Salmonella Infection and Pathogenesis

Abstract

Salmonella genus represents most common food borne pathogens isolated from food producing animals and is responsible for causing zoonotic infections in humans and other animal species, including birds. As a result, Salmonella diseases are among the most common problems for the humans, animals, and food industry around the world. Despite rising attention about other pathogens, Salmonella continues to be the most prominent cause of food borne disease worldwide. Salmonella can be transferred to humans at any point along the farm-to-fork chain, most commonly through infected animal-derived foods such as poultry and poultry related products (eggs), pork, fish, and so on. Some Salmonella serotypes have been confined to a single serovar and are known as “host-restricted” while the others have a wide host spectral range and are known as “host-adapted” serotypes. Globally Salmonella infection causes huge mortality and the infection plays a huge role in immune response by evolving multiple mechanism to subvert immunity to its own benefit. Numerous infectivity markers and determinants have indeed been reported to play essential role in Salmonella pathogenesis to colonize its host by invading and avoiding the host’s intestinal shielding system.

1. Introduction

Salmonella is a species in the genus with worldwide public health implications and is the major cause of foodborne disease, accounting for deaths of thousands of people worldwide [1, 2, 3, 4, 5, 6, 7, 8, 9]. Salmonella is anaerobic in nature and is a Gram-negative, rod-shaped bacterium belonging to the Enterobacteriaceae family. Salmonella is divided into two species: Salmonella enterica and Salmonella bongori. More than 2600 S. enterica serovars have been defined so far, with most of these serotypes likely to cause diseases in both humans and animals [10], whereas a few S. enterica variants, such as Salmonella Gallinarum (SG) and Salmonella Pullorum (SP), are non-flagellated and non-motile, the large percentage of Salmonella members are motile by peritrichous flagella. The SG and SP are linked to clinical disease in poultry and cause significant economic losses to poultry farming, particularly in developing countries [11, 12, 13]. According to recent data from the United States, Europe, and Low and Middle Income Countries (LMICs), Salmonella is frequently occurring international cause of foodborne disease. Salmonella also enhances food contamination in many natural environments [14]. Salmonella enteric found in the gut of food animals more persistently, is characterized by chronic transmitters which remove the bacterium with their own fecal matter. As a result, these carriers act as a reservoir for future bacterial contamination, allowing Salmonella to spread through infected milk, meat, eggs, and other agricultural products fertilized and developed in Salmonella-infested manure [14]. Salmonella have been isolated from variety of animals and their food products. These include poultry, ovine, porcine, bovine, lizards and snakes (Figure 1). This book chapter attempts to discuss different aspects of Salmonella serovars and Salmonella infection in different animals, with special emphasis to understand the mechanism of its pathogenesis.

Figure 1.

Sources of Salmonella enterica.

2. Brief history, morphology, physical and biochemical characteristics

In mid of nineteenth century, Salmonella was first reported by Eberth, which was followed by Gaffky who isolated and demonstrated that Bacillus causes human typhoid fever [15]. In 1885, Theobald Smith and Daniel Elmer Salmon from the gut of pigs isolated Bacillus infected with swine fever (hog cholera) [15, 16]. An American pathologist, Dr. Daniel Elmeri Salmon, in collaboration with Smith gave the name Salmonella [17]. Most reference centres of Salmonella all over the world, including Centers for Disease Control (CDC), use Salmonella nomenclature system of World Health Organization (WHO) [18].

Salmonella are anaerobic, chemo-organotrophic, rod-shaped with size 0.2–1.5 × 2–5 μm and are Gram negative in nature [19]. Except a few serovars viz S. choleraesuis, all other members of this genus produce hydrogen sulphide and majority of them do not perform lactose fermentation [20]. This crucial trait has been used to produce a number of selective and differential media for Salmonella culture, isolation, and presumptive identification. Salmonella-Shigella agar (SS), brilliant green agar (BGA), xylose lysine deoxycholate (XLD) agar, Hektoen enteric (HE) agar, MacConkey agar, lysine iron agar (LIA), and triple sugar iron (TSI) agar are among the media frequently used [21, 22].

Salmonella is non-fastidious, as outside the living hosts it can grow and multiply in a variety of environments. Salmonella is heat-sensitive, and is frequently killed at temperatures of 70°C or above. The majority of serotypes thrive and grow in temperatures ranging from 5 to 47°C with an optimum of 32 to 35°C. Few serotypes, however, may thrive at temperatures as low as 2–4°C and as high as 54°C [23]. Salmonella grow at pH ranging from 4 to 9, with optimum range of 6.5–7.5. Salmonella require high water activity of about 0.99–0.94 for survival. At pH greater than 3.8, water activity greater than 0.94 and temperature higher than 70°C, it shows no growth [23]. While almost all serotypes do not make indole, hydrolyze urea, or deaminate phenylalanine or tryptophan, the majority of serotypes rapidly convert nitrate to nitrite, ferment a range of carbohydrates with acid production [20].

3. Salmonella nomenclature, taxonomy and serovars

The nomenclature system of Salmonella is a complex process. This genus is composed of two main species, Salmonella enterica and Salmonella bongori. Salmonella enteric is further divided into 06 subspecies on the basis of biochemical properties and genomic relatedness [24]. The subspecies are denoted by Roman numerals: I. S. enterica subsp. enterica; II. S. enterica subsp. salamae; III. S. enterica subsp. arizonae; IIIa. S. enterica subsp. diarizonae; IV. S. enterica subsp. houtenae; V. S. enterica subsp. indica. The S. enterica subsp. enterica (I) is most common subspecies of Salmonella and is found to be predominantly associated with around 99% of Salmonella infections in humans & warm blooded animals. The remaining 05 subspecies and S. bongori are mainly attributed to Salmonella infections in cold blooded animals and are rarely found in humans [25].

For serotypes in subspecies (I), CDC uses names i.e. Enteritidis, Typhimurium, Choleraesuis and Typhi while as for the unnamed serotypes described post 1966 antigenic formulae are used in subspecies II, IV, VI and S. bongori. The name generally refers to the location (geographic) where the serovar/serotype was isolated first. In order to avoid any confusion between species and serotype, the first letter of the named serotype is written in capital and is not italicized. At the first citation of a serotype, the genus name is given first, followed by the word “serotype” or abbreviated form “ser” and finally the serotype name is written. One of the examples is Salmonella serotype or ser. Typhimurium. Afterwards the genus name can be directly written followed by serotype name (e.g. Salmonella Typhimurium or S. Typhimurium [26, 27].

4. Salmonella infection

Infection with Salmonella causes morbidity and mortality all over the world, with the host immune response varied depending on whether the infection is acute or systemic. In addition to this, anatomical location of Salmonella infection plays a huge role in immune response as it evolves multiple mechanisms to subvert immunity to its own benefit.

4.1 In humans

S enterica subsp. enterica continues to be a leading source of disease in humans and livestock around the world. The transmission of pathogens caused a huge portion of public health and economic loss. As agricultural production began to increase after World War II, Salmonellosis become more prevalent in different countries as was the case with Europe. Despite the fact that the genus Salmonella contains over 2600 serovars, only 05–08 serovars cause the majority of human Salmonellosis cases in the United States. As per CDC, Salmonella enteric ser Enteritidis (24.7%), S. ser Typhimurium (23.5%), S. ser Newport (6.2%), and S. ser. Heidelberg were responsible for approximately 60% of human cases. That year, 04 serotypes accounted for 46.4% of non-human isolates. Main reason for infection in humans and other mammals is S enterica which is responsible for 99% of overall infection [28]. Non-invasive non-typhoidal Salmonellosis, Invasive non-typhoidal Salmonellosis, and typhoid fever are the three principal diseases produced by Salmonella in humans, and these are all covered in greater depth below.

4.1.1 Non-invasive, non-typhoidal Salmonellosis

The non-typhoidal Salmonellosis (NTS) is associated with all the diseases of humans caused by Salmonella serotype except for the distinct typhoidal serotypes: Typhi and Paratyphi A-C. Salmonellosis is contracted orally through contaminated food or water. About 1.3 billion cases are reported annually of Salmonellosis gastroenteritis, causing huge mortality, approximately 03 million deaths globally [29]. According to the recent reports, NTS gastroenteritis is infecting developing countries. Acute enterocolitis is a symptom of Salmonellosis, and it is often followed by inflammatory diarrhea, which is only seen in people infected with invasive serovars (S. Typhi). The symptoms appear usually between 6 and 72 h. Primary symptoms of this disease are abdominal pain, diarrhea with or without blood, nausea, and vomiting.

4.1.2 Invasive non-typhoidal Salmonellosis

In Sub-Saharan Africa, a new Salmonella strain is emerging, with pathogenesis that is distinct from its genetic equivalents. This novel pathogen is known as Salmonella invasive non-typhoidal (iNTS). Salmonella serotypes S. Typhimurium and S. Enteritidis are the most typically connected with invasive NTS, however other serotypes such as Choleraesuis and Dublin have also been found to produce invasive illness in humans. [30, 31]. In Africa it has been found that invasive isolates have dominating genotype with several biological variations from the isolated strain (ST313) which proves that its genotype has surfaced new pathogenic clade in Sub-Saharan Africa and it may be the reason of invasive disease in humans [32]. In different parts of world other strains have also evolved which include S. Typhimurium ST313 strain, which gave an idea that this disease is spreading globally [33]. It was reported that iNTS are the main cause of bloodstream infections in African children [34]. Soon after the detection of AIDS in Africa, iNTS have also been reported in kids and adults and thus prompting a possible link between HIV and iNTS [34]. In New Jessey, first epidemiological link of iNTYS and AIDS was made with iNTS remained a prevalent bacterial bloodstream infection of kids and adults in Sub-Saharan Africa [32].

4.1.3 Typhoid fever

The main causative agent of Typhoid Fever is Salmonella Typhi. Every year about 21 million cases are being reported with almost 200,000 deaths globally. The yearly death rate increased by 39% from year 1990 to 2010 [35]. It has been reported that death rate caused by Salmonella Typhi in developing countries is comparatively similar to the death rate caused by breast cancer, prostate cancer, and leukemia in North America [36, 37]. Polysaccharide capsular agent allows S. Typhi to adapt to the acidic environment of stomach soon after infection as S. Typhi (acapsular) being less virulent [38, 39]. Unlike NTS, which has broad host specificity, S. Typhi is only found in humans. [40]. Salmonella Typhi inhabits and duplicates in host cells, these cells are used to translocate bacteria to liver, spleen and bone marrow. These cells include dendritic cells, neutrophils and macrophages [41].

4.2 In livestock

Salmonella infections can be seen in reptiles such as turtles, lizards, and snakes; birds like domestic pigeons and parrots; amphibians such as frogs and mammals such as dogs and cats. These infections are not frequent in small captive animals. Infection may be undetectable in reptiles, canines, and kittens although Salmonella could be identified in the stools of healthy animals. The guts of some animals can happily support these creatures which become the carrier animals of Salmonella. Diarrhea and enteritis are the common symptoms of Salmonellosis. Septicaemia can also be caused by Salmonella’s invasion in the host. This intrusion causes rise in body temperature, which is usually associated with Salmonella infection-induced enteritis. Drowsiness, loss of appetite and diarrhea are the clinical signs of Salmonella infection. The diarrhea could be severe, and typically domestic dogs and cats could become extremely ill and unknowingly pollute the residence. While in birds, this disease is seldom visible. However, animals or birds that are juvenile, aged, or weak may be badly harmed by the diarrhea-induced exhaustion. They develop sepsis and expire. Most of the affected organisms may experience diarrhea for a short period of time but the majority make a full recovery. Any recuperating animal can act as a vector of infection for a period of time. Salmonella can dwell in low numbers in the gastrointestinal system and lymphatic system, especially in locations like caecum of birds. Salmonella infection may recur if the organism develops another disease [42].

4.3 In domestic fowl and poultry

Salmonella causes four types of infections in poultry, all of which are serious: Pullorum serovars of S. enterica causes Pullorum disease, S. Gallinarum causes fowl typhoid, arizonae subspecies of S. enterica causes arizonosis [43] and several subspecies of Salmonella like S. Infantis, S. Enteritidis and S. Typhimurium cause paratyphoid. The unique S. enterica serovars Pullorum and Gallinarum seen in poultry have been largely eliminated from European and North American industries. Nonetheless, these serovars pose a greater hazard to avian safety and wellness in areas of the globe which have low industry development, particularly in areas with inadequate protection. Despite the fact that these two serovars of S. enterica are normally found in chicks, spontaneous occurrences caused by these serovars. Residential chicken are among the major reservoirs of this bacteria, posing a risk to human health through the intake of contaminated foods have indeed been reported in other birds like guinea fowl and turkeys.

Poultry products have been found as a major origin of Salmonellosis on numerous occasions. Across the year 2000, an approximate 182,060 Americans were sick with S. Enteritidis after eating tainted eggs [44]. During 1985 and 1999, eggs were blamed for about 80,010 S. Enteritidis cases in the United States [45]. In addition, consumption of infected chicken has been recognized as a major potential cause for S. Enteritidis transmission [46]. Several of the serovars which are frequent in humans are also abundant in poultry, demonstrating the relevance of livestock as a source for the spread of Salmonella in people [47]. Salmonella’s potential to infect chicken is highly linked to the transmitting serotype, as well as the maturity and genetic lineage of the bird. The disease caused by Gallinarum serovar of S. enterica, Fowl Typhoid (FT) spreads mostly through fecal-oral route [48]. There are also diseases mostly restricted to the gut caused by different Salmonella serovars in poultry [43]. Salmonellosis is the most common symptom of Typhimurium serovar infection in small birds. Fatality rates differ greatly, ranging from as low as 10% to as high as 80% in extreme cases.

4.4 In cattle

Salmonellosis is a leading cause of death and disease in livestock, some of which are commonly detected which are infected sub-clinically. As a result, cattle serve as a significant storehouse for diseases infecting humans. Several studies have been published during last decade with an emphasis on multi drug resistance variants and significance of Salmonella for food sector [49, 50]. Surprisingly, although extensive research was done on Salmonellosis, the infection and its associated risks remain un-addressed [43]. Salmonellosis is still a disease that affects livestock all over the world and is largely caused by the S. Dublin and S. Typhimurium. Additional serotypes have been linked to cattle infections on a stochastic basis [48]. Studies documented the identification of 101 distinct serotypes of Salmonella in cattle, most of which had a reduced incidence [43, 51]. In late 1960s, Salmonella infections in the livestock sector of Britain peaked with over 4000 cases reported in 1969 [48, 51]. Seven (07) serovars of Salmonella were found in 48% of the 730 isolated Salmonella from livestock in the United States [50]. There is a risk of novel strains being imported which was reported in United Kingdom as 10 Salmonella serovars were identified which were of non GB origin [43].

4.5 In pigs and sheep

S. enterica serovars Choleraesuis was first detected in swine when it was thought to be the causative agent of swine fever (hog cholera). The susceptibility of swine to Salmonella is determined by a number of parameters like the infecting serotypes and the pig’s age. Further, the incidence of Salmonellosis varies from region to region and is weakly linked to swine population, farming techniques, and their mixing [43]. Salmonella serovars linked to clinical illness in swine can be separated into two categories: Choleraesuis like host specific serovars and S. Typhimurium like ubiquitous. However, the presence of S. Choleraesuis has substantially decreased since then, and it is currently only spotted occasionally whereas Typhimurium still remains a severe threat to the swine sector especially in United States. Several serovars like Typhimurium, Copenhagen, Agona, Derby and Heidelberg were by far the most prevalent serovars in swine in the United States in the first decade of this century. Three of these serovars were isolated from humans during this time span [52]. During last 02 decades, research studies on other serovars have increased either due to improved surveillance or due to increased occurrence of infection.

Sheep Salmonellosis appears to be frequent in nations with considerable sheep population, including United Kingdom, Australia, New Zealand, and the United States of America. The seasonality of Salmonellosis spread and the incidence of diseases caused by widespread serovars is usually linked to sheep mobility and transportation [53, 54]. The Ovis strains of Serovar Abortus with restricted hosts are predicted to be introduced into diseased sheep flocks and spread via the fecal oral route [55]. However, there is no strong evidence that bacteria are transferred by drinking, nutrition, or the wastes of other hosts. Transmission of grazing livestock through the nasal channel may be possible due to many serovars causing pneumonia in lamb. Pulmonary discharge may transmit the bacteria to other animals.

4.6 In horses, dogs and cats

Salmonella Typhimurium was initially reported as the causative agent of Colitis in late 1910 and subsequently prevailed as a cause of Salmonellosis in horses throughout the world. Antibiotic use in conjunction with hospitalization stresses has been shown to have a significant impact on the horse’s sensitivity to Salmonella infection. The only host suitable for hooved animals is Salmonella Abortusequi, which causes horse paratyphoid disease. The surge and decline in prevalence of disease by distinct serovars has become a significant characteristic of the epidemiology of horse salmonellosis in the United States. This could lead to an increase in herd immunity and decrease in the pathogenicity of the individual serovar.

Salmonella infection in cats and dogs can be subclinical, with just occasional shedding. The infection fluctuates, ranging from moderate to severe gastritis, with the possibility of miscarriage, systemic dissemination, or sepsis [56]. Salmonella can be excreted over a month by healed animals, and persistent transmission with intervals of re-emergence is conceivable. Salmonella have the ability to propagate zoonotic infections and may play a role in the establishment of antibiotic resistance in bacteria [57]. The majority of the infestations were medically quiet, however some developed moderate diarrhea. Recent research has shown that dogs fed with uncooked meat can eliminate the bacterium in their stools for a longer period.

5. Salmonella pathogenesis

The favorable outcome of a pathogen is based on its capability to enter a host, evade host defense barrier and initiate infection. Salmonella has developed contrasting schedule to destabilize normal host cellular functions that allow it to get involved in and multiple inside the host cell. Depending upon the serotype of Salmonella involved and health status of human host, the acuteness of Salmonella infection varies. Elderly people, immune-suppression patients and children below 05 years of age are more prone to Salmonella infection. The ability of Salmonella to invade, replicate and remain alive within the human host makes it more morbific that finally results into harmful mortal disease.

Salmonella produces different virulence factors that play an important role in its pathogenicity. These involve (1) the potential to invade the cell (2) a perfect lipopolysaccharide coat (3) to replicate intra-cellularly and (4) feasibly the secretion of toxins [58]. The organisms establish a colony in ileum and colon after ingestion followed by occupying the intestinal epithelium and grows rapidly within the epithelium and lymphoid follicles. Salmonella invasion mechanism is partially understood. On epithelial cell surface there is the presence of specific receptors. When the organism incursion occurs, enterocyte membrane goes through disarrangement that results in pinocytosis of organism. Invasion depends on rearrangement of cell cytoskeleton and may be entailed to increase in cellular inositol phosphate and calcium. After invasion, organism has ability to proliferate intra-cellularly thereby escalating to mesenteric lymph nodes and all over the body by systematic circulation; absorbed by reticulo-endothelial cells that limits and checks the expansion of an organism. There is a perceptible genetic control involving multiple genes in both chromosomes and plasmids for attachment and invasion. Some organisms has the ability to infect liver, spleen, gall bladder, bone, meninges etc. depending upon the host defense. Human Salmonella (gastroenteritis) resides in intestine. However, most serotypes get perished on time. After invading the intestine, most of Salmonellae brings on an acute inflammatory response that may lead to ulceration, also they might elaborate cytotoxins that forbid protein synthesis. It is not clear if these cytotoxins play a role in the inflammatory response or ulceration. On the other hand, invasion of the mucosa induces epithelial cells to produce and release pro-inflammatory cytokines such as IL-1, IL-6, IL-8, TNF-2, IFN-U, MCP-1, and GM-CSF. These trigger an acute inflammatory response in the body and may also be accountable to harm the intestine [59]. Due to the inflammatory reaction, symptoms such as fever, chills, stomach pain, leukocytosis, and diarrhea are frequent. Polymorphonuclear leukocytes, blood, and mucus may be seen in the stool.

One of the features of Salmonella is non-phagocytic nature on human host cells during invasion [60], where it literally induces its own phagocytosis in order to gain access to its host cell. Salmonella pathogenicity islands (SPIs), gene clusters positioned at the major chromosomal DNA region and encoding for the structures required in the invasion activity, provide the remarkable genetics that enable this brilliant technique [61]. Bacteria tend to infiltrate the epithelial cells of the intestinal wall when they enter the digestive tract via contaminated water or food. Type III secretion systems, or SPIs, are multi-channel proteins that allow Salmonella to infuse its effectors into the cytoplasm via the intestinal epithelial cell membrane. The bacterial effectors subsequently activate the signal transduction pathway and lead the host cell’s actin cytoskeleton to be rebuilt, causing the epithelial cell membrane to ruffle outward and engulf the bacteria. The membrane ruffle’s morphology is similar to the process of phagocytosis [62].

The ability of the Salmonella strains to remain in the host cell is important for pathogens as strains lacking this capability are non-virulent [63]. After the host cell engulfs Salmonella, the bacterium is enclosed in a membrane compartment called a vacuole, which is formed of the host cell membrane. The presence of the bacterial foreign body activates the host cell immune response under normal circumstances, which result in the fusion of the lysosomes and the secretion of the digestive enzymes to break down the intracellular bacteria. Although, Salmonella uses the type III secretion system to inject other effector proteins into the vacuole, it causes the modification of the compartment structure. The re-assembled vacuole obstructs the fusion of the lysosomes and this allows the intracellular survival and replication of the bacteria inside the host cells. The ability of the bacteria to continue within macrophages allows them to be carried in the reticulo-endothelial system (RES) [64].

The mechanisms of Salmonella gastroenteritis and diarrhea are well known now. Only strains that infiltrate the intestinal mucosa are associated with the appearance of an acute inflammatory reaction and diarrhea; the secretion of fluid and electrolytes by the small and large intestines causes the diarrhea. Even though, the secretion is not just an indication of tissue destruction and ulceration, the mechanisms of secretion are indistinct. Unlike Shigella and invasive Escherichia coli, Salmonella infiltrates the intestinal epithelial cells but, do not escape the phagosome. Therefore, the extent of intercellular spread and ulceration of the epithelium is much less. From the basal side of epithelial cells, Salmonella escapes into the lamina propria. Systemic spread of the organisms can occur that causes the enteric fever. Following the invasion of the intestinal mucosa, activation of mucosal adenylate cyclase occurs; that results in the increase in cyclic AMP that causes secretion. It is not understood that by which mechanism adenylate cyclase is stimulated; it might involve local production of prostaglandins or other components of the inflammatory reaction.

6. Conclusion

Globally, Salmonellosis is the main cause of bacterial disease in all living creatures. All over the world it is posing very serious public health concerns and compromising the yield and output of animal husbandry production. The effort of isolating, identifying, and reporting Salmonella serotypes must continue for diagnostic, therapeutic, and public health objectives, despite the fact that the nomenclature for Salmonella is constantly evolving and the argument over the naming for the type species is still ongoing. Salmonella outbreaks have been linked to a variety of foods, and researchers are scrambling to figure out how this infection impacts humans and animals. This infection is a leading source of morbidity and mortality worldwide, with the host immune response differing depending on nature of infection. The genetic makeup of Salmonella made it possible for its strains to adapt to different environmental conditions. The implications of this infectious disease in humans vary depending on its serotype and the health level of the human host. Thus for better understanding the genetics of Salmonella and to investigate the mechanisms that contribute to pathogenesis evolution, a lot of work has been done. Occurrence of two different, potentially complementary evolutionary approaches to host range and virulence were evaluated using genome sequencing of Salmonella serovars. It includes horizontal gene transfer, gene loss which actually affects its ability to colonize. Gene acquisition by horizontal transfer (associated with SPIs, transposable elements, phages, and plasmids) and gene loss or loss of function, which affects host range. In spite of the presence a greater amount of research findings related to Salmonella infection and pathogenesis mechanism in host animals, several key queries remain intact viz the exact role of virulence genes and genomic islands of particular serovar in animal models. Thus the need of hour is to have an in depth understanding of Salmonella pathogenesis for developing intervention strategy to minimize the disease’s prevalence and spread, as well as assisting in the production of novel drugs and treatments which might lead to improved treatment of Salmonellosis in living creatures.

Conflict of interest

The authors declare that they have no competing interests.