Clinical Immunology 114 (2005) 248 – 255
www.elsevier.com/locate/yclim
Targeting of the actin cytoskeleton during infection by Salmonella strains
Donald G. Guiney*, Marc Lesnick1
Department of Medicine, UCSD School of Medicine, La Jolla, CA 92093-0640, United States
Received 19 July 2004; accepted with revision 22 July 2004
Available online 15 September 2004
Abstract
Many bacterial pathogens produce virulence factors that alter the host cell cytoskeleton to promote infection. Salmonella strains target
cellular actin in a carefully orchestrated series of interactions that promote bacterial uptake into host cells and the subsequent proliferation and
intercellular spread of the organisms. The Salmonella Pathogenicity Island 1 (SPI1) locus encodes a type III protein secretion system (TTSS)
that translocates effector proteins into epithelial cells to promote bacterial invasion through actin cytoskeletal rearrangements. SPI1 effectors
interact directly with actin and also alter the cytoskeleton through activation of the regulatory proteins, Cdc42 and Rac, to produce membrane
ruffles that engulf the bacteria. SPI1 also restores normal cellular actin dynamics through the action of another effector, SptP. A second TTSS,
Salmonella Pathogenecity Island 2 (SPI2), translocates effectors that promote intracellular survival and growth, accompanied by focal actin
polymerization around the Salmonella-containing vacuole (SCV). A number of Salmonella strains also carry the spv virulence locus,
encoding an ADP-ribosyl transferase, the SpvB protein, which acts later during intracellular infection to depolymerize the actin cytoskeleton.
SpvB produces a cytotoxic effect on infected host cells leading to apoptosis. The SpvB effect appears to promote intracellular infection and
may facilitate cell-to-cell spread of the organism, thereby enhancing virulence.
D 2004 Elsevier Inc. All rights reserved.
Keywords: Salmonella strains; Type III protein secretion system; Salmonella-containing vacuole
Introduction
The actin cytoskeleton determines shape and motility in
eukaryotic cells and is involved in a variety of crucial
cellular processes. Many bacterial pathogens employ virulence mechanisms to subvert the actin cytoskeleton and
promote infection. The induction of cytoskeletal rearrangements to facilitate bacterial invasion of host cells is a
common strategy in pathogenesis. Once inside the cell,
certain bacteria use actin to promote the intracellular phase
of infection. Listeria and Shigella strains induce focal actin
polymerization to propel the bacteria through the cytoplasm
* Corresponding author. Department of Medicine 0640, UCSD School
of Medicine, 9500 Gilman Drive, La Jolla, CA 92093-0640. Fax: +1 858
534 6020.
E-mail address: [email protected] (D.G. Guiney).
1
NIAID/NIH/DHHS Bethesda, MD 20892-7616. Dr. Lesnick’s work
was done prior to the NIH appointment. The views expressed in this article
do not necessarily represent the views of the NIAID, NIH, DHHS or the
United States.
1521-6616/$ - see front matter D 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.clim.2004.07.014
and into adjacent cells [1]. Members of the genus
Salmonella have evolved a particularly intricate process of
directing distinct actin rearrangements that facilitate different stages of intracellular infection. At least three different
major virulence loci in Salmonella encode crucial bacterial
proteins that target the actin cytoskeleton. The complex
orchestration of these successive attacks on actin provides a
fascinating illustration of the interrelationship between
mechanisms of pathogenesis and the signal transduction
pathways that regulate cell physiology.
Overview of the role of actin in Salmonella pathogenesis
Salmonella strains are common pathogens of reptiles,
birds, and mammals. Although they may colonize the
intestine of reptiles and birds without producing symptoms,
they are a prominent cause of enteritis in mammals.
Salmonella invade the intestinal mucosa and can also
produce systemic infection in a variety of hosts. Individual
Salmonella strains and genetic lineages differ greatly in
D.G. Guiney, M. Lesnick / Clinical Immunology 114 (2005) 248–255
their propensity to cause disease in different animal hosts
[2]. Host-adapted Salmonella strains produce a characteristic disease spectrum in their host species and are rarely
found in nature outside of that host. Other Salmonella have
a broad host-range and cause a variety of disease syndromes
in many different hosts. Genomic analysis is revealing
considerable variation in virulence gene loci among
Salmonella isolates [3,4]. In addition, host factors play a
major role in determining the outcome of Salmonella
infection. A number of genes affecting susceptibility to
Salmonella have been identified in mice [2]. In humans,
many medical conditions predispose to more severe
Salmonella infections, particularly defects in T cell function
as seen with HIV infection [5]. The ease of genetic
manipulation in Salmonella, together with a robust mouse
model of infection, has made Salmonella an attractive
model for the study of host–pathogen interactions.
Salmonella are facultative intracellular pathogens. The
failure of extracellular antibiotics such as gentamicin to
clear infection in animals or humans indicates that the
intracellular phase is crucial for pathogenesis [6]. Salmonella infection begins with ingestion of the organism and
traversion through the upper GI tract to gain access to the
distal small bowel and colon (see Fig. 1). The bacteria
invade intestinal epithelial cells, inducing fluid and electrolyte secretion and an intense inflammatory response [7]. The
ability to invade epithelial cells requires a cluster of genes
termed the Salmonella Pathogenicity Island 1 (SPI1) locus,
which encodes a type III protein secretion system (TTSS)
that delivers bacterial effector proteins into the cytoplasm of
the target host cell, inducing actin cytoskeletal rearrangements that facilitate uptake of the organism into a
Salmonella-containing vacuole (SCV) [8]. The function of
249
the SPI1 virulence system has been shown to be essential for
the production of enteritis in experimental animal systems
and all lineages of the genus Salmonella carry the SPI1
locus.
Salmonella are phagocytosed by neutrophils and macrophages in the intestinal mucosa (Fig. 1). Although neutrophils appear able to kill Salmonella [9], the organism
survives and proliferates in nonactivated macrophages [10].
The intracellular phase of Salmonella infection is characterized by processing of the SCV along a unique path of
vesicular trafficking, which requires expression of the genes
in Salmonella Pathogenecity Island 2 (SPI2), as well as loci
controlled by the two component regulatory system PhoPQ
[11,12]. SPI2 encodes a second type III protein secretion
system that delivers virulence effector proteins produced by
organisms in the SCV across the vacuolar membrane and
into the cytoplasm of the host cell [13]. A morphologic
feature of the SCV appears to be the focal condensation of
actin filaments surrounding the vacuole induced by an SPI2dependent mechanism. Both SPI2 and PhoPQ are required
for intracellular survival and proliferation, and all members
of the species Salmonella enterica carry the SPI2 locus. In
animal models of infection, both SPI2 and phoPQ are
essential for systemic disease. Salmonella bongori, which
lacks SPI2, is not a pathogen of humans or domestic
animals.
Certain strains of S. enterica possess an additional
genetic locus, termed spv, that enhances virulence by
profoundly altering actin physiology during intracellular
infection (Fig. 1). The spvB gene encodes a protein that
modifies actin monomers to prevent polymerization, such
that infected cells eventually lose all F-actin filaments [14–
16]. Among strains of S. enterica pathogenic for humans,
Fig. 1. Site of action of major Salmonella virulence loci that modulate the actin cytoskeleton. Bacteria on the luminal surface of intestinal epithelial cells use the
SPI1 TTSS to transport effector proteins into the host cell cytoplasm, inducing focal actin polymerizations that promote uptake of the organism into a
Salmonella-containing vacuole (SCV). Invasion is shown in the Peyer patch M cells that lack microvilli but is likely to occur through other epithelial cells as
well. The SPI2 TTSS and the spv genes are expressed in the intracellular environment and promote survival and replication of bacteria, as well as host cell
apoptosis. Bacteria are phagocytosed by adjacent subepithelial macrophages, and the organism can spread to regional lymph nodes, the liver, and the spleen.
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D.G. Guiney, M. Lesnick / Clinical Immunology 114 (2005) 248–255
carriage of the spv locus is associated with severe,
disseminated nontyphoid Salmonella infection [17,18]. In
animal models, the spv locus greatly increases systemic
virulence [2].
This overview indicates that Salmonella produces a
variety of virulence factors that induce seemingly diverse
and contradicting effects on the actin cytoskeleton of host
cells. However, the detailed analysis of this system of
pathogenesis presented in the subsequent sections reveals
carefully regulated and orchestrated sets of reactions that
function at different stages of the infectious cycle.
Actin cytoskeletal events associated with Salmonella
invasion
The hallmark of Salmonella enteritis involves invasion of
intestinal epithelial cells, induction of an intense inflammatory infiltrate, and eventual erosion of the normal mucosal
architecture [7]. Extensive genetic analysis has shown that
expression of the SPI1 TTSS and translocation of selected
effector proteins are required for bacterial invasion of
cultured epithelial cells as well as the production of enteritis
in animal models of disease [8,19]. Uptake of Salmonella by
the nonphagocytic cells of the intestinal epithelium involves
a complex series of actin cytoskeletal changes induced by
the translocated effector proteins. Three general biochemical
processes can be delineated: (1) the activation of the small
cytoskeletal regulatory proteins Cdc42 and Rac1, (2) the
interaction of bacterial effectors with actin to promote
polymerization and bundling of actin filaments, and (3) the
restoration of normal actin dynamics following invasion.
Salmonella invasion is characterized by the formation of
membrane ruffles leading to macropinocytosis and engulfment of the invading organism in a membrane-bound
vacuole, the SCV. The SPI1 TTSS effectors SopB, SopE,
and SopE2 are involved in activating the Rho-family
GTPases Cdc42 and Rac1 [20,21]. The specific G proteins
required for Salmonella invasion appear to differ depending
on cell type and orientation if the cells are polarized. SopB
is an inositol 3-phosphatase that increases cellular levels of
(1,4,5,6)P4, leading to Cdc42 activation [21]. SopE and
SopE2 are guanine nucleotide exchange factors for Cdc42
and Rac1 [20,22]. SopE is encoded on a lysogenic phage
and is only present in some serovar Typhimurium isolates,
while SopE2 is present in all Salmonella [22]. Activation of
Cdc42 and Rac1 leads to recruitment and activation of
WASP and Scar/WAVE family proteins together with the
Arp2/3 complex involved in initiating actin polymerization
[23]. Loss of the combined functions of SopB and SopE/E2
results in a significant impairment of invasion.
In concert with G protein activation, Salmonella translocates two actin-binding proteins, SipA and SipC [24,25].
SipC is also required for the function of SPI1 TTSS
translocation and is postulated to insert in the target cell
membrane. SipC has separate actin bundling and nucleating
domains, and its putative membrane location could direct
actin filament formation to the appropriate sites adjacent to
the invading bacteria [26,25]. The exact role of the actinbinding activity of SipC in invasion has not been established
genetically since SipC mutants are defective for the transport of all SPI1 TTSS effectors. Since SipA is not required
for TTSS function, genetic analysis could identify a function
in invasion. SipA mutants in cell culture assays show
delayed kinetics of invasion and are partially attenuated for
virulence in the calf enteritis model [19,27,28]. The detailed
three-dimensional structure of the SipA/actin complex has
recently been reported and indicates that SipA binds to actin
subunits on opposite strands of polymers, acting as a
bmolecular stapleQ to stabilize actin filaments [29]. It is
likely that SipA and SipC act in concert to promote actin
polymerization and bundling of F-actin at the site of
bacterial invasion. In this manner, the G protein-mediated
recruitment of the Arp2/3 machinery, together with localized
actin binding by SipA and SipC, results in the actin
polymerization that drives engulfment of the bacteria.
Salmonella also transports the SptP protein that is
involved in reversing the cytoskeletal rearrangements
induced during invasion [8]. SptP is a bifunctional protein
with a GTPase-activating (GAP) domain in the N-terminal
region and tyrosine phosphatase activity in the C-terminus
[30,31]. The GAP function reverses the activation of the G
proteins Cdc42 and Rac, and is responsible for restoring
normal cytoskeletal architecture following Salmonella
invasion [30]. Tyrosine phosphorylation of host proteins
also appears to be involved in Salmonella invasion, as well
as transducing nuclear signals inducing inflammatory gene
expression [31]. These processes are reversed by the SptP
C-terminal tyrosine phosphatase activity. Recent insight into
the temporal regulation of SptP activity in the host cell has
been provided by studies showing that SptP degradation by
the proteosome pathway is delayed compared to SopE [32].
These results suggest that SptP persists in the cytoplasm
after the breakdown of the SPI1 effectors that mediate actin
polymerization, restoring the actin cytoskeleton to its
normal dynamics.
Actin events associated with intracellular infection
Following invasion, Salmonella proliferation inside
epithelial cells is dependent on the function of the SPI2
TTSS [33]. Furthermore, Salmonella infection of the
intestinal mucosa leads to phagocytosis by macrophages,
and the development of systemic disease involves the
survival and growth of Salmonella inside macrophages by
a process that requires SPI2 [10,34,35]. The SPI2 TTSS
translocates a number of effector proteins across the
vacuolar membrane into the host cell, altering the physiology of the SCV [12,13]. These effects have been reported to
include the formation of filamentous endosomes [36],
decreased recruitment of NADPH oxidase [37], and a
change in the SCV fusion pathway [38]. In addition, a
SPI2-dependent effect on actin polymerization in the
D.G. Guiney, M. Lesnick / Clinical Immunology 114 (2005) 248–255
vicinity of the SCV has been described [39,40]. Within
several hours of Salmonella entry into cultured epithelial
cells, fibroblasts, or macrophages, a collection of F-actin
filaments around the SCV is observed. These structures are
referred to as VAP, for vacuolar-associated actin polymerizations. The formation of these localized F-actin networks
requires a functional SPI2 TTSS, although the individual
effector proteins responsible for this collection of filaments
have not been identified. Two effectors, SspH2 and SseI,
have been shown to colocalize with the polymerizing actin
cytoskeleton [40]. Both SspH2 and SseI interact through
homologous N-terminal domains with the host actin-binding
protein, filamin. SspH2 also contains a C-terminal domain
which may interact indirectly with profilin, another host
protein involved in actin polymerization. However, neither
SspH2 nor SseI is essential for the formation of F-actin
around the SCV, since single and double sspH2/sseI mutants
still show normal morphology of the VAP [40].
The functional significance of VAP formation has been
difficult to establish. Although it has been found in different
cell types and correlates with the intracellular replication of
Salmonella, there remains no direct evidence that VAP
formation plays an important role during intracellular
pathogenesis. The SPI2 secretion function is required for
VAP, but no effector protein(s) has been shown to be
essential for VAP formation. Therefore, no genetic data
support the importance of VAP. Treatment of macrophages
with actin depolymerizing agents such as latrunculin B or
cytocholasin D abolishes VAP formation and decreases
intracellular growth of Salmonella [39]. However, these
agents induce multiple deleterious changes in cell physiology, so that the observed decrease in bacterial replication
could be due to indirect effects. Therefore, additional work
is necessary to clarify the role of VAP formation in
Salmonella pathogenesis.
A number of Salmonella strains produce the SpvB
protein, an intracellular toxin that induces profound actin
depolymerization in host cells [14–16,41]. SpvB is encoded
by the spv locus (see Fig. 2), a regulon consisting of the
spvABCD structural genes controlled by spvR [42]. In the
251
Fig. 3. Schematic representation of domains within the SpvB protein. The
N-terminal region has homology with the N-terminal domains of a family
of insect toxins. However, each of these insect toxins has substantial
C-terminal extensions that do not share sequences with SpvB. The N- and
C-terminal regions of SpvB are connected by a polyproline bbridgeQ that is
likely to provide a flexible secondary structure connection between the
two domains. The C-terminus is homologous to the enzymatic region of
bacterial ADP-ribosyl transferases, being most closely related to the
toxins that modify cellular actin, including Vip2 from B. cereus, C2
from C. botulinum, and Iota from C. perfringens.
S. enterica subspecies I lineage, comprising the Salmonella strains most often isolated from humans and
domestic animals, the spv genes are located on virulence
plasmids. For several other Salmonella lineages, the spv
locus is encoded on the chromosome [43]. The subspecies
I plasmids carrying spv, found in serovars such as
Typhimurium, Enteriditis, Choleraesuis, Dublin, and Gallinarum-Pullorum, substantially increase the systemic virulence of these strains. Extensive genetic studies in the
mouse model of infection have shown that the spv genes
do not affect invasion in the intestinal tract but greatly
increase the ability of Salmonella to proliferate intracellularly during the systemic, extraintestinal phase of the
disease [44]. The spvR and spvB genes are essential for the
virulence phenotype, while spvA is not required, and spvC
and spvD appear to have partial virulence effects [45].
spvR encodes a LysR-type transcriptional activator
required together with the bacterial RpoS sigma factor to
express the spvABCD genes, as shown in Fig. 2 [46–48].
The SpvB protein is the major virulence effector of the
spv locus and consists of distinct N- and C-terminal
domains (Fig. 3) [49]. The N-terminal region is homologous
spvR
spvR
Fig. 2. The spv operon. The spvR gene encodes a transcriptional activator that is required together with the alternate sigma factor RpoS for expression of the
spvABCD operon and autoinduction of spvR. The spvR protein binds to sites just upstream from both the spvA and spvR promoters (PspvA and PspvR), inducing
its own expession as well as the spvABCD structural genes.
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D.G. Guiney, M. Lesnick / Clinical Immunology 114 (2005) 248–255
to a family of insect toxins initially identified in Photorhabdus luminescens and subsequently found in the
genomic sequences of several organisms, including species
of Serratia and Yersinia. However, the functional significance of these homologies remains unclear, since the
biochemical role of this N-terminal domain has not been
demonstrated for any member of the family.
In contrast, the C-terminal domain of SpvB contains a
clearly established ADP-ribosylating motif [14,15,50].
Purified SpvB was shown to use NAD as a substrate to
covalently attach ADP to G-actin monomers, a modification
that blocks actin polymerization. Due to a dynamic
equilibrium between actin polymerization and depolymerization in the eukaryotic cell, inhibition of monomer
polymerization leads to rapid loss of the F-actin polymers
in the cell (Fig. 4). As predicted, transfection of a vector
expressing SpvB into mammalian cells led to complete
depolymerization of the actin cytoskeleton [14].
Actin depolymerization due to SpvB is also seen in host
cells infected with Salmonella [15]. This effect was first
observed as cytotoxicity manifested by detachment of
infected human monocyte-derived macrophages [51] but
has been seen in a variety of cell lines. In addition, SpvBdependent actin depolymerization has been found in splenic
macrophages isolated from Salmonella-infected mice, confirming that this process occurs in vivo [Lesnick and
Guiney, unpublished results]. SpvB-induced actin depolymerization in cells is also associated with cytopathology
consistent with apoptosis, including activation of caspase 3,
cleavage of cytokeratin (in epithelial cells), DNA fragmentation, and nuclear condensation [33,51].
In contrast to VAP formation, SpvB-mediated actin
depolymerization has been clearly shown by genetic studies
to be essential for both the induction of cytopathology in
cell culture and for the spv virulence phenotype in vivo
[14,15]. A specific mutation in the NAD-binding site of
SpvB was constructed in the native spvB gene in serovars
Typhimurium and Dublin [14]. The mutant SpvB lacks
ADP-ribosylation activity and is unable to induce actin
depolymerization during Salmonella infection of human
epithelial cells or macrophages. Furthermore, the spvB
mutant Typhimurium and Dublin serovars are severely
attenuated for virulence in mice. Recent genetic evidence
also indicates that the SPI2 TTSS is required for SpvBmediated actin depolymerization in cultured cells. Strains
with mutations in the SPI2 genes ssaV and ssaJ, encoding
core components of the TTSS, are unable to induce actin
depolymerization in human macrophages [15]. These results
imply that SpvB is not able to gain access to the host cell
cytoplasm without the SPI2 TTSS function, but the
mechanistic relationship between the SPI2 system and
SpvB remains unclear. However, earlier genetic studies
comparing the attenuation of SPI2 and spv single and
double mutants suggested that SPI2 and spv act through
different virulence mechanisms [52]. This apparent conflict
between results from human cell cultures and mouse
infection experiments needs to be clarified in future studies.
Conclusions on the role of the actin cytoskeleton in
Salmonella pathogenesis
Fig. 4. Mechanism of F-actin disruption by bacterial ADP-ribosylating
toxins. In the host, there is a dynamic equilibrium between G-actin
monomers and polymerized F-actin filaments. The SpvB protein and other
actin modifying toxins catalyze the covalent attachment of ADP-ribose to
G-actin monomers, preventing their incorporation into polymers. Since
F-actin is continuously depolymerized by cytoskeletal regulatory proteins,
eventually the monomers are trapped in an inactive form, and no more
G-actin is available for polymerization. The cell becomes completely
depleted of F-actin filaments.
Three major Salmonella virulence systems target the
actin cytoskeleton of host cells, but the physiologic actions
of the bacterial effectors are quite different and potentially
antagonistic. The activities of the Salmonella effector
proteins must be carefully regulated and orchestrated during
the infectious process to avoid conflicting effects on the
cytoskeleton. Salmonella use focal actin polymerization to
gain access to the intracellular environment. In the case of
intestinal epithelial cells, this uptake requires the concerted
action of effectors transported by the SPI1 TTSS and is
reversed, in part, by the prolonged activity of another
effector, SptP. For macrophages, phagocytosis driven by
actin polymerization can occur in the absence of the SPI1
TTSS function, and SPI1 mutants remain virulent when
inoculated by routes that bypass intestinal invasion [53]. In
both intestinal invasion and phagocytosis, the inappropriate
activity of the SpvB protein would be predicted to
antagonize bacterial uptake. Extensive genetic evidence
indicates that SPI1 and spv are regulated by different
mechanisms [54]. In cell culture, SpvB is preferentially
expressed after bacterial entry into both phagocytic and
nonphagocytic cells [55,56]. Techniques that detect gene
expression in infected animals have found that spvB is
D.G. Guiney, M. Lesnick / Clinical Immunology 114 (2005) 248–255
253
Fig. 5. Model for the role of SpvB in Salmonella pathogenesis. After phagocytosis by the macrophage, SPI2 effectors and SpvB are secreted into the host cell
cytoplasm. SPI2 effectors alter key cellular processes to promote bacterial growth within the SCV. SpvB disrupts the actin cytoskeleton, further compromising
normal cellular physiology and initiating apoptosis. The apoptotic macrophage containing Salmonella is phagocytosed by surrounding uninfected
macrophages, thus perpetuating the intracellular infection. This model explains why Salmonella infections are not susceptible to aminoglycoside therapy, since
the organism is not exposed to the extracellular environment during the infection cycle.
expressed during systemic infection of mice [57]. However,
gene regulation is not the only factor that prevents
inappropriate action of SpvB. Purified SpvB added to cell
cultures is not active from the extracellular space [14] and
therefore must be delivered from Salmonella after they are
taken up into the SCV. Salmonella expressing SpvB before
phagocytosis are still efficiently engulfed by macrophages
[51], suggesting that transport of SpvB and depolymerization of cellular actin occurs only when Salmonella are
intracellular. The effect of SpvB on cellular actin is not seen
for a number of hours following cell entry [15,51]. The
detailed mechanisms regulating SpvB transport from the
SCV to the cytoplasm remain to be determined.
The effects of SPI2 and SpvB on the actin cytoskeleton
also appear to be potentially antagonistic. Focal actin
condensation (VAP) around the SCV is dependent on SPI2
TTSS function and is more pronounced in an SpvB mutant
[40]. The role of VAP during intracellular pathogenesis is
not clear, and it is possible that VAP is an epiphenomenon
induced by the SCV but not causally related to intracellular
proliferation. Alternatively, focal actin polymerization
around the SCV may affect vesicle mobility and fusion,
creating a more favorable environment for the bacteria. One
possible role for SpvB is to modulate VAP formation and
then remove the VAP after a certain amount of bacterial
growth has occurred. Focal actin polymerization has been
found to drive movement of intracellular vesicles, including
phagosomes, through the cytoplasm [58,59], and the
depolymerizing action of SpvB would be expected to
abolish this movement.
The SpvB protein produces a clear cytopathic effect in
the cell. About 10–12 h after Salmonella infection, human
monocyte-derived macrophages begin to lose F-actin
filaments, and by 20–24 h, many cells have no detectable
polymerized actin [15,51]. Cells detach from the culture
well and undergo DNA fragmentation in a process with
features of apoptosis. Loss of the actin cytoskeleton has
been shown in other culture systems to have global effects
on cell function and to induce apoptosis by a mechanism
involving BH3-only regulatory proteins [60]. By this
mechanism, the SpvB protein may provide Salmonella with
an exit strategy from the infected cell (see Fig. 5). The
observation that Salmonella infections are unresponsive to
aminoglycosides such as gentamicin implies that Salmonella spreads from cell-to-cell with little exposure to the
extracellular environment [6]. Death of infected cells by
necrosis must be minimal since necrotic cells quickly lose
their membrane function, and the intracellular bacteria
would be killed by aminoglycosides. However, infected
cells undergoing apoptosis retain membrane integrity,
excluding aminoglycosides from the intracellular bacteria.
These infected, apoptotic cells may be phagocytosed by
migrating, uninfected macrophages, thus perpetuating the
intracellular infection. According to this model, a major
function of the SpvB protein in vivo may be to ensure that
the infected cell is an attractive apoptotic target for
surrounding uninfected macrophages (Fig. 5).
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