Successful small intestine colonization of adult mice by Vibrio cholerae requires ketamine anesthesia and accessory toxins

Abstract

Vibrio cholerae colonizes the small intestine of adult C57BL/6 mice. In this study, the physical and genetic parameters that facilitate this colonization were investigated. Successful colonization was found to depend upon anesthesia with ketamine-xylazine and neutralization of stomach acid with sodium bicarbonate, but not streptomycin treatment. A variety of common mouse strains were colonized by O1, O139, and non-O1/non-O139 strains. All combinations of mutants in the genes for hemolysin, the multifunctional, autoprocessing RTX toxin (MARTX), and hemagglutinin/protease were assessed, and it was found that hemolysin and MARTX are each sufficient for colonization after a low dose infection. Overall, this study suggests that, after intragastric inoculation, V. cholerae encounters barriers to infection including an acidic environment and an immediate immune response that is circumvented by sodium bicarbonate and the anti-inflammatory effects of ketamine-xylazine. After initial adherence in the small intestine, the bacteria are subjected to additional clearance mechanisms that are evaded by the independent toxic action of hemolysin or MARTX. Once colonization is established, it is suggested that, in humans, these now persisting bacteria initiate synthesis of the major virulence factors to cause cholera disease. This adult mouse model of intestinal V. cholerae infection, now well-characterized and fully optimized, should serve as a valuable tool for studies of pathogenesis and testing vaccine efficacy.

Figure 1. Inoculation strategy affects successful colonization of C57BL/6 mice.

(A) Prior to inoculation, mice were treated as indicated on the X axis (S-1 mg/ml streptomycin in water; F-restriction of food overnight; A-anesthesia with ketamine-xylazine intraperitoneally at time of inoculation; B-intragastric delivery of sodium bicarbonate, and L-inoculation of bacteria in the logarithmic phase of growth. Control mice (+all) were infected using all five parameters. Mice were then inoculated with 1−5×10⁷ CFU V. cholerae El Tor O1 strain P27459 and colonization of the small intestine assessed after 20 hr. Data are pooled from 3 experiments so the input variation between experiments was normalized by dividing the recovered CFU by the input CFU and data are thereby plotted as a log colonization index with dashed line at 0 indicating that CFU recovered was identical to the input CFU (i). Markers above the dashed line are indicative of growth in the small intestine while markers below indicate clearance. Values below the solid line (d.l.) were below the detection limit of 500 CFU in the small intestine. The number of mice that cleared V. cholerae from the small intestine (n) over the total of mice in the group (n°) is also indicated. The experiment in panel B was performed as above except an additional parameter of anesthesia with inhaled isoflurane (+I) was added and data are pooled from two experiments. Significant and borderline P values are shown compared to control for survival (calculated by a χ2 test) and for colonization (calculated by a Mann-Whitney non-parametric t-test comparing medians).

Figure 2. Effect of mouse and V. cholerae strain variation on colonization.

(A) Mouse strains as indicated were inoculated with 1.1×10⁷ CFU El Tor O1 strain P27459. (B) C57BL/6 mice were inoculated with 0.5−1×10⁷ CFU of V. cholerae strain as indicated. (C) Streptomycin treated and non-treated C57BL/6 mice were inoculated with either 2−3×10⁷ or 1.5−3.9×10⁸ CFU of V. cholerae Classical O395. All mice were inoculated with the ABL strategy as described in the text and mice were assessed for colonization of the small intestine 20 hr after inoculation. Values are reported as a Col. Index as described in Fig. legend 1. The number of mice that cleared V. cholerae from the small intestine (n) over the total of mice in the group (n°) is also indicated. Only statistically significant p values calculated by a Mann-Whitney non-parametric t-test comparing medians are shown. P values in panels A and B are compared to El Tor O1 P27459 infection in C57BL/6 mice (Panel B) and in panel C to O395 control at 10⁷ CFU.

Figure 3. Colonization of the small intestine depends on accessory toxins.

Data represent merged results from 4 experiments, wherein C57BL/6 mice were inoculated by the ABL strategy with a low dose between 4×10⁵ to 4×10⁶ CFU and colonization of the small intestine was assessed after 48 hr. Values are reported as a Col. Index as described in Fig. legend 1. Despite variation in input, there was no correlation between increased inoculum and improved colonization in these data sets. The number of mice that cleared the infection over the total mice in the experimental group is shown in the graph as number cleared (n) over number inoculated n°. Number of mice varies since single mutant knockout infections were repeated more often with more mice per group to assure that observed differences in clearance frequency initially observed were reproducible and not representative of experiment to experiment variation or due to differences in the inoculum.