Salmonella enterica serovar-host specificity does not correlate with the magnitude of intestinal invasion in sheep

Abstract

The colonization of intestinal and systemic tissues by Salmonella enterica serovars with different host specificities was determined 7 days after inoculation of 1 to 2-month-old lambs. Following oral inoculation, S. enterica serovars Abortusovis, Dublin, and Gallinarum were recovered in comparable numbers from the intestinal mucosa, but serovar Gallinarum was recovered in lower numbers than the other serovars from systemic sites. The pattern of bacterial recovery from systemic sites following intravenous inoculation was similar. The magnitude of intestinal invasion was evaluated in ovine ligated ileal loops in vivo. Serovars Dublin and Gallinarum and the broad-host-range Salmonella serovar Typhimurium were recovered in comparable numbers from ileal mucosa 3 h after loop inoculation, whereas the recovery of serovar Abortusovis was approximately 10-fold lower. Microscopic analysis of intestinal mucosae infected with serovars Typhimurium and Dublin showed dramatic morphological changes and infiltration of inflammatory cells, whereas mucosae infected with serovars Abortusovis and Gallinarum were indistinguishable from uninfected mucosae. Together these data suggest that Salmonella serovar specificity in sheep correlates with bacterial persistence at systemic sites. Intestinal invasion and avoidance of the host's intestinal inflammatory response may contribute to but do not determine the specificity of serovar Abortosovis for sheep. Intestinal invasion by serovar Abortusovis was significantly reduced after mutation of invH but was not reduced following curing of the virulence plasmid, suggesting that the Salmonella pathogenicity island 1 influences but the virulence plasmid genes do not influence the ability of serovar Abortusovis to invade the intestinal mucosa in sheep.

FIG. 1

Rectal temperatures of lambs following oral inoculation with 5 × 10⁸ CFU of Salmonella serovars Abortusovis (closed circles), Dublin (closed squares), or Gallinarum (open triangles). Each datum point is the mean of temperatures from five animals plus or minus the standard error of the mean (SEM).

FIG. 2

Recovery of Salmonella serovars from intestinal walls, MLNs, and systemic sites of lambs infected orally with 5 × 10⁸ CFU of Salmonella serovars Dublin (black bars), Abortusovis (grey bars), and Gallinarum (white bars). Each bar represents the mean of triplicate samples from five animals ± SEM. Recovery of serovar Gallinarum from spleen was significantly lower than that of Salmonella serovars Dublin (P = 0.02) and Abortusovis (P = 0.03).

FIG. 3

Rectal temperatures of lambs inoculated i.v. with 5 × 10⁶ CFU of Salmonella serovars Abortusovis (closed circles), Dublin (closed squares) or Gallinarum (open triangles). Each datum point represents the mean of five animals ± SEM.

FIG. 4

Recovery of Salmonella serovars from MLNs and systemic sites of lambs infected i.v. with 5 × 10⁶ CFU of Salmonella serovars Dublin (closed bars), Abortusovis (stippled bars), and Gallinarum (open bars). BLN, bronchial lymph node; HLN, hepatic lymph node. Each bar represents the mean of triplicate samples from five animals ± SEM.

FIG. 5

Relative invasiveness of Salmonella serovars Typhimurium (strain 4/74), Dublin (strain SD2229), Abortusovis (strain SS44), and Gallinarum (strain G9) in ovine ileal loops; each bar represents the mean from 21, 12, 21, and 12 loops tested ± SEM, respectively. Three samples were analyzed from each loop.

FIG. 6

Relative levels of invasiveness of Salmonella serovars Typhimurium (strains 4/74 and 4/74 InvH) and Abortusovis (strains SS44, SS44 InvH, and plasmid-cured SU40) in ovine ileal loops. The bars represent the mean bacterial recoveries from nine loops tested ± SEM. Three samples were analyzed from each loop. SS44 versus SS44 InvH, P = 0.002; SS44 versus SU40, P = 0.406; 4/74 versus 4/74 InvH, P < 0.001.

FIG. 7

Cross sections of intestinal mucosa from ovine ileal loops stained with hematoxylin and eosin and incubated for 3 h with Salmonella serovars. (A) Untreated control; intestinal mucosa infected with (B) serovars Abortusovis (strain SS44), (C) Dublin (strain SD2229), (D) Gallinarum (strain G9), (E) Typhimurium strain 4/74, and (F) Typhimurium strain 4/74 InvH. A minimum of 10 sections were examined for each loop infected (n = 6) with each Salmonella strain. Representative sections are shown. Magnification, ×400.