The Possible Influence of Non-synonymous Point Mutations within the FimA Adhesin of Non-typhoidal Salmonella (NTS) Isolates in the Process of Host Adaptation

The gut microbiota induces melanin deposits that act as substrates for fimA-mediated aggregation of Salmonella Typhimurium and enhance infection of the German cockroach vector

When Salmonella Typhimurium is ingested by German cockroaches, the bacteria replicate in the gut and persist for at least 7 d, enabling transmission in the feces. However, the mechanisms that facilitate survival and persistence in the cockroach gut remain poorly detailed. We previously reported the formation of biofilm-like aggregate populations of S. Typhimurium in the gut of cockroaches upon ingestion. We also reported that deletion of the type-1 fimbrial subunit of S. Typhimurium, fimA, leads to a reduced bacterial load in the cockroach gut. Here, we link these observations and provide further insight into the mechanism and function of S. Typhimurium aggregation in the gut of the cockroach. We show that S. Typhimurium but not Escherichia coli forms aggregated populations in the cockroach gut, and that aggregate formation requires fimA but not the biofilm formation-related genes csgA and csgD. Furthermore, we show that S. Typhimurium aggregates are formed using small granular deposits present in the cockroach gut, which exhibit properties consistent with melanin, as substrates. These melanin deposits are prevalent in the guts of both immature and adult cockroaches from laboratory colonies and are correlated with increased gut bacterial density while being entirely absent in gnotobiotic cockroaches reared without exposure to environmental bacteria, indicating they are induced as a response to the gut microbiota. When cockroaches lacking melanin deposits in the gut are fed S. Typhimurium, they exhibit lower rates of infection than those harboring melanin deposits, demonstrating that microbiota-induced melanin deposits enhance infection of the gut of the vector. IMPORTANCE Cockroaches, including the German cockroach (Blattella germanica), can be both mechanical and biological vectors of pathogenic bacteria. Together, our data reveal a novel mechanism by which S. Typhimurium interacts with the cockroach gut and its microbiota that promotes infection of the vector. These findings exemplify the emerging but underappreciated complexity of the relationship between cockroaches and S. Typhimurium.

A Severe Gastroenteritis Outbreak of Salmonella enterica Serovar Enteritidis Linked to Contaminated Egg Fried Rice, China, 2021

Salmonella contamination of eggs and egg shells has been identified as a public health problem worldwide. Here, we reported an outbreak of severe gastrointestinal symptoms caused by Salmonella enterica serovar Enteritidis (S. enteritidis) in China. We evaluated the outbreak by using epidemiological surveys, routine laboratory testing methods, and whole genome sequencing (WGS). This outbreak occurred in a canteen in Beijing, during March 9–11, 2021, 225 of the 324 diners who have eaten at the canteen showed gastrointestinal symptoms. The outbreak had characteristical epidemiological and clinical features. It caused a very high attack rate (69.4%) in a short incubation time. All patients developed diarrhea and high fever, accompanied by abdominal pain (62.3%), nausea (50.4%), and vomiting (62.7%). The average frequency of diarrhea was 12.4 times/day, and the highest frequency of diarrhea was as high as 50 times/day. The average fever temperature was 39.4°C, and the highest fever temperature was 42°C. Twenty strains of S. enteritidis were recovered, including 19 from the patients samples, and one from remained egg fried rice. Antibiotic susceptibility test showed that the 20 outbreak strains all had the same resistance pattern. PFGE results demonstrated that all 20 strains bore completely identical bands. Phylogenetic analysis based on WGS revealed that all 20 outbreak strains were tightly clustered together. So the pathogenic source of this food poisoning incident may was contaminated egg fried rice. Resistance gene analysis showed that the outbreak strains are all multi-drug resistant strains. Virulence gene analysis indicated that these outbreak strains carried a large number of virulence genes, including 2 types of Salmonella pathogenicity islands (SPI-1 and SPI-2). Other important virulence genes were also carried by the outbreak strains, such as pefABCD, rck and shdA. And the shdA gene was not in other strains located in the same evolutionary branch as the outbreak strain. We speculated that this is a significant reason for the serious symptoms of gastroenteritis in this outbreak. This outbreak caused by S. enteritidis suggested government should strengthen monitoring of the prevalence of outbreak clone strains, and take measures to mitigate the public health threat posed by contaminated eggs.

The Invasin and Complement-Resistance Protein Rck of Salmonella is More Widely Distributed than Previously Expected

The rck open reading frame (ORF) on the pefI-srgC operon encodes an outer membrane protein responsible for invasion of nonphagocytic cell lines and resistance to complement-mediated killing. Until now, the rck ORF was only detected on the virulence plasmids of three serovars of Salmonella subsp. enterica (i.e., Bovismorbificans, Enteritidis, and Typhimurium). The increasing number of Salmonella genome sequences allowed us to use a combination of reference sequences and whole-genome multilocus sequence typing (wgMLST) data analysis to probe the presence of the operon and of rck in a wide array of isolates belonging to all Salmonella species and subspecies. We established the presence of partial or complete operons in 61 subsp. enterica serovars as well as in 4 other subspecies with various syntenies and frequencies. The rck ORF itself was retrieved in 36 subsp. enterica serovars and in two subspecies with either chromosomal or plasmid-borne localization. It displays high conservation of its sequence within the genus, and we demonstrated that most of the allelic variations identified did not alter the virulence properties of the protein. However, we demonstrated the importance of the residue at position 38 (at the level of the first extracellular loop of the protein) in the invasin function of Rck. Altogether, our results highlight that rck is not restricted to the three formerly identified serovars and could therefore have a more important role in virulence than previously expected. Moreover, this work raises questions about the mechanisms involved in rck acquisition and about virulence plasmid distribution and evolution.

IMPORTANCE The foodborne pathogen Salmonella is responsible for a wide variety of pathologies depending on the infected host, the infecting serovars, and its set of virulence factors. However, the implication of each of these virulence factors and their role in the specific host-pathogen interplay are not fully understood. The significance of our research is in determining the distribution of one of these factors, the virulence plasmid-encoded invasin and resistance to complement killing protein Rck. In addition to providing elements of reflection concerning the mechanisms of acquisition of specific virulence genes in certain serotypes, this work will help to understand the role of Rck in the pathogenesis of Salmonella.