Pigeon-associated strains of Salmonella enterica serovar Typhimurium phage type DT2 have genomic rearrangements at rRNA operons

Increased phage resistance through lysogenic conversion accompanying emergence of monophasic Salmonella Typhimurium ST34 pandemic strain

Salmonella enterica serovar Typhimurium (S. Typhimurium) comprises a group of closely related human and animal pathogens that account for a large proportion of all Salmonella infections globally. The epidemiological record of S. Typhimurium in Europe is characterized by successive waves of dominant clones, each prevailing for approximately 10-15 years before replacement. Succession of epidemic clones may represent a moving target for interventions aimed at controlling the spread and impact of this pathogen on human and animal health. Here, we investigate the relationship of phage sensitivity and population structure of S. Typhimurium using data from the Anderson phage typing scheme. We observed greater resistance to phage predation of epidemic clones circulating in livestock over the past decades compared to variants with a restricted host range implicating increased resistance to phage in the emergence of epidemic clones of particular importance to human health. Emergence of monophasic S. Typhimurium ST34, the most recent dominant multidrug-resistant clone, was accompanied by increased resistance to phage predation during clonal expansion, in part by the acquisition of the mTmII prophage that may have contributed to the fitness of the strains that replaced ancestors lacking this prophage.

Salmonella serotype assignment by sequencing analysis of intergenic regions of ribosomal RNA operons

Salmonella laboratorial detection is usually carried out by bacteriological culture and serological methods. Salmonella isolates are then classified into more than 2,650 serotypes with different somatic (O) and flagellar (H) antigenic combinations. More recently, DNA analysis methods were developed and applied for the identification of Salmonella serotypes, including intergenic spacer regions (ISRs) that separates DNA-encoding ribosomal subunits (rRNA gene) in Salmonella genomes. The present study aimed to evaluate the nucleotide diversity of the ISRs in 2 rRNA operons (rrnB and rrnH) for the assignment of Salmonella serotypes. A total of 63 Salmonella isolates (bacterial cultures) from 21 serotypes were obtained in the period of 2014 to 2017. DNA was extracted, and PCRs were used to detect the genus Salmonella and 4 important serotypes: Enteritidis, Gallinarum, Heidelberg, and Typhimurium. ISRs of the operons rrnB and rrnH were amplified by PCR and then sequenced. All sequence results were submitted to BLASTn search and were aligned in comparison to 72 Salmonella reference nucleotide sequences. The results demonstrated that 60 (95.2%) samples returned a sequence of the same serotype (determined by the traditional serological procedure) after searching in BLASTn and/or in the alignment with the reference sequences for both operons (rrnB and rrnH). These PCR-sequencing procedures had a general agreement index of 0.792 based on the Kappa analysis, 98.7% sensitivity value, 100% specificity, and 98.4% accuracy. Three different phylogenetic trees were generated from the alignments with the sequences of rrnH, rrnB, and rrnH plus rrnB and isolates clustered in specific branches according to the serotypes.

Infection, colonization and shedding of Campylobacter and Salmonella in animals and their contribution to human disease: A review

Livestock meat and offal contribute significantly to human nutrition as sources of high-quality protein and micronutrients. Livestock products are increasingly in demand, particularly in low- and middle-income settings where economies are growing and meat is increasingly seen as an affordable and desirable food item. Demand is also driving intensification of livestock keeping and processing. An unintended consequence of intensification is increased exposure to zoonotic agents, and a contemporary emerging problem is infection with Campylobacter and Salmonella spp. from livestock (avian and mammalian), which can lead to disease, malabsorption and undernutrition through acute and chronic diarrhoea. This can occur at the farm, in households or through the food chain. Direct infection occurs when handling livestock and through bacteria shed into the environment, on food preparation surfaces or around the house and surroundings. This manuscript critically reviews Campylobacter and Salmonella infections in animals, examines the factors affecting colonization and faecal shedding of bacteria of these two genera as well as risk factors for human acquisition of the infection from infected animals or environment and analyses priority areas for preventive actions with a focus on resource-poor settings.

Host specificity of bacterial pathogens

Most pathogens are able to infect multiple hosts but some are highly adapted to a single-host species. A detailed understanding of the basis of host specificity can provide important insights into molecular pathogenesis, the evolution of pathogenic microbes, and the potential for pathogens to cross the species barrier to infect new hosts. Comparative genomics and the development of humanized mouse models have provided important new tools with which to explore the basis of generalism and specialism. This review will examine host specificity of bacterial pathogens with a focus on generalist and specialist serovars of Salmonella enterica.

Genetic characterization of atypical Citrobacter freundii

The ability of a bacterial population to survive in different niches, as well as in stressful and rapidly changing environmental conditions, depends greatly on its genetic content. To survive such fluctuating conditions, bacteria have evolved different mechanisms to modulate phenotypic variations and related strategies to produce high levels of genetic diversity. Laboratories working in microbiological diagnosis have shown that Citrobacter freundii is very versatile in its colony morphology, as well as in its biochemical, antigenic and pathogenic behaviours. This phenotypic versatility has made C. freundii difficult to identify and it is frequently confused with both Salmonella enterica and Escherichia coli. In order to determine the genomic events and to explain the mechanisms involved in this plasticity, six C. freundii isolates were selected from a phenotypic variation study. An I-CeuI genomic cleavage map was created and eight housekeeping genes, including 16S rRNA, were sequenced. In general, the results showed a range of both phenotypes and genotypes among the isolates with some revealing a greater similarity to C. freundii and some to S. enterica, while others were identified as phenotypic and genotypic intermediary states between the two species. The occurrence of these events in natural populations may have important implications for genomic diversification in bacterial evolution, especially when considering bacterial species boundaries. In addition, such events may have a profound impact on medical science in terms of treatment, course and outcomes of infectious diseases, evading the immune response, and understanding host-pathogen interactions.

Intracontinental spread of human invasive Salmonella Typhimurium pathovariants in sub-Saharan Africa

A highly invasive form of non-typhoidal Salmonella (iNTS) disease has recently been documented in many countries in sub-Saharan Africa. The most common Salmonella enterica serovar causing this disease is Typhimurium (Salmonella Typhimurium). We applied whole-genome sequence-based phylogenetic methods to define the population structure of sub-Saharan African invasive Salmonella Typhimurium isolates and compared these to global Salmonella Typhimurium populations. Notably, the vast majority of sub-Saharan invasive Salmonella Typhimurium isolates fell within two closely related, highly clustered phylogenetic lineages that we estimate emerged independently ∼52 and ∼35 years ago in close temporal association with the current HIV pandemic. Clonal replacement of isolates from lineage I by those from lineage II was potentially influenced by the use of chloramphenicol for the treatment of iNTS disease. Our analysis suggests that iNTS disease is in part an epidemic in sub-Saharan Africa caused by highly related Salmonella Typhimurium lineages that may have occupied new niches associated with a compromised human population and antibiotic treatment.

Chromosomal rearrangements in Salmonella enterica serovar Typhi strains isolated from asymptomatic human carriers

Host-specific serovars of Salmonella enterica often have large-scale chromosomal rearrangements that occur by recombination between rrn operons. Two hypotheses have been proposed to explain these rearrangements: (i) replichore imbalance from horizontal gene transfer drives the rearrangements to restore balance, or (ii) the rearrangements are a consequence of the host-specific lifestyle. Although recent evidence has refuted the replichore balance hypothesis, there has been no direct evidence for the lifestyle hypothesis. To test this hypothesis, we determined the rrn arrangement type for 20 Salmonella enterica serovar Typhi strains obtained from human carriers at periodic intervals over multiple years. These strains were also phage typed and analyzed for rearrangements that occurred over long-term storage versus routine culturing. Strains isolated from the same carrier at different time points often exhibited different arrangement types. Furthermore, colonies isolated directly from the Dorset egg slants used to store the strains also had different arrangement types. In contrast, colonies that were repeatedly cultured always had the same arrangement type. Estimated replichore balance of isolated strains did not improve over time, and some of the rearrangements resulted in decreased replicore balance. Our results support the hypothesis that the restricted lifestyle of host-specific Salmonella is responsible for the frequent chromosomal rearrangements in these serovars.

Salmonella typhimurium phage typing for pathogens

Phage typing provides a rapid, accurate, and cheap method of investigating Salmonella strains for epidemiological use. Salmonella strains within a particular serovar may be differentiated into a number of phage types by their pattern of susceptibility to lysis by a set of phages with different specificity. Characterization based on the pattern of phage lysis of wild strains isolated from different patients, carriers, or other sources is valuable in epidemiological study. The phages must have well-defined propagation strains that allow reproducible discrimination between different Salmonella Typhimurium strains. Different schemes have been developed for this serovar in different countries. The Felix/Callow (England) and Lilleengen typing systems (Sweden) used for laboratory-based epidemiological analysis were helpful for control of salmonellosis. More recently, the extended phage-typing system of Anderson (England) that distinguishes more than 300 definitive phage types (DTs) has been used worldwide in Europe, the United States, and Australia. The use of this method for decades show us that some phage types (DT204 in the 1970s and DT104 in the 1990s) have a broad host range and are distributed worldwide, other phage types such as DT2 or DT99 are frequently associated with disease in pigeons, indicative of a narrow host range.

Linkage of avian and reproductive tract tropism with sequence divergence adjacent to the 5S ribosomal subunit rrfH of Salmonella enterica

The 183 bp between the end of the 23S rrlH rRNA gene and the start of the 5S rrfH rRNA gene (ISR-1) and the 197 bp between the end of the rrfH rRNA gene and the start of the transfer RNA aspU (ISR-2) of Salmonella enterica ssp. enterica serotypes Enteritidis, Typhimurium, Pullorum, Heidelberg, Gallinarum, Typhi and Choleraesuis were compared. ISR-1s of D1 serotypes (Pullorum, Gallinarum and Enteritidis), B serotypes (Typhimurium and Heidelberg) and the C2 serotype Newport and the enteric fever pathogens serotype A Paratyphi and serotype D1 Typhi formed three clades, respectively. ISR-2 further differentiated the avian-adapted serotype Gallinarum from avian-adapted Pullorum and Salmonella bongori from S. enterica. The results suggest that serotypes Heidelberg and Choleraesuis share some evolutionary trends with egg-contaminating serotypes. In addition, ISR-1 and ISR-2 sequences that confirm serotype appear to be linked to clinically relevant host associations of the Salmonellae.

The genome of Salmonella enterica serovar gallinarum: distinct insertions/deletions and rare rearrangements

Salmonella enterica serovar Gallinarum is a fowl-adapted pathogen, causing typhoid fever in chickens. It has the same antigenic formula (1,9,12:--:--) as S. enterica serovar Pullorum, which is also adapted to fowl but causes pullorum disease (diarrhea). The close relatedness but distinct pathogeneses make this pair of fowl pathogens good models for studies of bacterial genomic evolution and the way these organisms acquired pathogenicity. To locate and characterize the genomic differences between serovar Gallinarum and other salmonellae, we constructed a physical map of serovar Gallinarum strain SARB21 by using I-CeuI, XbaI, and AvrII with pulsed-field gel electrophoresis techniques. In the 4,740-kb genome, we located two insertions and six deletions relative to the genome of S. enterica serovar Typhimurium LT2, which we used as a reference Salmonella genome. Four of the genomic regions with reduced lengths corresponded to the four prophages in the genome of serovar Typhimurium LT2, and the others contained several smaller deletions relative to serovar Typhimurium LT2, including regions containing srfJ, std, and stj and gene clusters encoding a type I restriction system in serovar Typhimurium LT2. The map also revealed some rare rearrangements, including two inversions and several translocations. Further characterization of these insertions, deletions, and rearrangements will provide new insights into the molecular basis for the specific host-pathogen interactions and mechanisms of genomic evolution to create a new pathogen.