Araceli Santiago, PhD

Araceli Santiago, M.D.Diarrheal diseases account for 1 in 9 child deaths worldwide, making diarrhea the second leading cause of death among children under the age of 5. While there are many etiological agents responsible for diarrhea, pathogenic E. coli and Shigella are major contributors.

My research focuses on understanding the mechanism of regulation of AraC/XylS family members in pathogenic E. coli strains. AraC/Xyls is a large family of transcriptional regulators in Gram-negative bacteria controlling metabolic, cellular stress, and virulence. With the development of bioinformatics programs, new uncharacterized AraC/XylS members have been identified. Recently, nearly 2000 known and putative AraC/XylS members in 149 bacterial genomes have been identified.

Recently, we uncovered and characterized a new family of negative AraC/Xyls regulators (ANR) (AraC Negative Regulators) in Gram-negative bacteria. ANR family comprise at least 50 hypothetical proteins, present in 7 major species: Escherichia coli., Citrobacter spp., Haemophilus spp., Pasteurella spp., Mannheimia spp., Pantoea spp., and Aggregatibacter spp.

In pathogenic E. coli strains, ANR were subdivided into two main families (SFs), SF1 and SF2. SF1 comprised the ARN members found in enterohemorrhagic E. coli (EHEC), enteropathogenic E. coli (EPEC), Shiga-toxin producing E. coli (STEC), enterotoxigenic E. coli (ETEC), extra-intestinal pathogenic E. coli (ExPEC), enteroinvasive E. coli (EIEC), while that SF2 family comprised homologs mainly found in enterotoxigenic E. coli (ETEC), enteroaggregative E. coli (EAEC) and Shiga-toxin producing EAEC.

Members of the ANR family are small molecules (4.36-9.54 kDa). Alignment of ANR members revealed conserved alpha helical motifs with several highly conserved residues. Some of the ANR members characterized in our group include orf60 (termed Aar) in EAEC, orf02851 (termed Rnr) in Citrobacter rodentium, orf0450 and orf01070 (termed Cnr) in ETEC. Aar, Cnr and Rnr downregulate the expression of positive regulator partners AggR, CfaD/Rns and RegA, respectively, all members of the AraC/XylS family of regulators. Aar, in EAEC prototype strain 042 down regulates AggR and at least 44 genes identified as part of the AggR regulon, including AAF fimbria, dispersin and its secretion system aat, and type VI secretion.

My long-term goal is to identify the molecular regulatory mechanism of this new family of negative regulators and explore their potential in the treatment of diarrheal diseases. During last 4 years, I have been involved in the training of undergraduate students from SRIP Summer program at UVA.

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ANR (AraC Negative Regulators) is a new family of regulators in Gram-negative bacteria. Recently, we have characterized three ANR members: Aar in EAEC, Rnr in Citrobacter rodentium, and Cnr in ETEC. Aar, Cnr and Rnr downregulate the expression of positive regulator partners AggR, CfaD/Rns and RegA, respectively, all members of the AraC/XylS family of regulators and their corresponding regulons. (Click to enlarge)

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Analysis of fimbrial expression in 042 derivatives by TEM and qRT-PCR. Fimbrial structures were analyzed in 042 derivatives by TEM. 042aar was found to be hyper-fimbriated (Panel B and E), and distinguishable from the parent strain (Panel A and D), or the 042aar mutant complemented in trans (Panel C and F). Accordingly, high levels of fimbrial encoding genes were observed in the 042aar mutant. Complementation in trans of 042aar mutant abolished the expression of aaf genes (Panel G). (Click to enlarge)

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Blast analysis of Aar identified nearly 50 hypothetical proteins, present in 7 major genera/species: Escherichia coli, Citrobacter spp., Haemophilus spp., Pasteurella spp., Mannheimia spp., Pantoea spp. and Aggregatibacter spp (Panel A). In pathogenic E. coli strains, ANR were subdivided into two main families (SFs), SF1 and SF2 (Panel B). (Click to enlarge)

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ANR members in pathogenic bacteria were aligned by using PROMALS3D algorithm. The alignment revealed conserved alpha helical motifs with several highly conserved residues along the family (Panel A). Three alpha helices were determined along the Aar protein (also known as pAA060) by using Phyre2 algorithm (Panel B). (Click to enlarge)

Relevant publications.

1- Morin N, Santiago AE, Ernst RK, Guillot SJ, Nataro JP. Characterization of the AggR regulon in enteroaggregative Escherichia coli. Infect Immun. 2013 Jan;81(1):122-32. PMCID: PMC3536136

2- Santiago A, Ruiz-Perez F, Jo NY, Vijayakumar V, Gong MQ, Nataro JP. A large family of antivirulence regulators modulates the effects of transcriptional activators in Gram-negative pathogenic bacteria. PLOSPathogens. 2014 May; 10(5): e1004153. Doi:10.137/journal.ppat.1004153.

Contact Information:
Phone:  434-243-1902
Email:  aes8j@virginia.edu