Revisiting Persistent Salmonella Infection and the Carrier State: What Do We Know?

One characteristic of the few Salmonella enterica serovars that produce typhoid-like infections is that disease-free persistent infection can occur for months or years in a small number of individuals post-convalescence. The bacteria continue to be shed intermittently which is a key component of the epidemiology of these infections. Persistent chronic infection occurs despite high levels of circulating specific IgG. We have reviewed the information on the basis for persistence in S. Typhi, S. Dublin, S. Gallinarum, S. Pullorum, S. Abortusovis and also S. Typhimurium in mice as a model of persistence. Persistence appears to occur in macrophages in the spleen and liver with shedding either from the gall bladder and gut or the reproductive tract. The involvement of host genetic background in defining persistence is clear from studies with the mouse but less so with human and poultry infections. There is increasing evidence that the organisms (i) modulate the host response away from the typical Th1-type response normally associated with immune clearance of an acute infection to Th2-type or an anti-inflammatory response, and that (ii) the bacteria modulate transformation of macrophage from M1 to M2 type. The bacterial factors involved in this are not yet fully understood. There are early indications that it might be possible to remodulate the response back towards a Th1 response by using cytokine therapy.

A bioinformatic approach to identify core genome difference between Salmonella Pullorum and Salmonella Enteritidis

S. Pullorum and S. Enteritidis are closely related in genetic terms, but they show very different pathogenicity and host range. S. Enteritidis infects many different hosts, usually causing acute gastroenteritis, while S. Pullorum is restricted to avian, where it causes systemic disease in young animals. The reason why they differ in host range and pathogenicity is unknown. The core-genome denotes those genes that are present in all strains within a clade, and in the present work, an automated bioinformatics workflow was developed and applied to identify core-genome differences between these two serovars with the aim to identify genome features associated with host specificity of S. Pullorum. Results showed that S. Pullorum unique coding sequences (CDS) were mainly concentrated in three regions not present in S. Enteritidis, suggesting that such CDS were taken up probably during the separation of the two types from their common ancestor. One of the unique regions encoded Pathogenicity Islands 19 (SPI-19), which encodes a type VI secretion system (T6SS). Single-nucleotide polymorphism (SNP) analysis identified 1791 conserved SNPs in coding sequences between the two serovars, including several SNPs located in a type IV secretion system (T4SS). Analyzing of 100 bp regions upstream of coding sequences identified 443 conserved SNPs between the two serovars, including SNP variations in type III secretion system effector (T3SE). In conclusion, this analysis has identified genetic features encoding putative factors controlling host-specificity in S. Pullorum. The novel bioinformatic workflow and associated scripts can directly be applied to other bacteria to uncover the genome difference between clades.

Virulence Gene Distribution of Salmonella Pullorum Isolates Recovered from Chickens in China (1953-2015)

Salmonella enterica subspecies enterica serovar Gallinarum biovar Pullorum (Salmonella Pullorum) has strict host specificity for poultry, and pullorum disease seriously threatens the poultry industry. Virulence genes play a central role in Salmonella pathogenicity, but very few reports are available on the distribution of virulence genes in Salmonella Pullorum. In this study, we investigated 304 Salmonella Pullorum isolates recovered from chickens in China between 1953 and 2015 for the presence of 25 Salmonella virulence genes (invA, orgA, prgH, sitC, spaN, sifA, spiA, ttrC, mgtB, misL, siiE, spi4D, pipA, sipB, sopB, sefA, cdtB, pagC, shdA, msgA, lpfC, tolC, iroN, pefA, and spvB), including pathogenicity island genes, fimbriae genes, and virulence plasmid genes. PCR showed that 15 of the 25 virulence genes were present in all isolates tested, whereas cdtB was not present in any isolate. The presence rates of the remaining genes ranged from 97.7% to 99.7%. The variation rates of these virulence genes was low, and no significant differences were identified in the distribution of virulence genes over time. On the basis of the distribution of these virulence genes, the 304 Salmonella Pullorum isolates were divided into 10 virulence genotypes. The major genotype, which comprised 93.4% of all isolates, included isolates that carried 24 of the virulence genes assessed. The results of this study will help in the characterization of Salmonella Pullorum and in the study of the correlation between virulence genotypes and pathogenicity.