Abstract A082: Identification of commensal bacterial strains that provide resistance to L. monocytogenes infection
Authors: Becattini Simone , Kim Sohn G. , Carter Rebecca A. , Ling Lilan , Leiner Ingrid M. , Pamer Eric G. 
Published: November 2016

Listeria monocytogenes is an important cause of foodborne infections, particularly in patients with hematologic malignancies, causing septicemia and meningoencephalitis. Although most strains of L. monocytogenes are antibiotic sensitive, antibiotic treatment is frustratingly ineffective at curing highly immunocompromised patients, resulting in a mortality rate of approximately 30%. The gut microbiota, which consists of commensal bacterial populations that inhabit the intestine, normally confers protection against orally acquired pathogens by exerting colonization resistance. Changes in microbiota composition induced by disease, pharmacological or antibiotic therapies can lead to dysbiosis, thereby impairing commensal-mediated colonization resistance and predisposing to infection. Our preliminary data show that bacterial species represented in a healthy microbiota can efficiently eliminate Listeria monocytogenes from the gut lumen, thus preventing the pathogen from translocating across the intestinal epithelium and spreading systemically. This mechanism of protection is particularly important in the context of congenic or acquired (chemotherapy-driven) immune deficiencies. Indeed, mice lacking T and B lymphocyets as well as innate lymphoid cells (Rag2/Il2rg Double Knockout mice, or Raggc), are highly susceptible to oral doses of Listeria that are non-lethal in WT mice. Although generally implicated in susceptibility to Listeriosis, the adaptive immune system is not involved in the in vivo clearance of the pathogen from the intestine, and the increased susceptibility to infection of these mice mainly results from the absence of innate immune cells producing interferon gamma at early stages, which leads to bacterial dissemination. Interestingly, Raggc mice are capable of surviving Listeria infection if challenged with low doses, which likely to mimicks the bacterial dosage acquired by immunocompromised patients. However, Raggc mice pre-treated with antibiotics experience uncontrolled intestinal growth of Listeria and subsequent systemic spread thus succumbing to low inoculum infection. This phenotype could be reproduced in mice treated with a combination of doxorubicin and cyclophosphamide, a cocktail often used in the treatment of cancer; although chemotherapy seemed to have an effect on the ability of the microbiota to restrict instestinal growth of Listeria, chemotherapy-treated mice became highly susceptible to low doses of Listeria only when pre-treated with antibiotics. Because antibiotics are often required for patients receiving cancer treatment, our these findings have potential clinical relevance. We are identifying specific commensal bacterial taxa and molecular mechanisms that mediate protection against L. monocytogenes. Our data indicate that Lactobacillus in the small intestine and Clostridium in the large intestine restrict Listeria growth in the gut lumen, thus lowering the bacterial burden and systemic spread. Germ-free (GF) mice reconstituted with a consortium of putative protective bacteria have greater resistance to infection than untreated GF mice or GF mice reconstituted with microbiota of antibiotic-treated mice, suggesting that gut commensals are independent and crucial contributors to host defense against Listeria. Our ultimate goal is to identify bacterial species that can be developed as probiotics for cancer patients to prevent intestinal L. monocytogenes carriage and infection. Combined shotgun sequencing and ex vivo assays on stool samples from cancer patients will be used to correlate loss of specific commensal bacteria with a predisposition to L. monocytogenes infection. Ultimately, this work will 1.) provide a novel approach to identifying subjects at high risk for L. monocytogenes infection and 2.) identify and characterize probiotic bacterial species to prevent and/or treat L. monocytogenes infections in cancer patients.

Citation Format: Simone Becattini, Sohn G. Kim, Rebecca A. Carter, Lilan Ling, Ingrid M. Leiner, Eric G. Pamer. Identification of commensal bacterial strains that provide resistance to L. monocytogenes infection [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A082.