seawater microcosms were exposed to artificial sunlight for 12h and then placed in the dark for 12h. Transcript abundance was measured at 0, 2, 6, 12, and 24h in the sunlit microcosm and a dark control using microarrays. Culturable E. faecalis concentrations decreased 6-7 orders of magnitude within the first 6h of light exposure. After 12h in the dark, no evidence of dark-repair was observed. Expression data collected after 12h of sunlight exposure revealed a difference in transcript abundance in the light relative to dark microcosms for 35 unique ORFs, 33 ORFs showed increased transcript abundance and 2 ORFs showed reduced transcript abundance. A majority (51%) of the ORFs with increased transcript abundance in the sunlit relative to dark microcosms encoded hypothetical proteins; others were associated with protein synthesis, oxidative stress and DNA repair. Results suggest that E. faecalis exposed to sunlight actively transcribe RNA in response to photostress.