Evolution of Salmonella nomenclature: a critical note

Detection of Salmonella by competitive ELISA of lipopolysaccharide secreted into the culture medium

Detection of Salmonella in food is topical due to known cases of salmonellosis epidemics. Immunochemical methods including ELISA are widely used for Salmonella detection. Traditionally, commercial ELISA kits are based on sandwich technique and detect lipopolysaccharide (LPS), which is considered to be the component of the outer membrane of Gram-negative bacteria. Our aim was elaboration of competitive ELISA test for Salmonella detection in food with improved parameters. It was shown that in the Salmonella culture after the standard sample preparation procedure LPS is present mainly outside cells as a component of outer membrane vesicles. Improved sample preparation procedure includes separation of bacteria from the medium and analysis of the medium, which increases analytical sensitivity. Immobilization of the bovine serum albumin (BSA)-LPS conjugate in microplate wells allows to obtain a more homogeneous coating than immobilization of LPS itself. Thus, we have developed test system for Salmonella detection in food by competitive ELISA of LPS secreted into the culture medium with the immobilized BSA-LPS conjugate and monoclonal antibodies (mAb) to LPS core in the liquid phase. New competitive ELISA test is high sensitive, give reproducible results, allows the detection of any Salmonella serotype and is important for the protection of human health.

Salmonella gas-forming pyomyositis in an immunocompetent patient: a case report and review

The primary etiology of pyomyositis is predominantly Staphylococcus aureus, although Gram-negative bacteria may also be involved on rare occasions. The prognosis for pyomyositis caused by Gram-negative bacteria is more unfavorable than that of infections caused by Staphylococcus aureus. Among Gram-negative bacteria, members of the Enterobacteriaceae family, including Escherichia coli, Klebsiella species, and Salmonella species, have the capacity to produce gas. Gas-forming myositis is a rare phenomenon that primarily affects immunocompromised patients and is associated with a poor prognosis. To the best of our knowledge, no previous reports of gas-forming pyomyositis in an immunocompetent patient exist. We herein present the first documented case of Salmonella-induced gas-forming pyomyositis of the iliacus muscle in a healthy young man with no underlying diseases or comorbidities. Additionally, we reviewed cases of gas-forming pyomyositis in the literature and cases of pyomyositis caused by Gram-negative bacteria in immunocompetent patients.

Salmonella enterica Serovar Infantis in Broiler Chickens: A Systematic Review and Meta-Analysis

Salmonella enterica subsp. enterica serovar Infantis poses a growing threat to public health, due to its increasing prevalence worldwide and its association with high levels of antimicrobial resistance. Among livestock, S. Infantis is especially isolated from broilers. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, a systematic review was conducted by searching in three databases (Web of Science, Scopus, and PubMed) for English-language studies (1957-2023) that reported the prevalence of S. Infantis in broiler farms. Eligible studies included epidemiological investigations conducted in broiler chickens by sampling the house environment (flock-level prevalence) or the birds (individual-level prevalence). A random-effect model was applied to calculate S. Infantis pooled prevalence estimates with 95% confidence intervals (CIs). Furthermore, to assess between-study heterogeneity, the inconsistency index statistic (I2) was calculated. Among 537 studies retrieved, a total of 9 studies reporting flock-level prevalence of S. Infantis and 4 reporting individual-level prevalence were retained for analysis. The flock-level pooled prevalence was estimated to be 9% (95% CI: 1-26%) and a high between-study heterogeneity was found (I2 = 99%, p < 0.01). Concerning individual-level prevalence, a meta-analysis was not performed due to the scarcity of eligible studies. The data presented underscore the significant occurrence of S. Infantis in broilers at the farm level. By summarizing the existing literature, this work provides useful insights for conducting future surveys of Salmonella spp. in live broiler chickens as a preliminary step for developing more efficient control strategies.

Combatting extensively drug-resistant Salmonella: a global perspective on outbreaks, impacts, and control strategies

Antibiotic resistance in typhoid fever poses a critical public health problem due to the emergence of extensively drug-resistant (XDR) Salmonella, resulting in prolonged illness and treatment failure. Salmonella enterica serovar Typhi is the most predominant among all serotypes and can acquire resistance. The emergence of XDR Salmonella in various regions globally, particularly Pakistan, presents a concerning trend. However, limited data availability impedes a comprehensive understanding of the outbreaks and hinders the development of real-time solutions. Here, we have provided an updated overview of the current outbreaks of XDR Salmonella in epidemic and endemic regions. Treatments of XDR Salmonella infections are challenging, as there are records of treatment failure in humans and animals. However, intensive prevention techniques can be implemented pending the advent of novel antibiotics. Emphasis on antimicrobial stewardship and frequent surveillance of the pathogen should be made to keep track of potential outbreaks in both human and animal populations. Although progress is being made to combat XDR Salmonella within some regions, a unified and efficient effort on an international scale is required to curtail the XDR outbreak before it escalates and leads us back to the pre-antibiotic era.

Bridging Classical Methodologies in Salmonella Investigation with Modern Technologies: A Comprehensive Review

Advancements in genomics and machine learning have significantly enhanced the study of Salmonella epidemiology. Whole-genome sequencing has revolutionized bacterial genomics, allowing for detailed analysis of genetic variation and aiding in outbreak investigations and source tracking. Short-read sequencing technologies, such as those provided by Illumina, have been instrumental in generating draft genomes that facilitate serotyping and the detection of antimicrobial resistance. Long-read sequencing technologies, including those from Pacific Biosciences and Oxford Nanopore Technologies, offer the potential for more complete genome assemblies and better insights into genetic diversity. In addition to these sequencing approaches, machine learning techniques like decision trees and random forests provide powerful tools for pattern recognition and predictive modeling. Importantly, the study of bacteriophages, which interact with Salmonella, offers additional layers of understanding. Phages can impact Salmonella population dynamics and evolution, and their integration into Salmonella genomics research holds promise for novel insights into pathogen control and epidemiology. This review revisits the history of Salmonella and its pathogenesis and highlights the integration of these modern methodologies in advancing our understanding of Salmonella.

Relationship of Salmonella Typhimurium 14028 strain and its dam and seqA mutants with gut microbiota dysbiosis in rats

Introduction. Disruptions in gut microbiota, known as dysbiosis, have been increasingly linked to pathogenic infections, with Salmonella Typhimurium being a notable contributor to these disturbances.Hypothesis. We hypothesize that the S. Typhimurium 14028 WT strain induces significant dysbiosis in the rat gut microbiota and that the dam and seqA genes play crucial roles in this process.Aim. In this study, it was aimed at investigating the dysbiotic activity of the S. Typhimurium 14028 WT strain on the rat gut microbiota and the roles of dam and seqA genes on this activity.Method. Changes in the rat gut microbiota were determined by examining the anal swap samples taken from the experimental groups of these animals using 16S rRNA high-throughput sequencing technology.Results. In the experimental groups, the dominant phyla were determined to be Firmicutes and Bacteroidetes (P<0.05). However, while the rate of Bacteroidetes was significantly reduced in those treated with the WT and seqA mutants, no significant difference was observed in the dam mutant compared to the control group (P<0.05). In all experimental animals, the dominant species was determined to be Prevotella copri, regardless of the experiment time and application. The analysis results of the samples taken on the third day from the rat groups infected with the S. Typhimurium 14028 WT strain (W2) presented the most striking data of this study.Conclusion. Through distance analysis, we demonstrated that a successful Salmonella infection completely changes the composition of the microbiota, dramatically reduces species diversity and richness in the microbiota and encourages the growth of opportunistic pathogens.

Serotype Screening of Salmonella enterica Subspecies I by Intergenic Sequence Ribotyping (ISR): Critical Updates

(1) Background: Foodborne illness from Salmonella enterica subspecies I is most associated with approximately 32 out of 1600 serotypes. While whole genome sequencing and other nucleic acid-based methods are preferred for serotyping, they require expertise in bioinformatics and often submission to an external agency. Intergenic Sequence Ribotyping (ISR) assigns serotype to Salmonella in coordination with information freely available at the National Center for Biotechnology Information. ISR requires updating because it was developed from 26 genomes while there are now currently 1804 genomes and 1685 plasmids. (2) Methods: Serotypes available for sequencing were analyzed by ISR to confirm primer efficacy and to identify any issues in application. Differences between the 2012 and 2022 ISR database were tabulated, nomenclature edited, and instances of multiple serotypes aligning to a single ISR were examined. (3) Results: The 2022 ISR database has 268 sequences and 40 of these were assigned new NCBI accession numbers that were not previously available. Extending boundaries of sequences resolved hdfR cross-alignment and reduced multiplicity of alignment for 37 ISRs. Comparison of gene cyaA sequences and some cell surface epitopes provided evidence that homologous recombination was potentially impacting results for this subset. There were 99 sequences that still had no match with an NCBI submission. (4) The 2022 ISR database is available for use as a serotype screening method for Salmonella enterica subspecies I. Finding that 36.9% of the sequences in the ISR database still have no match within the NCBI Salmonella enterica database suggests that there is more genomic heterogeneity yet to characterize.

Metabolomic Profiles of Multidrug-Resistant Salmonella Typhimurium from Humans, Bovine, and Porcine Hosts

Simple Summary

The global threat that is imposed by the resistance the pathogens develop to antimicrobial drugs is escalating. Tools to detect the resistance (with evidence on molecular and cellular outcomes) would reveal intricate mechanisms through which novel drugs could be developed. Approaches such as metabolomics, which involve metabolite detection, provide scientific evidence of metabolite expression of antimicrobial-resistant pathogens. The current study involved metabolomics of antimicrobial-resistant Salmonella Typhimurium collected from various hosts (human, porcine, bovine) and were exposed to antimicrobial drugs—ampicillin, chloramphenicol, streptomycin, sulfisoxazole, and tetracycline—as one set of the experiment. The same isolates were also cultured with no drug exposure as a comparison. There are certain pathways of metabolite expression that are impacted by drug exposure when compared to no drug exposure, meaning that the expressed metabolites could be potential targets for drug companies for the treatment of antimicrobial-resistant pathogens.

Abstract

Antimicrobial resistance (AMR) is a global public health threat, yet tools for detecting resistance patterns are limited and require advanced molecular methods. Metabolomic approaches produce metabolite profiles and help provide scientific evidence of differences in metabolite expressions between Salmonella Typhimurium from various hosts. This research aimed to evaluate the metabolomic profiles of S. Typhimurium associated with AMR and it compares profiles across various hosts. Three samples, each from bovine, porcine, and humans (total n = 9), were selectively chosen from an existing library to compare these nine isolates cultured under no drug exposure to the same isolates cultured in the presence of the antimicrobial drug panel ACSSuT (ampicillin, chloramphenicol, streptomycin, sulfisoxazole, tetracycline). This was followed by metabolomic profiling using UPLC and GC–mass spectrometry. The results indicated that the metabolite regulation was affected by antibiotic exposure, irrespective of the host species. When exposed to antibiotics, 59.69% and 40.31% of metabolites had increased and decreased expressions, respectively. The most significantly regulated metabolic pathway was aminoacyl-tRNA biosynthesis, which demonstrated increased expressions of serine, aspartate, alanine, and citric acid. Metabolites that showed decreased expressions included glutamate and pyruvate. This pathway and associated metabolites have known AMR associations and could be targeted for new drug discoveries and diagnostic methods.

Screening the Presence of Non-Typhoidal Salmonella in Different Animal Systems and the Assessment of Antimicrobial Resistance

Salmonella is a major bacterial foodborne pathogen that causes the majority of worldwide food-related outbreaks and hospitalizations. Salmonellosis outbreaks can be caused by multidrug-resistant (MDR) strains, emphasizing the importance of maintaining public health and safer food production. Nevertheless, the drivers of MDR Salmonella serovars have remained poorly understood. In this study, we compare the resistance profiles of Salmonella strains isolated from 4047 samples from domestic and wild animals in Chile. A total of 106 Salmonella strains (2.61%) are isolated, and their serogroups are characterized and tested for susceptibility to 16 different antimicrobials. The association between antimicrobial resistance (AMR) and a subset of independent variables is evaluated using multivariate logistic models. Our results show that 47 antimicrobial-resistant strains were found (44.3% of the total strains). Of the 47, 28 correspond to single-drug resistance (SDR = 26.4%) and 19 are MDR (17.9%). S. Enteritidis is highly persistent in animal production systems; however, we report that serogroup D strains are 18 times less likely to be resistant to at least one antimicrobial agent than the most common serogroup (serogroup B). The antimicrobials presenting the greatest contributions to AMR are ampicillin, streptomycin and tetracycline. Additionally, equines and industrial swine are more likely to acquire Salmonella strains with AMR. This study reports antimicrobial-susceptible and resistant Salmonella in Chile by expanding the extant literature on the potential variables affecting antimicrobial-resistant Salmonella.

A diet-specific microbiota drives Salmonella Typhimurium to adapt its in vivo response to plant-derived substrates

Background

Little is known about the complex interactions between the diet, the gut microbiota, and enteropathogens. Here, the impact of two specific diets on the composition of the mouse gut microbiota and on the transcriptional response of Salmonella Typhimurium (S. Typhimurium) was analyzed in an enteritis model.

Results

Mice were fed for two weeks a fibre-rich, plant-based diet (PD), or a Westernized diet (WD) rich in animal fat and proteins and in simple sugars, and then infected with an invasin-negative S. Typhimurium strain ST4/74 following streptomycin-treatment. Seventy-two hours post infection, fecal pathogen loads were equal in both diet groups, suggesting that neither of the diets had negatively influenced the ability of this ST4/74 strain to colonize and proliferate in the gut at this time point. To define its diet-dependent gene expression pattern, S. Typhimurium was immunomagnetically isolated from the gut content, and its transcriptome was analyzed. A total of 66 genes were more strongly expressed in mice fed the plant-based diet. The majority of these genes was involved in metabolic functions degrading substrates of fruits and plants. Four of them are part of the gat gene cluster responsible for the uptake and metabolism of galactitol and D-tagatose. In line with this finding, 16S rRNA gene amplicon analysis revealed higher relative abundance of bacterial families able to degrade fiber and nutritive carbohydrates in PD-fed mice in comparison with those nourished with a WD. Competitive mice infection experiments performed with strain ST4/74 and ST4/74 ΔSTM3254 lacking tagatose-1,6-biphosphate aldolase, which is essential for galactitol and tagatose utilization, did not reveal a growth advantage of strain ST4/74 in the gastrointestinal tract of mice fed plant-based diet as compared to the deletion mutant.

Conclusion

A Westernized diet and a plant-based diet evoke distinct transcriptional responses of S. Typhimurium during infection that allows the pathogen to adapt its metabolic activities to the diet-derived nutrients. This study therefore provides new insights into the dynamic interplay between nutrient availability, indigenous gut microbiota, and proliferation of S. Typhimurium.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-021-00082-8.

Mitracarpus frigidus (Rubiaceae) inhibits inflammatory and oxidative stress mediators in Salmonella sp. mouse infection

OBJECTIVES

Evaluation of the in-vivo anti-inflammatory activity of the methanolic extract obtained from the aerial parts of Mitracarpus frigidus (MFM) in the infection caused by two Salmonella strains and its chemical fingerprint by UFLC-quadrupole time of flight-MS.

METHODS

The efficacy of MFM was investigated in a classical in-vivo Salmonella infection mouse model. A Salmonella reference strain (ATCC 13311) and a clinical isolate were used to infect mice and then MFM was orally administered during 14 days. At the end of the treatment with MFM, the infection and inflammatory levels were assayed.

KEY FINDINGS

MFM treatment showed a significant reduction in mice mortality by Salmonella infection and, also, did not cause alterations in the liver function. Inhibitions of inflammatory and oxidative stress mediators [malondialdehyde (MDA), catalase, and metalloproteinase] were possibly involved in the observed effects. Chlorogenic acid, clarinoside, quercetin-pentosylhexoside, rutin, kaempferol-3O-rutinoside, kaempferol-rhamnosylhexoside and 2-azaanthraquinone were identified in MFM.

CONCLUSIONS

MFM was effective in some inflammatory parameters, in the experimental conditions that were used in the study. The results presented in this study and the previous in-vitro anti-Salmonella activity reported by our research group reinforce the importance of MFM studies to considerer it as an alternative treatment for salmonellosis.

Quantitative LAMP and PCR Detection of Salmonella in Chicken Samples Collected from Local Markets around Pathum Thani Province, Thailand

Salmonella is a bacterium that infects people when they consume contaminated food or liquids. To prevent humans from becoming ill, it is useful to have an efficient method of detecting Salmonella before the disease is passed on through the food chain. In this research, the efficiency of Salmonella detection was compared using the following four methods: conventional loop-mediated isothermal amplification (LAMP), PCR, quantitative LAMP (qLAMP), and qPCR. The artificial infection of chicken samples started with incubating of 10 mL of 10⁸ CFU of S. typhimurium for 6 hr. and enriching for 2 hr. to represent real contamination of the samples. The results show that the sensitivity of Salmonella DNA detection in PCR, qPCR, LAMP, and qLAMP were 50 ng, 5 ng, 50 pg, and and 500 fg, respectively. Thirty samples of 10 g chicken were collected from 10 markets in Pathum Thani, Thailand; then, the infection was detected. The conventional LAMP, qLAMP, and qPCR methods detected Salmonella in all the chicken samples. However, the conventional PCR method detected Salmonella infection in only eight of the samples. Overall, the qLAMP method had the highest sensitivity of Salmonella DNA detection.

DNA sequencing, genomes and genetic markers of microbes on fruits and vegetables

The development of DNA sequencing technology has provided an effective method for studying foodborne and phytopathogenic microorganisms on fruits and vegetables (F & V). DNA sequencing has successfully proceeded through three generations, including the tens of operating platforms. These advances have significantly promoted microbial whole‐genome sequencing (WGS) and DNA polymorphism research. Based on genomic and regional polymorphisms, genetic markers have been widely obtained. These molecular markers are used as targets for PCR or chip analyses to detect microbes at the genetic level. Furthermore, metagenomic analyses conducted by sequencing the hypervariable regions of ribosomal DNA (rDNA) have revealed comprehensive microbial communities in various studies on F & V. This review highlights the basic principles of three generations of DNA sequencing, and summarizes the WGS studies of and available DNA markers for major bacterial foodborne pathogens and phytopathogenic fungi found on F & V. In addition, rDNA sequencing‐based bacterial and fungal metagenomics are summarized under three topics. These findings deepen the understanding of DNA sequencing and its application in studies of foodborne and phytopathogenic microbes and shed light on strategies for the monitoring of F & V microbes and quality control.

Genetic boundaries delineate the potential human pathogen Salmonella bongori into discrete lineages: divergence and speciation

BACKGROUND

Salmonella bongori infect mainly cold-blooded hosts, but infections by S. bongori in warm-blooded hosts have been reported. We hypothesized that S. bongori might have diverged into distinct phylogenetic lineages, with some being able to infect warm-blooded hosts.

RESULTS

To inspect the divergence status of S. bongori, we first completely sequenced the parakeet isolate RKS3044 and compared it with other sequenced S. bongori strains. We found that RKS3044 contained a novel T6SS encoded in a pathogenicity island-like structure, in addition to a T6SS encoded in SPI-22, which is common to all S. bongori strains so far reported. This novel T6SS resembled the SPI-19 T6SS of the warm-blooded host infecting Salmonella Subgroup I lineages. Genomic sequence comparisons revealed different genomic sequence amelioration events among the S. bongori strains, including a unique CTAG tetranucleotide degeneration pattern in RKS3044, suggesting non-overlapping gene pools between RKS3044 and other S. bongori lineages/strains leading to their independent accumulation of genomic variations. We further proved the existence of a clear-cut genetic boundary between RKS3044 and the other S. bongori lineages/strains analyzed in this study.

CONCLUSIONS

The warm-blooded host-infecting S. bongori strain RKS3044 has diverged with distinct genomic features from other S. bongori strains, including a novel T6SS encoded in a previously not reported pathogenicity island-like structure and a unique genomic sequence degeneration pattern. These findings alert cautions about the emergence of new pathogens originating from non-pathogenic ancestors by acquiring specific pathogenic traits.

Nontyphoidal Salmonella purulent pericarditis presenting with pericardial tamponade in a patient on infliximab therapy

Infection with nontyphoidal Salmonella is traditionally characterized by intestinal manifestations. However, extra-intestinal infections are known to occur, with purulent pericarditis associated with cardiac tamponade being rare. This case report is of a 57-year-old male with Crohn's disease initiated on infliximab therapy two months prior to presentation. He presented with recurrent chest pain and a single occurrence of fever. A Computed Tomography (CT) scan of the chest revealed a pericardial effusion. An echocardiogram confirmed the presence of the fluid with tamponade physiology, requiring immediate surgical decompression. The pericardial fluid culture grew Salmonella enterica, despite the patient having only a single episode of fever, disproportionate to the severity of the infection. Conceivably, the lack of systemic symptoms may be attributed to recent infliximab therapy. Upon conducting a literature review, immunosuppressive factors seem to play a significant role in nontyphoid Salmonella enterica pericardial effusion presenting with cardiac tamponade.

Draft Genome Sequences of Three Salmonella enterica Serovar 4,[5],12:i:- Strains and One S. enterica Serovar Typhimurium Strain, Isolated in Brazil

Draft genomes of three Salmonella enterica 4,[5],12:i:− (STi) strains isolated from human infections were obtained using Illumina sequencing. They were negative for the fljBA operon but positive for hin, and k-mer analyses revealed their identity as S. enterica 4,[5],12:i:− 08-1736 and S.

Draft genomes of three Salmonella enterica 4,[5],12:i:− (STi) strains isolated from human infections were obtained using Illumina sequencing. They were negative for the fljBA operon but positive for hin, and k-mer analyses revealed their identity as S. enterica 4,[5],12:i:− 08-1736 and S. Typhimurium. A draft S. Typhimurium sequence is described for comparison.

Campylobacteriosis, Salmonellosis, Yersiniosis, and Listeriosis as Zoonotic Foodborne Diseases: A Review

Zoonoses are diseases transmitted from animals to humans, posing a great threat to the health and life of people all over the world. According to WHO estimations, 600 million cases of diseases caused by contaminated food were noted in 2010, including almost 350 million caused by pathogenic bacteria. Campylobacter, Salmonella, as well as Yersinia enterocolitica and Listeria monocytogenes may dwell in livestock (poultry, cattle, and swine) but are also found in wild animals, pets, fish, and rodents. Animals, often being asymptomatic carriers of pathogens, excrete them with faeces, thus delivering them to the environment. Therefore, pathogens may invade new individuals, as well as reside on vegetables and fruits. Pathogenic bacteria also penetrate food production areas and may remain there in the form of a biofilm covering the surfaces of machines and equipment. A common occurrence of microbes in food products, as well as their improper or careless processing, leads to common poisonings. Symptoms of foodborne infections may be mild, sometimes flu-like, but they also may be accompanied by severe complications, some even fatal. The aim of the paper is to summarize and provide information on campylobacteriosis, salmonellosis, yersiniosis, and listeriosis and the aetiological factors of those diseases, along with the general characteristics of pathogens, virulence factors, and reservoirs.

Phenotypic and genotypic profile of clinical and animal multidrug-resistant Salmonella enterica isolates from Mexico

AIMS

The objective of this study was to obtain a phenotypic and genotypic profile of Salmonella enterica including multidrug-resistant (MDR) isolates from food-producing animals and clinical isolates, as well as their genetic relatedness in two different States of Mexico (Jalisco and State of Mexico).

METHODS AND RESULTS

A total of 243 isolates were evaluated in terms of antimicrobial resistance (AMR) and related genes through a disk diffusion method and PCR respectively; we found 16 MDR isolates, all of them harbouring the bla_CMY gene but not qnr genes, these isolates represent less than 10% of the collection. The pulsed-field gel electrophoresis revealed a higher genotypic similitude within isolates of State of Mexico than Jalisco.

CONCLUSIONS

A low percentage of Salmonella isolates were resistant to relevant antibiotics in human health, nevertheless, the AMR and involved genes were similar despite the different serovars and origin of the isolates.

SIGNIFICANCE AND IMPACT OF THE STUDY

This investigation provided an insight of the current status of AMR of Salmonella isolates in two States of Mexico and pinpoint the genes involved in AMR and their epidemiological relationship, the information could help to determine an adequate therapy in human and veterinary medicine.

Genetic Characterization of the Galactitol Utilization Pathway of Salmonella enterica Serovar Typhimurium

Galactitol degradation by salmonellae remains underinvestigated, although this metabolic capability contributes to growth in animals (R. R. Chaudhuri et al., PLoS Genet 9:e1003456, 2013, https://doi.org/10.1371/journal.pgen.1003456). The genes responsible for this metabolic capability are part of a 9.6-kb gene cluster that spans from gatY to gatR (STM3253 to STM3262) and encodes a phosphotransferase system, four enzymes, and a transporter of the major facilitator superfamily. Genome comparison revealed the presence of this genetic determinant in nearly all Salmonella strains. The generation time of Salmonella enterica serovar Typhimurium strain ST4/74 was higher in minimal medium with galactitol than with glucose. Knockout of STM3254 and gatC resulted in a growth-deficient phenotype of S Typhimurium, with galactitol as the sole carbon source. Partial deletion of gatR strongly reduced the lag phase of growth with galactitol, whereas strains overproducing GatR exhibited a near-zero growth phenotype. Luciferase reporter assays demonstrated strong induction of the gatY and gatZ promoters, which control all genes of this cluster except gatR, in the presence of galactitol but not glucose. Purified GatR bound to these two main gat gene cluster promoters as well as to its own promoter, demonstrating that this autoregulated repressor controls galactitol degradation. Surface plasmon resonance spectroscopy revealed distinct binding properties of GatR toward the three promoters, resulting in a model of differential gat gene expression. The cyclic AMP receptor protein (CRP) bound these promoters with similarly high affinities, and a mutant lacking crp showed severe growth attenuation, demonstrating that galactitol utilization is subject to catabolite repression. Here, we provide the first genetic characterization of galactitol degradation in Salmonella, revealing novel insights into the regulation of this dissimilatory pathway.

IMPORTANCE

The knowledge of how pathogens adapt their metabolism to the compartments encountered in hosts is pivotal to our understanding of bacterial infections. Recent research revealed that enteropathogens have adapted specific metabolic pathways that contribute to their virulence properties, for example, by helping to overcome limitations in nutrient availability in the gut due to colonization resistance. The capability of Salmonella enterica serovar Typhimurium to degrade galactitol has already been demonstrated to play a role in vivo, but it has not been investigated so far on the genetic level. To our knowledge, this is the first molecular description of the galactitol degradation pathway of a pathogen.

Efficient and Specific Detection of Salmonella in Food Samples Using a stn-Based Loop-Mediated Isothermal Amplification Method

The Salmonella enterotoxin (stn) gene exhibits high homology among S. enterica serovars and S. bongori. A set of 6 specific primers targeting the stn gene were designed for detection of Salmonella spp. using the loop-mediated isothermal amplification (LAMP) method. The primers amplified target sequences in all 102 strains of 87 serovars of Salmonella tested and no products were detected in 57 non-Salmonella strains. The detection limit in pure cultures was 5 fg DNA/reaction when amplified at 65°C for 25 min. The LAMP assay could detect Salmonella in artificially contaminated food samples as low as 220 cells/g of food without a preenrichment step. However, the sensitivity was increased 100-fold (~2 cells/g) following 5 hr preenrichment at 35°C. The LAMP technique, with a preenrichment step for 5 and 16 hr, was shown to give 100% specificity with food samples compared to the reference culture method in which 67 out of 90 food samples gave positive results. Different food matrixes did not interfere with LAMP detection which employed a simple boiling method for DNA template preparation. The results indicate that the LAMP method, targeting the stn gene, has great potential for detection of Salmonella in food samples with both high specificity and high sensitivity.