Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2009
Publication Date: 1/8/2010
Citation: Kyle, J.L., Parker, C., Goudeau, D.M., Brandl, M. 2010. Transcriptome Analysis of Escherichia coli O157:H7 Exposed to Lysates of Lettuce Leaves. Applied and Environmental Microbiology. 76(5):1375-1387
Interpretive Summary: Outbreaks of Escherichia coli O157:H7 illness have been linked increasingly to leafy greens, particularly to processed lettuce. We present here evidence that numerous genes are specifically involved in the adaptation and growth of the pathogen in Romaine lettuce leaf lysates used as a model system to investigate the behavior of EcO157 in lesions, such as those caused by produce processing. We demonstrate that several regulons involved in oxidative stress, osmotic stress, and resistance to antimicrobial and toxic compounds were upregulated in the lysates. These genes were also increased in transcription on shredded lettuce as confirmed by QRT-PCR. Exposure to the lettuce lysates enhanced the resistance of EcO157 to hydrogen peroxide and chlorine. This increased resistance to chlorine combined with the growth of the pathogen at injured sites on the leaves may help explain the association of processed leafy greens with outbreaks of EcO157 in the past years.
Technical Abstract: The impact of Escherichia coli O157:H7 (EcO157) on human health in the U.S. has grown dramatically in past decades. Between 1995 and 2008, 27 outbreaks of EcO157-associated disease were linked to leafy produce. Harvesting and processing of leafy greens inherently cause plant tissue damage, creating additional niches for human pathogens on leaves. We previously demonstrated that EcO157 multiplies more rapidly on shredded than on intact leaves. In this study, we used whole genome transcriptional profiling to characterize gene expression patterns in EcO157 after a 15- and 30-min exposure to lettuce lysates obtained by homogenizing Romaine lettuce leaves. After a period of adaptation, EcO157 completed ten cell generations in the lysates under warm temperatures. Microarray analysis revealed that 21.6% and 19.8% of the genome was upregulated over 2-fold as early as 15 and 30 min postinoculation, respectively. Befitting the hypothesis that EcO157 cells need to adapt to stressful physicochemical conditions in damaged lettuce tissue, we observed the upregulation of multiple genes associated with osmotic stress and oxidative stress, of genes belonging to the Mar antibiotic resistance regulon, and of others involved in detoxification of noxious compounds. Upregulation of oxidative stress and antimicrobial resistance genes in EcO157 in lettuce lysates was confirmed on shredded lettuce by QRT-PCR. We further demonstrate that this adaptation to the presence of reactive oxygen species imparts the pathogen with increased resistance to two sanitizers, namely, hydrogen peroxide and calcium hypochlorite. This increased resistance to chlorine combined with the growth of the pathogen at injured sites on the leaves may help explain the association of processed leafy greens with outbreaks of EcO157 in the past years.