Overview: 3 bacterial strategies for infection
1. Bacteria adhere to and subvert the host cell without invading
2. Bacteria invade host cell and remain in peripheral tissues: localized infection
3. Bacteria invade host cells and disseminate to other body sites: systemic infection
Adaptation to an invasive lifestyle
Pathogens can be divided into 3 groups according to their lifestyle preferences:
• Extracellular: tend to avoid penetrating and living within host cells
• Obligately intracellular: can only live within host cells
• Facultatively intracellular: may have a preference for intracellular or extracellular living,
but are capable of both
What can bacteria invade?
o Blood vessel walls
o Lympathic system
- Extracellular matrix
o May secrete collagenases to allow digestion of the ECM
o “Trojan horse” strategy
o True invasion? Debatable!
How can bacteria gain access to these tissues?
- Organism may be capable of breaching the epithelium of its own accord
- Organism may erode the tooth structure
- Organism is injected into the host by an arthropod
- Organism gains access to the underlying tissues following trauma to the epithelium Invasion: to enter for hostile purposes.
Actually more like… pathogens often behave more like confidence tricksters; the host cell
usually invites them inside!
Inside host cells: nutrients are abundant, no competition with extracellular bacteria, immune
system is circumvented
The host cell cytoskeleton
o Dynamic structure: constantly being rearranged to meet the needs of the host cell;
o Involved in: cellular movement, vesicle trafficking, mitosis, formation of a range of
o 3 main components: actin filaments, microtubules, intermediate filaments
- Polymer of 43kDa globular protein, actin -> arranged to a form helical structure
- Organized in the form of linear bundles, 2D networks, 3D gels -> actions of
numerous accessory proteins
- Concentrated at the cell periphery
- Form a 3D network below the cytoplasmic membrane – “cell cortex” – determines cell
- Form transient structures known as filopodia (fingers) and lamellipodia (ruffles) these
are also structures that are invariably formed during bacterial invasion.
Invasion involving actin rearrangements
- Most common form of bacterial invasion
- Pathogens: Yersinia spp., Listeria monocytogenes, Shigella spp., Salmonella spp.
- All utilize microfilaments to gain access to cells
- Important differences in the actual process and fate of the invading organism Yersinia spp.
• 3 species are able to cause disease in humans: Yersinia pestis, Yersinia enterocolitica,
• Pestis invade systemically; enterocolitica and pseudotuberculosis invade locally.
Stages of Y. enterocolitica invasion:
a. Attachment mediated at many points through the bacterial adhesins “invasin”,
“Ail”, and “YadA”;
b. Crystal structure of invasion shown to be very similar to fibronectin -> despite
very different protein sequences!
c. Invasion has 100-fold greater affinity for host cell integrins than natural
a. Integrin binding by invasin forces bacterium and host into close contact. Why
i. Normally, integrins mediate binding of cells to other cells and ECM
ii. Binding of integrins initiates cell ‘spreading’ to cover a new surface b. Y. enterocolitica exploits natural integrin-mediated spreading to become
c. Integrin clustering leads to activation of second messengers, that end up causing
the formation of membrane protrusions and the engulfment of bacteria.
Howevers… in vivo vs in vitro integrins are found at the basal side of an epithelial cell, and
whether they are important for in vivo invasion by Y. enterocolitica is debatable!
In vivo, the likely route of invasion of Yersinia is the microfold cells (M cells) of Peyer’s patches.
What are M-cells?
- Specialized intestinal epithelial cells lack villi, far less abundant than IECs (intestinal
- Capable of actively transporting antigens across from the gut lumen
across the epithelium
- Antigens are then placed into direct contact with immune cells
- Process facilitates the initiation of mucosal immune reactions.
2 routes to invasion by Yersinia spp.
1. Invasion of M-cells
2. Invasion of IECs at the basal site.
Invasion by Listeria monocytogenes
- Facultatively intracellular
- Facultative anaerobe, motile at low temperatures
- Infects a wide variety of hosts -> mammals, birds, fish, insects
- Uncommon but serious form of food poisoning with mortality rate up to 70%
- One of few bacteria capable of crossing the placenta.
a. Adherence is mediated in part by the bacterial protein, ActA, which binds to
heparin and/or heparin sulfate. 2. Deployment of internalins (Inls)
a. Inls are targeted to the bacterial surface by a N-terminal signal sequence
b. N-terminus also has a stretch of leucine rich repeat units (LRRs)
c. LRR regions bind to host cell receptors such as E-cadherin (InlA)
d. Plenty of redundancy – listeria has lots of internalins and other internalization
The zipper mechanism
- Like Yersinia, L. monocytogenes is engulfed by actin rearrangements and ‘sinks’ into the
- This is known as the zipper mechanism after internalization, the host cell looks the
same as it did before internalization.
- Listeria disrupts the phagosomal membrane within minutes of invasion, and escapes to
- It uses a toxin known as listerilysin (LLO) to punch holes in the vacuole
- Once in the cytosol, Listeria replicates with a doubling time of ~50 minutes, and is
capable of using many host cytosol components as growth substrates.
- Listeria also needs iron to grow, but makes no siderophores
o Instead, it attacks host iron stores using a ferric iron reductase, then transports
the iron to its own transferrin binding protein
o People with iron overload are more susceptible to listeriosis.
The grass is always greener…
- Listeria likes to move onto pastures new to outrun the host immune system, to get
access to more nutrients;
- It uses actin to propel itself around the cytosol and from one host cell to the next;
- Bacterial protein responsible for motility is ActA -> part of this protein shares a high
degree of homology with the actin binding domain of human vinculin, a cytoskeletal
protein. Shigella spp.
- Gram negative, Facultatively anaerobic rod
- Different serotypes cause varying degrees of disease
- Shigellosis: severe, watery diarrhea, sometimes with blood and mucus; fever and
abdominal cramps usually self-limiting in adul