Preface

*Salmonella* spp. is a global pathogen responsible for millions of deaths each year. *Salmonella* is a complex genus comprising two species, S. enterica and S. bongori, and more than 2600 different serotypes. S. enterica is composed of six different subspecies: enterica (I), salamae (II), arizonae (IIIa), diarizonae (IIIb), houtenae (IV), and indica (VI). Serotypes of S. enterica subsp. enterica are responsible for more than 99% of human infections. Another way to classify *Salmonella* is according to the type of disease it causes. In general terms, there is typhoid fever caused by S. Typhi and infections caused by non-typhoidal *Salmonella*.

Typhoid fever is caused by the ingestion of water or food contaminated with fecal material carrying S. Typhi. As described by Al-Khafaji et al. in Chapter 1, a tool of mechanisms is responsible for the virulence of S. Typhi. The Vi capsule antigen can inhibit phagocytosis and complement C3. This capsule decreases the recognition of somatic antigens by antibodies. Flagella also contribute to virulence by interacting with host epithelial cells, macrophages, and immune evasion. S. Typhi also encodes in its genome some pathogenic island with the genes necessary to invade the host and effectively evade the immune system. Although typhoid fever is distributed worldwide, improvements in water supply and sewerage systems have resulted in a decreased incidence.

In low- and middle-income countries where sanitary conditions continue to be a problem, typhoid fever is still highly prevalent. In developed countries, typhoid fever is a travel-related disease. It is estimated that there are 26.9 million S. Typhi infections annually. However, these data are misleading. The fact that this disease presents non-specific symptoms along with the lack of diagnostic tests and underreporting of cases in some regions of the world suggests that the real prevalence of this disease is much greater. In Chapter 2, Sado and Sado describe the importance of enteric fever diagnosis in primary care. The development of nonspecific symptoms complicates disease diagnosis. It is therefore important to follow a series of criteria established by health authorities to facilitate diagnosis.

Prevention is undoubtedly the key and vaccines can certainly play a key role in this regard. Although there are some vaccines commercially available, their efficacy can still be improved. In Chapter 3, Rachmawati et al. provide an overview of the in silico approach for the S. Typhi epitope vaccine. There are three types of S. Typhi vaccines: live-attenuated, inactivated, and sub-unit vaccines. The authors further elaborate on the steps to be followed for developing a new type of vaccine using only the part of the subunit that is recognized by B and T cells of the immune system, the epitope area. The development of bioinformatic tools, omic technologies, and recombinant DNA technologies give these types of vaccines enormous potential. In Chapter 4, Mishra et al. describe how computational tools can help in the development of effective vaccines against multidrug-resistant S. enterica strains.

Nontyphoidal *Salmonella* primarily causes gastroenteritis, bacteremia, and focal infection, mainly related to the consumption of food contaminated with this pathogen. Livestock, especially poultry, can carry *Salmonella* in their gut without symptoms. Therefore, *Salmonella* can contaminate products produced from these animals, move through the food chain, and reach the consumer. In Chapter 5, Pandey and Goud highlight the importance of control measures to prevent the spread of this pathogen. Another cause for concern is the increase in antibiotic resistance. The Exponential increase in antibiotic resistance in recent years has resulted in increased hospitalization and deaths. In Chapter 6, Al-Hamadany describes how *Salmonella* must evade the host immune system to survive. The main serotypes involved in human cases of salmonellosis are S. Typhimurium and S. Enteritidis. The virulence and invasiveness of these and other serotypes are mainly related to the presence of *Salmonella* Pathogenicity Islands (SPIs), as discussed in Chapter 7 by Sarika. In these SPIs are located the main genes involved in invasion, survival, and extra-intestinal spread. These SPIs are also of great interest from an evolutionary point of view as they can be acquired by horizontal transmission. SPI1 and SPI2 are the main types of pathogenicity islands, and they encode the main virulent genes and Type III secretor systems necessary for host cell invasion. However, in *Salmonella* there are more than 10 different SPIs, some of which are specific to certain subspecies.

S. Enteritidis is one of the main serotypes causing infections in humans. As reviewed in Chapter 8 by Ogunremi et al., it is important to characterize the S. Enteritidis strain implicated in food salmonellosis. Although pulsed-field gel electrophoresis (PFGE) is a widely used typing tool, its discriminatory capacity in S. Enteritidis is low due to the clonality of this serotype. In the genomic era, whole genome sequencing is an attractive tool for a full and comprehensive characterization of the genetic attributes of bacteria. Despite massive information obtained with this technology, this approach is not useful for generating a useful nomenclature-based description of S. Enteritidis subtypes. In this sense, the characterization of 60 polymorphic loci by a single nucleotide-based genotypic polymerase chain reaction assay (SNP-PCR) allowed to define 25 circulating clades of S. Enteritidis. This approach is an ideal subtyping test, being highly discriminatory, low cost, rapid, and reproducible. It is useful to identify the subtype designation of an isolate for outbreak surveillance.

Non-enterica subspecies of S. enterica are mainly related to cold-blooded animals. However, it has been observed that these subspecies can also colonize warm-blooded animals. For example, in Chapter 9, Rubira et al. describe the adaption of S. enterica subsp. diarizonae serotype 61: k: 1,5, (7) to sheep. This serotype has become highly prevalent in sheep herds in Sweden, Norway, Switzerland, and the United Kingdom. In this sense, most animals are asymptomatic carriers and rarely show clinical symptoms. Studies carried out in slaughterhouses have isolated this serotype from the intestinal content and respiratory track of healthy animals. Rarely these S. enterica subsp. diarizonae serotype 61: k: 1,5, (7) cause health disorders such as chronic proliferative abortions, testicular lesions in rams, or tract disorders in young animals. This serotype is endemic in sheep herds in Sweden, with 40% of large herds being positive for this bacterium. In addition, 1.8% of sheep carcasses of the largest slaughterhouse in Sweden were positive for this serotype. This could suggest that there is a direct link between the consumption of sheep products and cases of human salmonellosis caused by S. enterica subsp. diarizonae serotype 61: k: 1,5, (7). However, the number of reported cases due to this serotype is residual. These data highlight the low pathogenicity of this serotype. Consequently, an exception for this serotype was made in the Swedish *Salmonella* control program. However, the pathogenicity of this serotype could change over time and an increase of human salmonellosis cases due to S. enterica subsp. diarizonae serotype 61: k: 1,5, (7) should result in the revision of this exception. A similar situation is observed with serotype S. enterica subsp. salamae 4, [5], 12:b:- (commonly known as S. Sofia) in Australia broiler flocks. This serotype has also become highly prevalent, but no direct relationship can be established with

**V**

an increase in cases of human salmonellosis due to this bacterium. This study reflects the adaption of non-enterica serotypes of S. enterica to livestock. Comparative genomic studies are necessary to fully understand the potential pathogenicity of these serotypes and if they can be considered as commensal microorganisms.

Antimicrobial resistance is one of the main challenges of global public health. The indiscriminatory use of antimicrobials in both humans and animals has increased the number of resistant bacteria exponentially. For that reason, it is necessary to make rational use of antibiotics and to find new alternatives to them. The food production chain is one of the points where it is important to reduce the use of antibiotics. The transmission of multidrug-resistant foodborne pathogens from food to consumers is one of the main areas of concern. In the last years, a great effort has been made to introduce new control measures from farm to fork.

Bacteriophages (phages) are prokaryotic viruses that can infect and kill bacterial pathogens. Phages were discovered in the beginning of the 20th century, but due to the discovery of antibiotics they were relegated to the background during the subsequent decade. However, the need for antibiotic alternatives has resulted in the rediscovery of phages. In Chapter 10, Thanki et al. describe the application of phages in different points of the poultry and swine production chain. They can be applied to farm animals through feed or water, in slaughterhouses to reduce *Salmonella* load in chicken and swine skin, or in food packaging to inhibit the growth of this pathogen during storage. There are some commercially available *Salmonella* phage cocktails approved by the US Food and Drug Administration to be used in the food chain. However, phage therapy needs to overcome some challenges such as host range, resistance development, and phage delivery. Tools like genetic engineering and phage encapsulation could help to solve the actual limitations of this antibiotic alternative.

In Chapter 11, Ruiz-Pérez et al. describe the potential of natural products to control the growth of *Salmonella* in the food production chain. Different bacteria-, fungus-, animal-, and plant-derived products have been tested against different foodborne pathogens such as *Salmonella*. Some of them have shown promising results, but commercial production and application of these products is still a limiting factor. Biofilm formation is another problem associated with *Salmonella* in the food production chain. In Chapter 12, Lamas et al. explain that bacteriocins and phages can be applied to combat biofilms. However, they also have some limitations to kill biofilm cells, mainly due to protection offered by the characteristic extracellular

This book offers a global vision of the *Salmonella* genus and its implications for human health, from host-specific S. Typhi to serotypes transmitted through the food chain. The guest editors would like to thank the editorial team for their

**Alexandre Lamas, Patricia Regal and Carlos Manuel Franco**

Department of Analytical Chemistry,

Universidade de Santiago de Compostela,

Nutrition and Bromatology,

Lugo, Spain

substances produced in this type of bacterial life.

invaluable assistance.

an increase in cases of human salmonellosis due to this bacterium. This study reflects the adaption of non-enterica serotypes of S. enterica to livestock. Comparative genomic studies are necessary to fully understand the potential pathogenicity of these serotypes and if they can be considered as commensal microorganisms.

Antimicrobial resistance is one of the main challenges of global public health. The indiscriminatory use of antimicrobials in both humans and animals has increased the number of resistant bacteria exponentially. For that reason, it is necessary to make rational use of antibiotics and to find new alternatives to them. The food production chain is one of the points where it is important to reduce the use of antibiotics. The transmission of multidrug-resistant foodborne pathogens from food to consumers is one of the main areas of concern. In the last years, a great effort has been made to introduce new control measures from farm to fork.

Bacteriophages (phages) are prokaryotic viruses that can infect and kill bacterial pathogens. Phages were discovered in the beginning of the 20th century, but due to the discovery of antibiotics they were relegated to the background during the subsequent decade. However, the need for antibiotic alternatives has resulted in the rediscovery of phages. In Chapter 10, Thanki et al. describe the application of phages in different points of the poultry and swine production chain. They can be applied to farm animals through feed or water, in slaughterhouses to reduce *Salmonella* load in chicken and swine skin, or in food packaging to inhibit the growth of this pathogen during storage. There are some commercially available *Salmonella* phage cocktails approved by the US Food and Drug Administration to be used in the food chain. However, phage therapy needs to overcome some challenges such as host range, resistance development, and phage delivery. Tools like genetic engineering and phage encapsulation could help to solve the actual limitations of this antibiotic alternative.

In Chapter 11, Ruiz-Pérez et al. describe the potential of natural products to control the growth of *Salmonella* in the food production chain. Different bacteria-, fungus-, animal-, and plant-derived products have been tested against different foodborne pathogens such as *Salmonella*. Some of them have shown promising results, but commercial production and application of these products is still a limiting factor. Biofilm formation is another problem associated with *Salmonella* in the food production chain. In Chapter 12, Lamas et al. explain that bacteriocins and phages can be applied to combat biofilms. However, they also have some limitations to kill biofilm cells, mainly due to protection offered by the characteristic extracellular substances produced in this type of bacterial life.

This book offers a global vision of the *Salmonella* genus and its implications for human health, from host-specific S. Typhi to serotypes transmitted through the food chain. The guest editors would like to thank the editorial team for their invaluable assistance.

> **Alexandre Lamas, Patricia Regal and Carlos Manuel Franco** Department of Analytical Chemistry, Nutrition and Bromatology, Universidade de Santiago de Compostela, Lugo, Spain

Section 1

An Overview of *Salmonella Typhi* and Vaccine Development
