Experimental reproduction of bovine Salmonella encephalopathy using a norepinephrine-based stress model

Neurological disease represents a sporadic but serious manifestation of bovine salmonellosis that is thought to be related to systemic infection. Salmonella enterica serovar Dublin (S. Dublin) is the serovar most associated with systemic infection in cattle, although reports of neurological disease associated with S. Dublin or any other serovar are rare and usually anecdotal. This study reports the involvement of three strains of S. enterica, serovars Saintpaul, Montevideo, and Enteritidis, in Salmonella encephalopathies. Encephalopathies were reproduced in calves using a norepinephrine-based stress model. Neurological signs were not observed in calves infected with control strains of S. enterica, including S. Dublin, or in calves infected with clinical strains in the absence of norepinephrine. Therefore, norepinephrine may play a role in Salmonella encephalopathies.

Involvement of a Salmonella genomic island 1 gene in the rumen protozoan-mediated enhancement of invasion for multiple-antibiotic-resistant Salmonella enterica serovar Typhimurium

Multiple-antibiotic-resistant Salmonella enterica serotype Typhimurium is a food-borne pathogen that may be more virulent than related strains lacking the multiresistance phenotype. Salmonella enterica serotype Typhimurium phage type DT104 is the most prevalent of these multiresistant/hypervirulent strains. Multiresistance in DT104 is conferred by an integron structure, designated Salmonella genomic island 1 (SGI1), while we recently demonstrated DT104 hyperinvasion mediated by rumen protozoa (RPz) that are normal flora of cattle. Hyperinvasion was also observed in other Salmonella strains, i.e., other S. enterica serovar Typhimurium phage types and other S. enterica serovars, like S. enterica serovar Infantis, possessing SGI1, while DT104 strains lacking SGI1 were not hyperinvasive. Herein we attempted to identify SGI1 genes involved in the RPz-mediated hyperinvasion of Salmonella strains bearing SGI1. Transposon mutagenesis, coupled with a novel reporter system, revealed the involvement of an SGI1 gene previously designated SO13. Disruption of SO13 expression led to an abrogation of hyperinvasion as assessed by tissue culture invasion assays and by bovine challenge experiments. However, hyperinvasion was not observed in non-SGI1-bearing strains of Salmonella engineered to express SO13. That is, SO13 and another SGI1 gene(s) may coordinately upregulate invasion in DT104 exposed to RPz.

Klebsiella to Salmonella gene transfer within rumen protozoa: implications for antibiotic resistance and rumen defaunation

The rumen has long been thought to be a site of gene transfer for microorganisms. Rumen protozoa (RPz) are active predators of bacteria that can harbor antibiotic resistance genes. In this study, RPz were assessed as sites of gene transfer between two bacterial species, Salmonella and Klebsiella. One Klebsiella isolate carried a plasmid bearing bla(CMY-2), encoding an extended-spectrum beta-lactamase conferring ceftriaxone resistance, while the Salmonella was susceptible to ceftriaxone yet capable of thriving within protozoa. In vitro studies revealed that ceftriaxone-resistant Salmonella could be isolated following co-incubation of Salmonella and Klebsiella with RPz obtained from adult cattle and goats. Ceftriaxone-resistant Salmonella were not recovered in the presence of an inhibitor of protozoa engulfment or when a protozoa-sensitive Salmonella was part of the co-incubation. This transfer event was additionally observed in vitro for protozoa-independent stressors although at a significantly lower frequency. The gene transfer event was related to bacterial conjugation since a conjugation inhibitor, nalidixic acid, perturbed the phenomenon. Ceftriaxone-resistant Salmonella were recovered from calves, sheep, and goats co-challenged with ceftriaxone-resistant Klebsiella and ceftriaxone-sensitive Salmonella. However, the transfer event was not observed in calves and sheep that were defaunated prior to the co-challenge. Moreover, Salmonella transconjugants were isolated from separate bovine in vivo studies involving a Klebsiella donor carrying a plasmid conferring colicin activity while no such transconjugants were obtained from defaunated calves. These results provide an important basis for evaluating and preventing the spread of antibiotic resistance and other selective advantages for pathogens present in ruminants.

Exposure to rumen protozoa leads to enhancement of pathogenicity of and invasion by multiple-antibiotic-resistant Salmonella enterica bearing SGI1

Multiple-antibiotic-resistant Salmonella enterica serotype Typhimurium is a food-borne pathogen that has been purported to be more virulent than antibiotic-sensitive counterparts. The paradigm for this multiresistant/hyperpathogenic phenotype is Salmonella enterica serotype Typhimurium phage type DT104 (DT104). The basis for the multiresistance in DT104 is related to an integron structure designated SGI1, but factors underlying hyperpathogenicity have not been completely identified. Since protozoa have been implicated in the alteration of virulence in Legionella and Mycobacterium spp., we attempted to assess the possibility that protozoa may contribute to the putative hypervirulence of DT104. Our study reveals that DT104 can be more invasive, as determined by a tissue culture invasion assay, after surviving within protozoa originating from the bovine rumen. The enhancement of invasion was correlated with hypervirulence in a bovine infection model in which we observed a more rapid progression of disease and a greater recovery rate for the pathogen. Fewer DT104 cells were recovered from tissues of infected animals when protozoa were lysed by preinfection chemical defaunation of the bovine or ovine rumen. The protozoan-mediated hypervirulence phenotype was observed only in DT104 and other Salmonella strains, including serovars Agona and Infantis, possessing SGI1.

SlyA regulates the collagenase-mediated cytopathic phenotype in multiresistant Salmonella

Salmonella enterica serotype Typhimurium phagetype DT104 (DT104) is a foodborne pathogen with a multiresistant phenotype conferred by a genomic-based integron structure designated as SGI1. Recently, a novel cytopathic phenotype was ascribed to several isolates of DT104 recovered from veal calves. This phenotype is dependent upon clg, a gene encoding a collagenase in Salmonella. Using a novel transposon system and an RT-PCR assay for detection of clg expression, we identified SlyA as a regulator of the collagenase-mediated phenotype. The function of SlyA, in regards to clg expression, is to repress the synthesis of Clg. Derepression ensued in the absence of SlyA or in the presence of a truncated version of SlyA with the latter being relevant for maintenance of another virulence aspect mediated by SlyA, i.e. survival within macrophages. The SlyA-mediated effect on clg expression was restricted to DT104 and other Salmonella phagetypes and serotypes possessing SGI1 thus suggesting co-regulation by an SGI1-specific component.