Salmonella Flashcards
Overview of Salmonella infection
Orally-ingested Salmonella survive the acidic pH of the stomach and preferentially enter M-cells. These M-cells transport the salmonella to the lymphoid cells in the underlying Peyer’s patches – lymphoid tissue that essentially performs immune surveillance for the gastrointestinal system.
Salmonella serotypes that are associated with systemic infection will enter intestinal macrophages and will be disseminated throughout the reticuloendothelial system. In contrast, non-typhoidal Salmonella induce a localized inflammatory response, resulting in the influx of PMNs to the intestinal lumen and diarrhoea.
Salmonella 2CSs - resistance to antimicrobial peptides
PhoP/Q & PmrA/B
both connected by PmrD which is activated by the PhoP/Q system. PmrD protects PmrA from dephosphorylation by PmrB, activating PmrA
PhoP/Q contributes to resistance to bile salts and is activated in the GI tract
PhoP/Q and PmrA/B both confer resistance to antimicrobial peptides
APs are + charged, and have high affinity for the neg charged bacterial cell envelope. PhoPQ/PmrAB modify the lipid A region by the addition of aminoarabinose which reduces the net negative charge and lowers the affinity for APs
PhoP/Q also activates SodC
phoP/phoQ-deficient Salmonella typhimurium are attenuated for virulence & macrophage survival, and can be used as live vaccines
Salmonella SodC
resistance to oxidative killing (the respiratory burst?)
two distinct periplasmic enzymes: SodCI and II
SodC is induced within macrophages and induction is mediated by PhoPQ
Whilst both enzymes are induced within macrophages and within the spleen during infection of mice, the level of induction of SodCI is greater. SodC is induced by PhoP/Q when magnesium concs are low
So not only does PhoPQ confer LPS modifications that help protect against antimicrobial peptides, it also boosts the bacteria’s oxidative stress response.
How might PhoQ help intracellular survival of salmonella?
Hypothesis:
Phagolysosomes have low Mg2+ concentrations, thus the PhoP/Q system is activated during phagocytosis
PhoP/Q activation induces Mg2+ transport mechanisms & LPS modifications that promote intracellular survival
the promotor is
the region of DNA within the genome that contains the consensus sequences that the RNA polymerase complex recognises and binds to
The stringent response
has been shown to be extremely important for virulence in various bacterial species, including Salmonella.
in salmonella infection Stressors encountered include:
acidic pH
Reactive oxygen & nitrogen species
Nutrient limitation
sigma factor 34 (RpoS) phenotypes in Salmonella
In addition to facilitating adaptation to stressors, RpoS can also directly regulate certain virulence factors
A protein called RssB is an adaptor protein for RpoS, the master regulator of general stress.
RpoE
is the envelope stress response in Salmonella. RpoE deficient mutants showed impaired resistance to ROS and decreased survival in macrophages
Defence
Salmonella acquires ferric iron by secreting the siderophores enterobactin and salmochelin
Enterobactin uses two membrane receptors IroN and FepA to pull the siderophore back but the receptors can also pull in other species exochelins
Required by salmonella to evade nutritionally immunity in macrophages and cause persistent infection in mice
T3 and T4SS are activated by acidification of the phagosome
e.g. salmonella have T3SS which are activated by phagosome acidification
Salmonella pathogenicity island II
Secreted effectors and virulence
Salmonella’s effectors mediate uptake through macrophages via a T3SS
1,Salmonella cells attach to the intestinal epithelium by means of adhesins, such as those encoded within SPI-3 and SPI-4 (salmonella pathogenicity islands 3 and 4)
2 and 3, Invasion of bacteria follows, and engulfment by M cells is mediated by virulence factors encoded within SPI-1 (T3SS) and SPI-5.
Salmonella does this by secreting the effector SipA into the host which causes actin rearrangment leading to salmonella uptake
5, Once taken up by phagocytes inside the cytoplasm, Salmonella replicates within the SCV. Factors encoded within SPI-2 and the pSLT plasmid are essential for survival in the phagosome (SifA prevents maturation of the phagolysosome).
7, Bacteria are internalized within phagocytes and located again within an SCV, where SPI-3, in addition to SPI-2 and the pSLT plasmid, play an important role
T3SS and T4SS:
Translocon at the tip of the injection apparatus forms a pore in host cell membranes and injects the effectors directly, to mediate manipulation of the immune response (e.g. siderophores) Approx 13 SPIs discovered, activated at different stages of host cell invasion.
*PhoP/Q represses SPI 1!
Virulent salmonella also have a plasmid that encodes for factors that are important for surviving inside the macrophage. A spiC (effectors) mutant of Salmonella shows reduced survival in macrophages due to interference with vesicular trafficking and lyzosome fusion.
reading
Salmonella enterica serovar Typhimurium is a primary enteric
pathogen infecting both humans and animals. Infection begins
with the ingestion of contaminated food or water so that salmonellae reach the intestinal epithelium and trigger gastrointestinal
disease. In some patients the infection spreads upon invasion of
the intestinal epithelium, internalization within phagocytes, and
subsequent dissemination. In that case, antimicrobial therapy,
based on fluoroquinolones and expanded-spectrum cephalosporins as the current drugs of choice, is indicated. To accomplish the
pathogenic process, the Salmonella chromosome comprises several virulence mechanisms. The most important virulence genes
are those located within the so-called Salmonella pathogenicity
islands (SPIs). Thus far, five SPIs have been reported to have a
major contribution to pathogenesis. Nonetheless, further virulence traits, such as the pSLT virulence plasmid, adhesins, flagella,
and biofilm-related proteins, also contribute to success within the
host. Several regulatory mechanisms which synchronize all these
elements in order to guarantee bacterial survival have been described. These mechanisms govern the transitions from the different pathogenic stages and drive the pathogen to achieve maximal
efficiency inside the host
almost all Salmonella organisms that
cause disease in humans and domestic animals belong to S. enterica subspecies enterica (I) (20–22).
Alternatively, S. enterica strains can also be classified on the
basis of the O (lipopolysaccharide [LPS]) surface antigen into 67
serogroups and into 2,557 serotypes or serovars when strains are
differentiated by both their O and H (flagellar) antigens
Two major clinical syndromes caused by Salmonella infection in
humans are enteric or typhoid fever and colitis/diarrheal disease.
Clinical manifestations include fever, headache, abdominal pain, and transient diarrhea or constipation, and
infection can produce fatal respiratory, hepatic, spleen, and/or
neurological damage. Without treatment, the mortality is 10 to
20%, decreasing to 1% among patients treated with the appropriate antibiotics
Another particular aspect of the clinical impact of Salmonella
infections is the so-called carrier state. This condition, which is
valid for NTS infections in both humans and livestock and for
typhoid fever, corresponds to a persistent colonization of the gut,
established durably upon the initial infection (over 10 weeks
postinfection). Biofilm production is often among the virulence
traits supporting such chronic persistence. These carriers are characterized by a symptom-free condition and can act as reservoirs
and hence contribute to the propagation of the disease
salmonella invasion
The first obstacle to
overcome within the host is the acidic pH of the stomach. To
protect itself against severe acid shock, S. Typhimurium activates
the acid tolerance response (ATR), which provides an inducible
pH-homeostatic function to maintain the intracellular pH at values higher than those of the extracellular environment (53). After
entering the small bowel, salmonellae must reach and traverse the
intestinal mucus layer before encountering and adhering to intestinal epithelial cells. In mice, salmonellae appear to preferentially
adhere to and enter the M cells of the Peyer’s patches (PPs) in the
intestinal epithelium
Shortly after adhesion,
the invasion process appears as a consequence of engaged host cell
signaling pathways leading to profound cytoskeletal rearrangements (56, 57). These internal modifications disrupt the normal
epithelial brush border and induce the subsequent formation of
membrane ruffles that engulf adherent bacteria in large vesicles
called Salmonella-containing vacuoles (SCVs) (58–60), the only
intracellular compartment in which Salmonella cells survive and
replicate (58, 59).
SCVs are initially integrated within the early endocytic pathway. However, they need to be later uncoupled to bypass delivery
of lysosomal enzymes. This action depends on Salmonella-directed changes in host endocytic trafficking and function to eventually avoid fusion with secondary lysosomes
SCV maturation, Salmonella induces de novo formation of an Factin meshwork around bacterial vacuoles, a process which is
termed vacuole-associated actin polymerization (VAP) and is important for maintenance of the integrity of the vacuole membrane
(65). SCVs then migrate to a perinuclear position, in close proximity to the Golgi apparatus, presumably to facilitate interception
of endocytic and exocytic transport vesicles to obtain nutrients
and/or membrane fragments. This event appears to be essential
for bacterial replication (66, 67). In addition, it has been observed
that intracellular Salmonella can induce the formation of long
filamentous membrane structures called Salmonella-induced filaments (SIFs) (68, 69). SIFs are tubular aggregates along a scaffold
of microtubules and originate from the SCVs and extend throughout the cell. Although the biological role of the induction of SIFs is
not completely understood, it has been postulated that this process may lead to an increased availability of nutrients that may
otherwise be limited within the SCV (70)
Thereafter, a fraction of these SCVs transcytose to the basolateral membrane. Once across the intestinal epithelium, salmonellae are engulfed by phagocytes. Three types of phagocytes are reported to interact with these invading bacteria: (i) neutrophils, (ii)
inflammatory monocytes which differentiate into macrophages,
and (iii) dendritic cells, another type of monocytes which function
as antigen-presenting cells. The first two types of cells are both
recruited from blood in response to the inflammatory signal
Salmonellae are then phagocytosed and internalized again
within SCVs, triggering a response similar to that reported inside
epithelial and M cells to ensure bacterial survival and replication
(25, 78, 79). Migration of these infected phagocytes, predominantly macrophages, facilitates systemic dissemination of the bacteria via the bloodstream to several additional tissues, such as the
spleen and liver, where this pathogen preferentially replicates (2
SPIs.
SPI-1 encodes several effector proteins which mostly trigger
invasion of epithelial cells by mediating actin cytoskeletal rearrangements and hence internalization of the bacteria. These effectors are translocated into the host cell by means of a type III secretion system (T3SS), termed T3SS-1, also encoded within SPI-1
, SPI-2 harbors four types of genes which are important for virulence: ssa, the genes encoding the T3SS-2 apparatus;
ssr, encoding the regulators; ssc, encoding the chaperones; and sse,
encoding the effectors
SPI-3 encodes proteins with no obvious functional relationship to each other, since it is involved in
both initial attachment and long-term persistence (MisL) as well
as survival during systemic dissemination (MgtCB
SPI-4 contains only six ORFs, arranged in
a single operon termed siiABCDEF, and plays a role during the
initial interaction with the intestinal epithelium and possibly contributes to long-term persistence
clever salmonella
Nutrient access. Stecher et al. have reported that the inflammatory host response induced by S. Typhimurium changes the composition of the microbiota and suppresses its growth, thereby offering Salmonella a growth advantage (147). Particularly, the
absence of flagella or chemotactic movement attenuates disease
and reduces the fitness of salmonellae in the inflamed gut (9).
Since mucosal inflammation provides a localized source of highenergy nutrients, motility and chemotaxis allow Salmonella to efficiently access these nutrients and accumulate in proximal areas,
resulting in faster replication and a fitness benefit over the microbiota (148).
Tetrathionate respiration. Colonic bacteria produce large
quantities of H2S, a highly toxic compound which is converted to
thiosulfate by the cecal mucosa as a protective response (149).
However, during Salmonella-induced gut inflammation processes, reactive oxygen species (ROS) are released by the neutrophils recruited. These compounds react with thiosulfate to form a
new respiratory electron acceptor, tetrathionate, which, in the
presence of the ttr genes located in SPI-2, can be utilized as an
electron acceptor. This advantageous ability is particularly important under the anaerobic growth conditions encountered in the
intestinal mucus layer, since it confers the opportunity to outgrow
the fermenting commensal competitors (86).
