Salmonella Pathogenicity Islands 19 (SPI19) encodes a type VI secretion system (T6SS). SPI19 is only present in few serovars of S. enterica, including the host-adapted serovar S. Dublin and the host-specific serovar S. Gallinarum. The role of the SPI19 encoded T6SS in virulence in these serovar is not fully understood. Here we show that during infection of mice, a SPI19/T6SS deleted strain of S. Dublin 2229 was less virulent than the wild type strain after oral challenge, but not after IP challenge. The mutant strain also competed significantly poorer than the wild type strain when co-cultured with strains of E. coli, suggesting that this T6SS plays a role in pathogenicity by killing competing bacteria in the intestine. No significant difference was found between wild type S. Gallinarum G9 and its ΔSPI19/T6SS mutant in infection, whether chicken were challenged orally or by the IP route, and the S. Gallinarum G9 ΔSPI19/T6SS strain competed equally well as the wild type strain against strains of E. coli. However, contrary to what was observed with S. Dublin, the wild type G9 strains was significantly more cytotoxic to monocyte derived primary macrophages from hens than the mutant, suggesting that SPI19/T6SS in S. Gallinarum mediates killing of eukaryotic cells. The lack of significant importance of SPI19/T6SS after oral and systemic challenge of chicken was confirmed by knocking out SPI19 in a second strain, J91. Together the results suggest that the T6SS encoded from SPI19 have different roles in the two serovars and that it is a virulence-factor after oral challenge of mice in S. Dublin, while we cannot confirm previous results that SPI19/T6SS influence virulence significantly in S. Gallinarum.

Salmonella enterica is a leading foodborne pathogen worldwide, posing significant public health concerns. However, its prevalence in mountainous regions in China remains unclear. This study analyzed 384 Salmonella strains isolated from 2,540 samples collected between 2017 and 2024 from humans, food, and environmental sources in tropical mountainous areas of China. Whole-genome sequencing and antimicrobial-susceptibility testing were performed to investigate genomic characteristics and antimicrobial resistance profiles. The results showed that S . I 1,4 [5],12:i:- (ST34, 43.5%) was the predominant serovar among diarrheal patients, while S . Enteritidis (ST11, 40%) was the primary causative serovar associated with systemic infections characterized by bacteremia. In food and environmental samples, S . Typhimurium (ST19, 39.1%) and S . Enteritidis (ST11, 35.1%) were most frequently isolated, respectively. Antimicrobial susceptibility testing revealed high resistance rates to ampicillin (63.8%), tetracycline (62.7%), ciprofloxacin (17.7%), cefotaxime (16.4%), and ceftazidime (15.6%), with 115 isolates (29.9%) identified as multidrug-resistant. Notably, six isolates were tigecycline-resistant, including two harboring tet(X4 ) genes on an IncFIA(HI1)-IncHI1A- IncHI1B(R27) hybrid plasmid. The distribution of virulence genes varied across strains from different sources and serovars. Genes encoding carbonic anhydrase ( mig-5 ), plasmid-encoded fimbriae ( pefA , pefB , pefC , and pefD ), and type III secretion system effectors ( spvB , spvC , and spvD ) were significantly more prevalent in strains from patients with systemic infection than in those from diarrhea cases. The cdtB gene encoding typhoid toxin was identified in 19 non-typhoidal strains. This study presents the first comprehensive report on the prevalence and genetic dynamics of S. enterica in tropical mountainous regions of southern China, offering insights into novel interventions. IMPORTANCE: Salmonella is a serious zoonotic foodborne pathogen and the third leading cause of death from human diarrheal diseases worldwide. In this study, we report, for the first time, a systematic surveillance of Salmonella prevalence among clinical patients, food, and environmental sources in the tropical mountainous regions of southern China from 2017 to 2024. Our findings illuminate the prevalence and genetic dynamics of S. enterica across diverse sources, as well as differences between patients with diarrheal disease and those with systemic infections characterized by bacteremia. Moreover, we observed high resistance to clinically important treatment drugs, including fluoroquinolones and third-generation cephalosporins, in these regions. These results provide critical evidence to inform more effective surveillance, treatment, and control strategies in tropical settings and underscore the need for continuous, real-time monitoring of pathogen dynamics.

Background: Hepatitis C virus (HCV) infection poses a major public health challenge globally, especially among injecting drug users. China has the world’s largest burden of HCV infections. However, little is known about the characteristics of transmission networks among drug user populations. This study aims to investigate the molecular epidemiology and transmission characteristics of HCV infections among drug users in Zhuhai, a bustling port city connecting Mainland China and its Special Administrative Regions. Methods: Participants enrolled in this study were drug users incarcerated at Zhuhai’s drug rehabilitation center in 2015. Their sociodemographic and behavioral information, including gender, promiscuity, drug use method, and so forth, was collected using a standardized questionnaire. Plasmas separated from venous blood were analyzed for HCV infection through ELISA and RT-PCR methods to detect anti-HCV antibodies and HCV RNA. The 5’UTR fragment of the HCV genome was amplified and further sequenced for subtype identifications and phylogenetic analysis. The phylogenetic tree was inferred using the Maximum Likelihood method based on the Tamura-Nei model, and the transmission cluster network was constructed using Cytoscape3.8.0 software with a threshold of 0.015. Binary logistic regression models were employed to assess the factors associated with HCV infection. Results: The overall prevalence of HCV infection among drug users was 44.37%, with approximately 19.69% appearing to clear the HCV virus successfully. Binary logistic regression analysis revealed that those aged over 40, engaging in injecting drug use, and being native residents were at heightened risk for HCV infection among drug user cohorts. The predominant HCV subtypes circulating among those drug users were 6a (60.26%), followed by 3b (16.7%), 3a (12.8%), 1b (6.41%) and 1a (3.85%), respectively. Molecular transmission network analysis unveiled the presence of six transmission clusters, with the largest propagation cluster consisting of 41 individuals infected with HCV subtype 6a. Furthermore, distinct transmission clusters involved eight individuals infected with subtype 3b and seven with subtype 3a were also observed. Conclusion: The genetic transmission networks revealed a complex transmission pattern among drug users in Zhuhai, emphasizing the imperative for a targeted and effective intervention strategy to mitigate HCV dissemination. These insights are pivotal for shaping future national policies on HCV screening, treatment, and prevention in port cities.