From Commensal to Pathogen: Translocation of Enterococcus faecalis from the Midgut to the Hemocoel of
Manduca sexta
Authors: Mason Katie L. , Stepien Taylor A. , Blum Jessamina E. , Holt Jonathan F. , Labbe Normand H. , Rush Jason S. , Raffa Kenneth F. , Handelsman Jo 
Published: May 2011

A dynamic homeostasis is maintained between the host and native bacteria of the gastrointestinal tract in animals, but migration of bacteria from the gut to other organs can lead to disease or death.
Enterococcus faecalis
is a commensal of the gastrointestinal tract; however,
Enterococcus
spp. are increasingly frequent causes of nosocomial infections with a high mortality rate. We investigated the commensal-to-pathogen switch undergone by
E. faecalis
OG1RF in the lepidopteran model host
Manduca sexta
associated with its location in the host.
E. faecalis
persists in the harsh midgut environment of
M. sexta
larvae without causing apparent illness, but injection of
E. faecalis
directly into the larval hemocoel is followed by rapid death. Additionally, oral ingestion of
E. faecalis
in the presence of
Bacillus thuringiensis
insecticidal toxin, a pore-forming toxin that targets the midgut epithelium, induces an elevated mortality rate. We show that the loss of gut integrity due to
B. thuringiensis
toxin correlates with the translocation of
E. faecalis
from the gastrointestinal tract into the hemolymph. Upon gaining access to the hemolymph,
E. faecalis
induces an innate immune response, illustrated by hemocyte aggregation, in larvae prior to death. The degree of hemocyte aggregation is dependent upon the route of
E. faecalis
entry. Our data demonstrate the efficacy of the
M. sexta
larval model system in investigating
E. faecalis
-induced sepsis and clarifies controversies in the field regarding the events leading to larval death following
B. thuringiensis
toxin exposure.

IMPORTANCE
This study advances our knowledge of
Enterococcus faecalis
-induced sepsis following translocation from the gut and provides a model for mammalian diseases in which the spatial distribution of bacteria determines disease outcomes. We demonstrate that
E. faecalis
is a commensal in the gut of
Manduca sexta
and a pathogen in the hemocoel, resulting in a robust immune response and rapid death, a process we refer to as the “commensal-to-pathogen” switch. While controversy remains regarding
Bacillus thuringiensis
toxin-induced killing, our laboratory previously found that under some conditions, the midgut microbiota is essential for
B. thuringiensis
toxin killing of
Lymantria dispar
(N. A. Broderick, K. F. Raffa, and J. Handelsman, Proc. Natl. Acad. Sci. U. S. A. 103:15196–15199, 2006; B. Raymond, et al., Environ. Microbiol. 11:2556–2563, 2009; P. R. Johnston, and N. Crickmore, Appl. Environ. Microbiol. 75:5094–5099, 2009). We and others have demonstrated that the role of the midgut microbiota in
B. thuringiensis
toxin killing is dependent upon the lepidopteran species and formulation of
B. thuringiensis
toxin (N. A. Broderick, K. F. Raffa, and J. Handelsman, Proc. Natl. Acad. Sci. U. S. A. 103:15196–15199, 2006; N. A. Broderick, et al., BMC Biol. 7:11, 2009). This work reconciles much of the apparently contradictory previous data and reveals that the
M. sexta-E. faecalis
system provides a model for mammalian sepsis.