Module 8.6.2.2_2005_archived

Invasive Salmonellosis in Humans

SATHEESH NAIR,¹ DEBORAH HOUSE,² ANNE BISHOP,² AND JOHN WAIN^1*

[SECTION EDITOR: GORDON DOUGAN]

Posted August 26, 2005

Archived August 18, 2008

The Wellcome Trust Sanger Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SA,¹ and Centre for Molecular Microbiology and Infection, Imperial College, London SW7 2AZ,² United Kingdom

*Corresponding author. Phone: +44 (0) 1223 494828, Fax: +44 (0)1223 494919, E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

( see updated version 8.6.2.2 )

INTRODUCTION

Salmonellosis in the human host is generally associated with Salmonella enterica from subspecies enterica (also termed subspecies I). Acute infections can present in one of four ways: enteric fever, gastroenteritis, bacteremia, and extraintestinal (EI) focal infection. As with other infectious diseases the course and outcome of the infection depend upon a variety of factors, including inoculating dose, immune status of the host, and genetic background of both host and infecting organism. Here we review the salmonellae for which invasive disease in humans is an important aspect of their population biology, and we focus on the bacterial factors that may contribute to the "invasiveness" of the different serotypes in the human host.

Broadly speaking the S. enterica from human infections can be subdivided onto two groups: the enteric fever (typhoidal) group and the nontyphoidal salmonellae (NTS), which typically cause gastroenteritis but which can cause invasive disease under certain conditions. Five serotypes of Salmonella are associated with enteric fever: Salmonella enterica subspecies enterica serotype Typhi (serotype Typhi), serotype Paratyphi A, serotype Paratyphi B, serotype Paratyphi C, and serotype Sendai. Serotype Typhi forms a genetically homogenous group as does serotypes Paratyphi A and Sendai together, whereas serotypes Paratyphi B and C are heterogenous (72). Serotype Typhi is the causative agent of typhoid fever, while serotypes Paratyphi A, B, and C cause paratyphoid fever. Together typhoid and paratyphoid fever are known as enteric fever. It is clear that the enteric fever salmonellae are intrinsically invasive in the human host. Where an infection is established, these strains usually cause systemic disease and are rarely associated with gastroenteritis, possibly with the exception of certain strains of serotype Paratyphi B. The NTS, on the other hand, are more typically associated with gastroenteritis and rarely cause extraintestinal (EI) infections in the human host. This difference in clinical outcome is thought to be due, in part, to the way the different serotypes interact with the gut epithelium, with the NTS causing an acute inflammatory reaction that prevents systemic spread of the bacteria (Fig. 1). Although rare, NTS can cause systemic disease, typically when the host’s defense is compromised by underlying disease or immunodeficiency, or when there are lesions that provide a favorable environment for colonization. The highest number of EI NTS infections are caused by the serotypes prevalent in the food chain and, consequently, most frequently associated with gastroenteritis (e.g., Salmonella enterica serotype Typhimurium and serotype Enteritidis). However, when the number of cases of bacteremia is expressed as a percentage of the number of cases of gastroenteritis (invasiveness index) within a defined geographical region, it becomes apparent that certain serotypes are more invasive than others (Table 1). For example, between 1996 and 1999 FoodNet in the United States documented 7,895 culture-confirmed cases of Salmonella gastroenteritis. Of these, 540 invasive Salmonella infections were identified: 135 (25%) were due to serotype Typhimurium and 71 (13%) were due to serotype Enteritidis. Serotype Typhimurium (135 invasive cases of 2,160 culture-confirmed gastrointestinal cases) and serotype Enteritidis (71 invasive cases of 1,094 culture-confirmed cases) were clearly the most prevalent strains among the invasive serotypes, however, their invasiveness index (number of invasive cases per number of gastrointestinal cases) was only 6%, compared with 71% for serotype Dublin (20 invasive cases of 28 culture-confirmed cases) and 52% for serotype Choleraesuis (12 invasive cases of 23 culture-confirmed cases). A further report from the United States also showed serotype Dublin to have a high invasive index (91%) and a high mortality rate (26%). Reports from England and Wales also show serotype Dublin and serotype Choleraesuis, as well as serotype Virchow and serotype Panama, to be invasive in the human host (77, 91).

Fig. 1Part 1A: Schematic diagram of the etiology of Salmonella gastroenteritis. (a) Following ingestion, the bacteria pass through the stomach and into the small intestine. The Salmonella colonize the gut and induce an inflammatory reaction (b), which prevents the spread of the bacteria to systemic sites and promotes their excretion in the stool. This infection can be prolonged with bacteria being detected in the stools for weeks after the initial acute episode of gastroenteritis. Part 1B: Schematic diagram of the etiology of enteric fever. (a) Following ingestion, the bacteria pass through the stomach and into the small intestine. The enteric Salmonella do not colonize the gut or induce an inflammatory reaction in the same way as the NTS and are thus able to attach to and penetrate the gut epithelium, possibly via a paracellular route or within phagocytic cells such as M cells or professional phagocytes (b). The Salmonella penetrate into the deeper tissues of the gut and disseminate throughout the body in the bloodstream. Following colonization of and replication within the tissues of the reticuloendothelial system (RES), the Salmonella are shed back into the bloodstream where they cause a secondary bacteremia. This coincides with the onset of symptoms and marks the end of the incubation period. At some point during the infection, the bacteria colonize the gall bladder and are shed back into the gut, via the bile duct, and excreted in the feces.

Table 1Invasiveness index of the most common nontyphoidal Salmonella serotypes associated with invasive disease in humans

The bacterial factors responsible for an invasive phenotype in humans are not known. Genetic comparisons of the different serotypes, including plasmid carriage (70), have given some clues as to which genes might be required for systemic spread in the human host (Table 2). However, the regulation of these genes may also be important and, as yet, few studies have compared the expression of known virulence determinants in the different serotypes.

Table 2Specific genetic traits and invasiveness of Salmonella serotypes in humans

ENTERIC FEVER SALMONELLAE

Serotype Typhi causes typhoid fever, which is the most common form of enteric fever; it was first described by both Eberth and Koch in 1880 and subsequently cultured by Gaffki in 1884. Agglutination with specific sera and the ability to ferment sugars allows serotype Typhi to be differentiated from nonpathogenic Escherichia coli. An inverse relationship between pathogenicity and the ability to ferment sugars has been described, with isolates from food poisoning being intermediate in both respects. Isolates resembling the food-poisoning bacteria have also been isolated from cases of enteric fever but these can be differentiated from other enteric-fever-causing organisms by culture on lead acetate media. These isolates were termed "B. paratyphosus" (93). An improved form of the lead acetate test is still in use (2, 52).

Enteric fever is a febrile, systemic illness. Fever itself is the only consistent symptom (63), although abdominal pain or discomfort, muscle and/or joint pain, and headache are frequently observed. All these resolve relatively quickly if a course of an appropriate antibiotic is administered, but they can persist for several weeks in the untreated patient (63). Some typhoid patients develop complications (21), the most severe of which is gastrointestinal (GI) hemorrhage and perforation (12, 23), a condition requiring both surgical and antimicrobial intervention and which carries a high risk of mortality (11). A minority of infected persons become asymptomatic chronic carriers and continue to intermittently excrete serotype Typhi in their stools for prolonged periods (>1 year).

SEROTYPE TYPHI

Serotype Typhi is restricted and adapted to the human host. It is identified by its serotype, O9 [Vi]: Hd, and specific biochemical reactions. It is less sacchrolytic than most other members of the S. enterica species and forms a discrete group when analyzed by the phylogenetically relevant methods of multilocus enzyme electrophoresis (MLEE) (72) and multilocus sequence typing (MLST) (45). The genome of serotype Typhi CT18 is predicted to contain more than 4,000 coding sequences and 204 pseudogenes. These are largely conserved in the second sequenced isolate serotype Typhi Ty2 (21), which supports the idea that genome degradation has contributed to the host restriction of serotype Typhi (62). When serotype Typhi became isolated in the human host the function of genes required for survival in other hosts could have been lost in the absence of selective pressure. The gain of host adaptation, the ability to cause invasive disease, may also be the result of a loss of gene function. By reducing the ability to cause a secretory response in the human gut (e.g., loss of gene function on the CS54 island, see below) the invasive nature of serotype Typhi infection may be favored. The acquisition of DNA has also played an important role in the evolution of salmonellae (46). The most noticeable genetic addition in serotype Typhi, when compared with other salmonellae, is the presence of Salmonella pathogenicity island 7 (SPI-7) (87). This island contains the genes responsible for the synthesis and transport of the Vi capsular polysaccharide (termed the viaB region) (34), the SPI-1 type III secretion system (TTSS) effector protein sopE (33), and the pil genes that encode a type IVB pilus implicated in bacterial self-association (58, 80) and interactions with epithelial cells (95). SPI-7 is present in a very similar form in serotype Paratyphi C and some isolates of serotype Dublin (64). Subtle differences may, however, affect the properties encoded by regions of the island, for example, serotype Paratyphi C, unlike serotype Typhi, can not self-associate via the products of the pil region. This is because sequence variations in the repeat regions flanking the serotype Paratyphi C shufflon cause the shufflon to be inactive, resulting in the stable production of PilV, a potential minor pilus protein that has been shown to have a negative effect on the self-association of serotype Typhi (76). The importance of pil-mediated serotype Typhi self-association in the pathogenesis of typhoid fever has yet to be demonstrated, although transfer of conjugative plasmids in vitro is enhanced by the ability to self-associate (58). Serotype Typhi CT18 has a unique repertoire of eight fimbrial operons (79). These are all present in various combinations with other fimbrial operons in several salmonellae and a simple correlation between fimbrial operons and host specificity, or disease syndrome, has not been demonstrated. Serotype Typhimurium LT2 has 13 fimbrial operons (55) and it is possible that a lack of fimbrae is associated with host restriction because of the increased need for immune evasion associated with dependence on a single host. Certainly of the human restricted salmonellae none possess the same major lipopolysaccharide (LPS) group. A fact interpreted as showing that the host immune response to the major serogroups (O-antigen epitopes) selects against less transmissible host restricted S. enterica of the same serotype (83) (Table 3). This may also explain the variation in fimbriae expressed by different serotypes. In serotype Paratyphi A SopE from the serotype Typhi specific SPI-7 is present, but not in the same site as in serotype Typhi, while the rest of SPI-7 is entirely absent (64). SPI-7 can be missing from serotype Typhi in culture collections (59), but it is almost always present in fresh clinical isolates (88). This suggests that SPI-7 plays an important role during the infective process. The absence of most of SPI-7 from serotype Paratyphi A, which causes a very similar disease to typhoid in humans (see below), is therefore of interest. A H-NS-regulated cytotoxin is one virulence factor shared by, and apparently restricted to, serotype Paratyphi A and serotype Typhi, the expression of which is enhanced in the serotype Typhi vaccine strain Ty21a (61).

Table 3Serotypes implicated in human invasive disease: antigenic formulas and indicators of variation

SEROTYPE PARATYPHI A

Serotype Paratyphi A is the second most common cause of enteric fever. The number of reported cases in several Asian countries is currently on the increase, although the risk factors for acquiring serotype Paratyphi A may differ from those for typhoid fever (85). This suggests that, even if the pathogenicity of serotype Typhi and serotype Paratyphi A are very similar, there may be some differences in transmission routes.

Serotype Paratyphi A is part of a clonally related group of bacteria (72), which also includes serotype Sendai (8). The recently completed genome sequence of serotype Paratyphi A revealed a very close relationship to serotype Typhi (54). There are 173 pseudogenes in serotype Paratyphi A compared with 204 in serotype Typhi. Of the 168 that could be identified as probable orthologues in both serotypes 28 were pseudogenes in both (54). Of these, 27 had different mutations causing the predicted loss of the open reading frame. Thus, these two pathogens, if adapting to the niche of the human host by loss of gene function, seem to be doing so along a path of convergent evolution. Three pseudogenes, shdA, ratB, and sivH, are on a single island, CS54, and are shared by serotype Typhi and serotype Paratyphi A. ShdA binds fibronectin and is involved in the persistence of serotype Typhimurium in the mouse gut (48). All three genes are involved in colonization of the mouse gut (Peyer’s patch or cecum), and ratB and shdA, but not sivH, are required for fecal shedding of serotype Typhimurium LT2 in mice (47). The combined effect of knocking out all three of these genes together has not been reported to date. The importance of these three pseudogenes to the clinical picture seen in enteric fever, that is, little early colonization of the gut and a lack of inflammatory response (37), is unclear. The other two genes present in CS54, ratA and sivI, have no detectable phenotype in serotype Typhimurium-mouse models of infection.

The type III secretion systems, SPI-1 and SPI-2, are intact in both serotype Typhi and serotype Paratyphi A, although some of the effector proteins (spoA, sopD2, sseJ, and slrP) are predicted to be pseudogenes in both serotypes, but once again the significance of this is unknown. One effector protein, SopE, is present in both serotype Typhi and serotype Paratyphi and in only some epidemic strains of serotype Typhimurium (25). This protein is very closely related to SopE2, which is intact in serotype Paratyphi A but present as a pseudogene in serotype Typhi. SopE would, therefore, seem to be worthy of further investigation.

SEROTYPE PARATYPHI B

Serotype Typhi and serotype Paratyphi A are both restricted to humans, whereas the serotype Paratyphi B group is more like the NTS in causing a range of infections in both humans and animals. In most clinical microbiology laboratories Salmonella with the serotype O4:Hb:1,2 isolated from the blood of patients with suspected enteric fever would be typed as serotype Paratyphi B. Salmonella with this serotype, however, are more frequently isolated from the stools of patients with gastroenteritis. The ability to ferment d-tartrate (dT) has become the diagnostic method to distinguish between the enteric fever serotype Paratyphi B, which does not ferment d-tartrate (dT^–), and the NTS-like serotype Paratyphi B var. java, which can ferment d-tartrate (dT⁺) (43). The ability to produce a slime wall has also been associated with the typhoidal strains; however, in a very detailed study of United Kingdom isolates, Chart et al. demonstrated that this distinction is not absolute and slime wall production can be the same for both dT⁺ and dT^– strains. Furthermore dT^– serotype Paratyphi B is more often associated with gastroenteritis than with enteric fever (13). Of 367 dT^– serotype Paratyphi B isolates only 12.3% were isolated from blood. This suggests that although dT^– serotype Paratyphi B is more invasive than serotype Paratyphi B var. java (1.3% of 1,007 cases), it is much less invasive than serotype Typhi or serotype Paratyphi A. Chart also noted that the patients with invasive disease had a history of travel and suggested that there may be an invasive strain of serotype Paratyphi B circulating in Asia which could be adapted to the human host. This observation, that serotype Paratyphi B is heterogeneous genetically, could explain the apparent low invasiveness index of SPB as a group. Pulsed-field gel electrophoresis (PFGE) has been used to distinguish between serotype Paratyphi B and serotype Paratyphi B var. java, but it could not discriminate between invasive and noninvasive isolates in Malaysia (26). However, another study, using a comprehensive German reference collection, has shown an association between certain chromosomal digest patterns and an ability to cause invasive disease (65). Characterization of the serotype Paratyphi B group by MLEE has revealed 23 electrophoretotypes (ETs) (71). All dT^– strains fall into a single ET, known as Pb1, whereas dT⁺ strains are found in all ETs. The restriction of dT^– strains to a single phylogenetic group underlines the importance of this discriminatory test. Tartrate metabolism in E. coli has been described in detail (66). Metabolism is oxygen sensitive and depends on two genes, ttdA and ttdB, which have orthologues in Salmonella; annotated as STM3355 and STM3354 in the serotype Typhimurium LT2 sequence, and STY3535 and STY3534 in the serotype Typhi CT18 sequence. A direct link between tartrate metabolism and invasive disease is unlikely but a rapid molecular test for the recognition of the genetic lesion should allow large-scale epidemiology to be carried out. Malorny et al. (52) have performed a detailed analysis of this region of the chromosome in serotype Paratyphi B and serotype Paratyphi B var. java and have described a point mutation in the putative start codon of an adjacent possible membrane transport protein STM3356 which correlates perfectly with the dT^– phenotype. This suggests that the protein may be essential for tartrate metabolism, although the function of this protein remains unknown. Within the sequence around the putative coding sequence (CDS) STM3356 there are several possible start codons, and in the annotation for serotype Typhi CT18 (STY3536) one such alternative is chosen. The biological significance of the single-base-pair difference in this CDS has yet to be determined. Given that serotype Paratyphi B strains from the MLEE group Pb1 are more often associated with gastroenteritis, even though the group contains strains that cause systemic disease, it seems likely that the ability to use tartrate has phylogenetic, but not necessarily biological, relevance. It is probable that there are invasive and noninvasive variants within the SPB Pb1 clonal group of salmonellae. The analysis of very closely related strains from a single MLEE group (Pb1) of S. enterica, which have totally different disease characteristics in the human host, such as the invasive and noninvasive strains of serotype Paratyphi B, may greatly facilitate the identification of a set of genes required for invasion across the human gut, invasion of the deeper tissues, and/or systemic dissemination and survival. So far comparisons of these two sets of isolates have revealed no differences in siderophores, serum resistance or surface structures (13), but invasive strains do possess a bacteriophage-encoded sopE, lack avrA, and have normal levels of SopB, but not SopD, production (65). Within the SPB serotype, strains adapted to poultry have been described (84), and there is extensive literature on human infection. This heterogeneous group of organisms should provide a fruitful area for the investigation of host adaptation in salmonellae.

SEROTYPE PARATYPHI C

By using biochemical tests, S. enterica isolates with the serotype 6,7:c:1,5 can be subdivided into several different subtypes: serotype Paratyphi C, serotype Typhisuis, serotype Choleraesuis, serotype Choleraesuis var. Kunzendorf, and serotype Choleraesuis var. Decatur (82). Serotype Paratyphi C is probably restricted to humans, but the difficulties in strain identification make this assertion unsure.

Serotype Typhisuis is a typical host-restricted serotype that is rarely isolated from humans. Infection in pigs, the natural host, ranges from enterocolitis (67) to chronic paratyphoid fever (81). Serotype Typhisuis is auxotrophic and forms a monophyletic group by MLEE (8, 72). Differentiation from serotype Paratyphi C is based on the prototrophic nature of the latter. The use of fermentation tests for utilization of arabinose and trehalose have been described (82). By using MLEE, two distinct groups of serotype Paratyphi C have been described, one of which is closely related to serotype Typhisuis and the other to serotype Miami (72) or serotype Montevideo (8). With the serotype Paratyphi C strains SARB48 and SARB49, it appears that the two MLEE groups can also be differentiated by ribotyping (82) and MLST, suggesting that they represent distinct phylogenetic groups. Extensive analyses of serotype Paratyphi C have been carried out with PFGE (41) and ribotyping (82), but an association between strain type and difference in disease has not been described. Sequencing of a serotype Paratyphi C isolate is under way.

The third member of this serotype, serotype Choleraesuis, is a more diverse group of organisms containing three biotypes and five MLEE groups (8, 72). Serotype Choleraesuis is a host-adapted, but not restricted, serotype that causes swine paratyphoid fever (16). It causes a highly invasive disease in humans with little involvement of the gastrointestinal tract (19). The genome sequencing of serotype Choleraesuis strain SC-B67 and its two plasmids (pSCV50 and pSC138) was recently completed at Chang Gung University, Taiwan, and analysis is under way (www.salmonella.org).

NONTYPHOIDAL SALMONELLOSIS

Improvements in diagnosis (14), the influence of HIV (15, 27), and/or the appearance of more virulent strains (31) may be responsible for the growing number of NTS serotypes reported as causing invasive disease syndromes in humans. Serotypes of NTS are widespread and are commonly associated with specific animals. They typically cause a self-limiting gastroenteritis in the human host, the symptoms of which include fever, diarrhea, vomiting, and stomach cramps. The illness can be accompanied by prolonged fecal shedding (>1 month). NTS gastroenteritis carries a significant burden of mortality, largely because of the association with extraintestinal (EI) infections (86). EI NTS infections can present as gastroenteritis with secondary bacteremia, or as a primary bacteremia or focal infection with no history of gastroenteritis (Fig. 2). EI infection carries a significant mortality which is highest for primary bacteremia and focal infections (74). A variety of focal infections have been documented, including meningitis, septic arthritis, osteomyelitis, and infectious endarteritis (40). In the otherwise healthy, immunocompetent host EI NTS infections are relatively rare; however, certain groups of individuals, <2 years and >65 years (74, 90), are at increased risk. Age-related differences in the type of EI infection also exist; children are more likely to develop secondary bacteremia and meningitis, whereas adults are more at risk of a primary bacteremia or focal infection, including endarteritis (3, 39). Focal infections have been linked to a variety of intrinsic and iatrogenic host factors that affect immune status; diabetes and malignancy, HIV infection, or receiving therapeutic immunosuppression (9, 38, 39). Several hypotheses have been proposed for the association between immunodeficiency and EI NTS infection, including reactivation of latent infection or hemolytic disease (94) or malaria-associated anemia (30, 32). The high risk of death in the elderly is most likely due to an increased prevalence of atherosclerotic plaques and aneurysms in this age group, which increases the risk of infectious endarteritis. The mortality rate for EI NTS infection in HIV-positive individuals is high, with recurrent infection occurring in a significant proportion of the survivors (27). An association with helminth infections suggests gut damage may predispose to invasive NTS (22). Enteric fever has a lower mortality than systemic NTS infections (28) and, although HIV-positive patients may be at a greater risk of acquiring typhoid fever (29), few cases of co-infection (53) have been reported and the expected increase in cases of typhoid fever has not taken place in regions where co-infection with HIV should occur (63). There are also several interesting aspects of the biology of the pathogens among the vast literature on the host factors involved in EI NTS infection.

Fig. 2Outcome of an infection with Salmonella depends on the invading serotype and the immune status of the host. The enteric salmonellae (serotypes Typhi and Paratyphi A, B, and C) cause enteric fever, while the NTS typically cause gastroenteritis in the immunocompetent host. However, NTS infection can become systemic if the host is compromised or the infecting serotype is particularly invasive. The systemic disease may follow an acute bout of gastroenteritis but it can occur in the absence of gastroenteritis, in particular, if the individual is immunocompromised as the result of HIV infection, disease-induced or iatrogenic immunosuppression.

SEROTYPE TYPHIMURIUM AND SEROTYPE ENTERITIDIS

In both the developed and less-developed countries serotype Enteritidis, in particular, Phage Type 4 (PT4), and serotype Typhimurium infection are the most commonly reported cause of EI NTS (9, 42). These systemic infections are most frequently seen in individuals with predisposing factors (9, 20, 32, 69, 86, 89), but bacterial factors include plasmid-borne multidrug resistance (MDR) to three or more antimicrobial agents (42) and chromosomally mediated quinolone resistance (35). Many NTS serotypes, including serotype Typhimurium and serotype Enteritidis, carry a plasmid not usually associated with resistance, the virulence plasmid, which encodes the spv genes that are essential for infection in laboratory rodents. The role of the virulence plasmid in gastroenteritis and invasive disease in humans is unclear. Some reports suggest that the spv genes promote dissemination of serotype Typhimurium from the gut (24), although in vitro experiments to date have not demonstrated a role for these genes in invasion of epithelial cells (10). Harboring of a virulence plasmid may facilitate systemic virulence in some S. enterica subspecies I serotypes, but the virulence level of the enteric disease will almost certainly vary with the bacterial strain and the host involved. The study of spv-containing pathogens needs to be complemented with epidemiological studies to determine whether virulence plasmids are important in the pathogenesis of NTS infections in humans (24).

SEROTYPE DUBLIN

The natural hosts for serotype Dublin are cattle. Infections in humans are not very common, although the reported incidence of invasive NTS infection caused by serotype Dublin is increasing in the United States (86). Reports from Europe vary: serotype Dublin has been reported in the United Kingdom and Denmark (49, 51, 77), but it does not seem to be a problem in Spain with no cases of serotype Dublin being reported among the 970 Salmonella isolates from humans collected between 1991 and 1996 (68). Also in Australia between 1995 and 2003, serotype Dublin has not been reported in humans although it is very common in cattle (surveillance reports from the National Enteric Pathogen Surveillance Scheme [NEPSS], Melbourne University, Victoria, Australia). There are few, if any reports of invasive cases of serotype Dublin being reported from Africa and Asia. Whether this patchy distribution of human cases is because of human factors, food animals, or variation in the bacterial population has not been studied in depth. Variation in the bacterial population has been demonstrated; by using MLEE, serotype Dublin can be divided into three clonal types that are very closely related to serotype Enteritidis (73). Though both serotypes are similar at the genetic level, serotype Dublin is largely host adapted for cattle and serotype Enteritidis has a wider range of hosts, with the strongest association with poultry and humans. Geographical differences also occur; type Du1 is globally distributed, whereas Du3 is restricted to France and the United Kingdom and contains all the capsulate strains (Vi positive) (73). The epidemiological data linking bacterial variation and human disease is limited but there is an association between ribotype and the ability of serotype Dublin to cause human invasive disease (17), and with strain type when using a combination of plasmid profiles, ribotyping, and PFGE (51). The data taken together suggest that some serotype Dublin strains may be more infectious for humans. Which genetic factors may be responsible for this has also been investigated. Vi-positive strains contain the SPI-7 type IV pili, used by Typhi for self-association (64). Disruption of this system reduces invasion across human cultured intestinal cells (57). Serotype Dublin also carries an spv-containing plasmid, which promotes enhanced intracellular proliferation in intestinal tissues and at extraintestinal sites in the natural host, cattle (50); however, no association has been found with human disease (17, 60).

Serotype Dublin therefore seems to be a polyphyletic serotype with two main clonal groups. One of these groups contains strains that are Vi positive and may be associated with the ability to cause invasive disease in humans.

SEROTYPE VIRCHOW

There have been numerous single-case reports of invasive serotype Virchow infections (5, 6, 78); this serotype is recognized as a significant cause of invasive salmonellosis in Israel (74, 90), Australia (1, 4), and the United Kingdom (78, 91). The increasing incidence of serotype Virchow PT26 is of particular concern because of its association with more invasive disease in humans (75, 92). In contrast to the United Kingdom and Australia, there have been no reports of invasive serotype Virchow infections from North America (44, 86) and this serotype was not mentioned among CDCs ten most frequently reported Salmonella from humans (Salmonella surveillance summary for 1986, CDC 1988). Little is known about the population structure of this serotype, but it can infect animals and seems to cause invasive disease in humans.

OTHER SEROTYPES ASSOCIATED WITH HUMAN INVASIVE DISEASE

Serotype Panama and serotype London infection in Liverpool, United Kingdom, have a high invasive index (91) and in Taiwan the majority of cases of bacteremia involving serotype Panama (79.5%) occur in children without predisposing conditions (94). Bacteremia due to serotype Heidelberg has been reported from the Unites States (77, 86) and Australia (NEPSS 2001–2003). It can be egg associated (36), and has an invasiveness index of 11% (60 of 551 serotype Heidelberg isolated were from invasive disease) was higher than that for serotype Typhimurium (6%) and serotype Enteritidis (6%) (86). However, the high blood invasion index for serotype Heidelberg is not consistent in different countries; 3.3% in the United Kingdom and 5.9% from Australia. Limited data on the population structure of this serotype suggest that it is monophyletic (8). In a study carried out in New Zealand, of a total of 62 cases of salmonellosis, 30 were culture-positive for serotype Brandenburg (48%). Of these cases, 13 (43%) were from extraintestinal sites. Eleven of these 13 (85%) cases did not have any risk factors for invasive disease (18). Serotype Schwarzengrund represented 11 (2%) of the 540 culture-confirmed invasive Salmonella infections identified by FoodNet in a surveillance carried out between 1996 and 1999 in the United States (86). Of 192 serotype Bredeney cases in Eastern Australia, noted in a report by NEPSS for the year 1995, 17 (9%) were invasive. In 2002 in Australia 178 cases of serotype Chester were reported, of which 7 (3.9%) were invasive, and 219 cases were reported in 2003, of which 10 (4.6%) were invasive (NEPSS 2001–2003).

CONCLUSIONS

For serotype Typhi and serotype Paratyphi A there is a clear association between serotype and systemic infection in humans. Comparisons made between sequenced strains of serotype Typhi and serotype Paratyphi A and microarray data investigating regions of intraserotype variation for serotype Typhi (7) and serotype Paratyphi A (54) have begun to shed light on genetic factors that are important to the ability of these related but distinct serotypes restricted to humans to cause systemic disease. The acquisition of islands of DNA or single genes may explain, in part, why these two serotypes are adapted to humans, but the presence of an overlapping, but not identical, set of pseudogenes caused by different mutational lesions suggests convergent evolution and genome degradation as part of adaptation to a common niche (54). Which of the many pseudogenes or regions of acquired sequence have contributed to the host restriction of both serotype Typhi and serotype Paratyphi A and/or to their ability to cause systemic disease in the human host remains to be discovered. For serotypes Paratyphi B and Paratyphi C, a good clinical description of the host and a detailed population of genetics of the pathogen are necessary before more detailed genetic studies of novel virulence factors can be initiated.

Serotype Typhimurium and serotype Enteritidis are the most common serotypes within the food chain and thus the high number of invasive infections associated with these serotypes is most likely due to exposure rather than to increased virulence of the pathogen. In Africa, however, a closely related group of strains of serotype Typhimurium may have become host adapted to humans (31). Some of the NTS associated with human invasive disease are not single clonal groups (81) and again a more detailed molecular analysis of the pathogen will be necessary before we can effectively search for bacterial virulence factors. The similarities between serotypes such as serotype Typhi and serotype Typhimurium suggest that they share some mechanisms for invasion and intracellular trafficking (56); however, it is also clear that salmonellae exhibit diversity in their mechanism of adaptation even to a single host, human beings (81).