To evaluate the role of eggs and other food vehicles as risk factors associated with Salmonella enteritidis (SE) outbreaks in order to address the endemicity of SE infections in the USA. We retrieved and analyzed data relating to all SE outbreaks reported to the Centers for Disease Control and Prevention (CDC) between 1990 and 2015. We then used descriptive and analytical statistical methods, including negative binomial regression models for the estimation of rate-ratios, to analyze the data. Analyses showed that egg-based dishes were the most common food vehicle associated with outbreaks of SE in the USA (273 cases [24%]); this was followed by several other food items, including meat (130 cases [11%]), vegetables (96 cases [8%]), chicken items (95 cases [8%]), dairy products (55 cases [5%]), and bakery items (8 cases [1%]). Compared to egg-based dishes, other food items such as meat (exp(β) = 0.51, 95% CI 0.37, 0.69), chicken (exp(β) = 0.42, 95% CI 0.30, 0.58), vegetables (exp(β) = 0.41, 95% CI 0.29, 0.55), and dairy items (exp(β) = 0.27, 95% CI 0.18, 0.40) were significantly associated with outbreaks of SE in the USA. Of 1144 SE outbreaks, 402 (35%) occurred in the Northeast region of the USA, followed by the South (253 [22%]), West (250 [22%]), and Midwestern regions (239 [21%]). Epidemiological and spatiotemporal trends analyses demonstrated that a significant proportions of Salmonella enteritidis outbreaks in the USA are attributed to food vehicles other than eggs. Our findings can be used to plan effective strategies to mitigate the increasing occurrence of foodborne SE outbreaks.

Many antibiotic-resistant bacteria carry resistance genes on conjugative plasmids that are transferable to commensals and pathogens. We determined the ability of multiple enteric bacteria to acquire and retransfer a broad-host-range plasmid RP4. We used human-derived commensal Escherichia coli LM715-1 carrying a chromosomal red fluorescent protein gene and green fluorescent protein (GFP)-labeled broad-host-range RP4 plasmid with ampR, tetR, and kanR in in vitro matings to rifampicin-resistant recipients, including Escherichia coli MG1655, Dec5α, Vibrio cholerae, Pseudomonas putida, Pseudomonas aeruginosa, Klebsiella pneumoniae, Citrobacter rodentium, and Salmonella Typhimurium. Transconjugants were quantified on selective media and confirmed using fluorescence microscopy and PCR for the GFP gene. The plasmid was transferred from E. coli LM715-1 to all tested recipients except P. aeruginosa. Transfer frequencies differed between specific donor–recipient pairings (10⁻² to 10⁻⁸). Secondary retransfer of plasmid from transconjugants to E. coli LM715-1 occurred at frequencies from 10⁻² to 10⁻⁷. A serial passage plasmid persistence assay showed plasmid loss over time in the absence of antibiotics, indicating that the plasmid imposed a fitness cost to its host, although some plasmid-bearing cells persisted for at least ten transfers. Thus, the RP4 plasmid can transfer to multiple clinically relevant bacterial species without antibiotic selection pressure.

Background. Antibiotic-resistant (AR) bacteria pose an increasing threat to public health, but the dynamics of antibiotic resistance gene (ARG) spread in complex microbial communities are poorly understood. Conjugation is a predominant direct cell-to-cell mechanism for the horizontal gene transfer (HGT) of ARGs. We hypothesized that commensal Escherichia coli donor strains would mediate the conjugative transfer of ARGs to phylogenetically distinct bacteria without antibiotic selection pressure in gastrointestinal tracts of mice carrying a human-derived microbiota with undetectable levels of E. coli. Our objective was to identify a mouse model to study the factors regulating AR transfer by conjugation in the gut. Methods. Two donor E. coli strains were engineered to carry chromosomally encoded red fluorescent protein, and an ARG- and green fluorescent protein (GFP)-encoding broad host range RP4 conjugative plasmid. Mice were orally gavaged with two donor strains (1) E. coli MG1655 or (2) human-derived mouse-adapted E. coli LM715-1 and their colonization assessed by culture over time. Fluorescence-activated cell sorting (FACS) and 16S rDNA sequencing were performed to trace plasmid spread to the microbiota. Results. E. coli LM715-1 colonized mice for ten days, while E. coli MG1655 was not recovered after 72 h. Bacterial cells from fecal samples on days 1 and 3 post inoculation were sorted by FACS. Samples from mice given donor E. coli LM715-1 showed an increase in cells expressing green but not red fluorescence compared to pre-inoculation samples. 16S rRNA gene sequencing analysis of FACS GFP positive cells showed that bacterial families Lachnospiraceae, Clostridiaceae, Pseudomonadaceae, Rhodanobacteraceae, Erysipelotrichaceae, Oscillospiraceae, and Butyricicoccaceae were the primary recipients of the RP4 plasmid. Conclusions. Results show this ARG-bearing conjugative RP4 plasmid spread to diverse human gut bacterial taxa within a live animal where they persisted. These fluorescent marker strategies and human-derived microbiota transplanted mice provided a tractable model for investigating the dynamic spread of ARGs within gut microbiota and could be applied rigorously to varied microbiotas to understand conditions facilitating their spread.

Anti-TNF-α therapy is widely used for inflammatory bowel disease (IBD), but response rates vary, and long-term efficacy declines in many patients. Given the limitations of existing treatments, novel therapeutic strategies are needed. This study investigates whether Lactobacillus murinus (L. murinus) attenuates tumor necrosis factor alpha (TNF-α)-induced pro-inflammatory responses in a human intestinal epithelial cell model of colitis by modulating the aryl hydrocarbon receptor (AHR). An in vitro model was established using Caco-2 cell monolayers treated with TNF-α to simulate intestinal inflammation. Cells were pre-treated with L. murinus or known AHR ligands, and the effects on AHR activation, barrier integrity, and inflammatory response were assessed via transepithelial electrical resistance (TEER) and IL-8 quantifications. As CYP1A1 is a well-established transcriptional target of AHR, its mRNA expression was used as a surrogate marker of AHR modulation in this model. TNF-α stimulation significantly disrupted epithelial barrier integrity and increased IL-8 secretion in a dose-dependent manner. L. murinus pre-treatment enhanced CYP1A1 expression and was associated with reduced TNF-α-induced barrier disruption and IL-8 secretion. Notably, the beneficial effects of L. murinus on epithelial integrity were not replicated by synthetic AHR ligands, suggesting ligand-selective differences in AHR related responses. These findings suggest that AHR-associated signaling induced by L. murinus may contribute to mitigation of TNF-α-induced epithelial barrier dysfunction and inflammation. This study identifies a potential probiotic-associated mechanism that warrants further investigation, including studies designed to establish a causal role of AHR dependency in the observed effects. In addition, future studies are needed to identify the specific L. murinus metabolites responsible for inducing CYP1A1 expression and activating the AHR pathway.