Bacterial Invasion and Intracellular Survival

Some bacteria do more than colonize a host surface: they enter host cells and survive inside them, gaining a protected niche shielded from antibodies, complement, and many antibiotics. Invasion requires the pathogen to trigger or hijack the host cell's own uptake machinery, and intracellular survival requires it to resist or remodel the destructive compartments into which it is taken.

Definition

Bacterial invasion is the active entry of a bacterium into a host cell, typically by inducing the host's cytoskeleton to internalize it; intracellular survival is the pathogen's capacity to persist and replicate within the host cell, either free in the cytosol or within a modified vacuole.

Scope

This topic covers how bacteria enter non-phagocytic host cells, the contrast between cytosolic and vacuolar intracellular lifestyles, and the strategies pathogens use to persist within cells. It is a reference topic in bacterial pathogenesis and does not provide clinical guidance.

Core questions

• How do bacteria induce non-phagocytic cells to take them up?
• What distinguishes a cytosolic from a vacuolar intracellular lifestyle?
• How do intracellular bacteria avoid lysosomal killing and exploit host resources?

Key concepts

• Invasins and triggered uptake
• Zipper versus trigger mechanisms of entry
• Type III secretion effectors
• Cytosolic lifestyle
• Vacuolar (modified phagosome) lifestyle
• Actin-based motility and cell-to-cell spread

Mechanisms

Invasion of non-phagocytic cells follows two broad routes. In the zipper mechanism, a bacterial surface protein (an invasin) binds a host receptor and progressively engages the membrane to engulf the bacterium; in the trigger mechanism, secreted effectors injected by a type III secretion system remodel the host actin cytoskeleton to produce membrane ruffles that sweep the bacterium in. Once inside, pathogens diverge: cytosolic bacteria escape the entry vacuole into the cytoplasm and may exploit host actin to move and spread cell-to-cell, while vacuolar bacteria remain within a membrane-bound compartment that they arrest or remodel to prevent fusion with destructive lysosomes. Across both lifestyles, intracellular pathogens subvert host-cell functions to acquire nutrients and evade killing.

Clinical relevance

An intracellular niche helps explain why certain infections are chronic, relapse, or are hard to clear, since the bacteria are sheltered from humoral immunity and from agents that penetrate cells poorly. This entry describes invasion and intracellular survival for reference and is not a basis for treatment decisions.

History

The molecular study of invasion advanced through the identification of invasin in Yersinia, which conferred entry into cultured cells, and through work on Salmonella, Shigella, and Listeria that defined the trigger and zipper mechanisms and the contrast between vacuolar and cytosolic lifestyles, establishing intracellular pathogens as a major theme of cellular microbiology.

Key figures

• Pascale Cossart
• Stanley Falkow
• Ralph Isberg
• Philippe Sansonetti

Seminal works

• finlay-1997-science
• pizarro-cerda-cossart-2006
• ray-2009

Frequently asked questions

Why would a bacterium live inside a host cell?

The intracellular space offers protection from antibodies, complement, and many antibiotics, plus access to host nutrients, giving the pathogen a sheltered niche in which to replicate.

What is the difference between a cytosolic and a vacuolar intracellular pathogen?

Cytosolic pathogens escape the entry vacuole and live free in the cytoplasm, often spreading cell-to-cell using host actin, whereas vacuolar pathogens remain inside a membrane-bound compartment that they modify to avoid lysosomal destruction.