When Legionella pneumophila enters a macrophage, it hijacks the phagosome and converts it into a completely different compartment, one that resembles the endoplasmic reticulum (ER) rather than the destructive, acidic environment that normally kills bacteria. This remodeled compartment, called the Legionella-containing vacuole (LCV), becomes a protected niche where the bacterium replicates freely. The entire process depends on roughly 300 bacterial proteins injected directly into the host cell.
How Legionella Avoids Destruction
Normally, after a macrophage engulfs a bacterium, the phagosome fuses with lysosomes within minutes. The resulting compartment drops to a highly acidic pH, fills with digestive enzymes and reactive oxygen species, and breaks down the trapped microbe. Legionella prevents this entirely.
The bacterium uses a specialized molecular syringe called the Dot/Icm type IV secretion system to inject effector proteins across the vacuole membrane and into the host cell’s cytoplasm. These effectors block the normal fusion machinery that would merge the phagosome with lysosomes. One effector, LegC3, disrupts a key step in membrane fusion by preventing the assembly of protein complexes (called trans-SNARE complexes) that physically pull two membranes together. Without those complexes forming, lysosomes simply cannot merge with the vacuole.
The result is striking: Legionella maintains a neutral pH inside its vacuole for at least six hours after infection. By contrast, vacuoles containing non-pathogenic bacteria or dead Legionella become acidified within 15 minutes. The LCV does eventually acidify, but only after 10 or more hours of infection, by which point the bacterium has already established its replicative niche.
Remodeling the Vacuole Into an ER-Like Compartment
Blocking lysosome fusion is only half the strategy. Legionella simultaneously recruits material from the host cell’s ER to rebuild its vacuole from the outside in. The phagosome, originally made of plasma membrane, is progressively studded with ER-derived vesicles and proteins until it becomes virtually indistinguishable from a segment of the ER itself.
A central player in this remodeling is the bacterial effector DrrA (also called SidM). DrrA grabs a host protein called Rab1, a small signaling molecule that normally directs vesicle traffic between the ER and Golgi apparatus, and forces it onto the LCV surface. Once Rab1 is in place, it redirects ER-derived vesicles toward the vacuole. DrrA also promotes an unusual form of membrane fusion: it brings together a vesicle-associated protein (Sec22b) with proteins normally found on the plasma membrane (syntaxin 2, 3, 4 and SNAP23), creating a non-standard pairing that efficiently stitches ER-derived vesicles onto the LCV.
Two other effectors, SidC and SdcA, anchor themselves to the vacuole membrane by binding a specific lipid signal and then act as tethers to pull additional ER proteins toward the vacuole surface. Meanwhile, the bacterium recruits another pair of host signaling molecules, Rab33B and Rab6A, through a cascade. Bacterial effectors from the SidE family chemically modify Rab33B, which then recruits Rab6A. Rab6A in turn interacts with ER-resident fusion proteins, including syntaxin 18, to physically dock the vacuole against the ER network. This Rab33B-Rab6A cascade essentially welds the LCV into the ER, giving Legionella intimate access to the cell’s protein-manufacturing infrastructure.
Transitioning From Smooth to Rough ER
The ER-like character of the LCV isn’t static. Early in infection, the vacuole resembles smooth ER. As the infection progresses, it transitions to resemble rough ER, the ribosome-studded form responsible for protein production. Host proteins Rab4, Rab10, and an ER membrane protein called BAP31 are critical for this shift. This transition likely gives the bacterium access to a richer supply of newly synthesized host proteins and amino acids.
Feeding Inside the Vacuole
Legionella is a nutritional parasite. It relies heavily on amino acids scavenged from its host rather than synthesizing its own from scratch. Isotope-tracing experiments have shown that amino acids made by the host cell are efficiently transported into the LCV and then into the bacteria, where they are directly incorporated into new bacterial proteins. Host amino acid transporters on the vacuole membrane appear to facilitate this transfer.
Most amino acids are used as-is. Only a few, including alanine, aspartate, and glutamate, show signs of being partially rebuilt by the bacterium, suggesting Legionella does minimal biosynthetic work and instead co-opts the host’s metabolic output wholesale.
Legionella also reshapes the host cell’s energy metabolism. It modulates mitochondrial dynamics in infected macrophages, triggering a shift toward a Warburg-like metabolic state, where the cell relies more on rapid glucose breakdown rather than efficient oxygen-dependent energy production. This metabolic reprogramming directly favors bacterial replication.
The Replication-to-Escape Switch
Inside the remodeled vacuole, Legionella exists in a replicative form: long, slender, non-motile rods with a wavy cell wall, focused entirely on growth and division. During this phase, virulence traits like motility and cytotoxicity are switched off. The bacteria are metabolically active but not infectious.
As the bacterial population expands and nutrients inside the LCV become depleted, Legionella undergoes a dramatic transformation. Nutrient starvation triggers what’s known as the stringent response, a tightly coordinated genetic program that converts replicating bacteria into a transmissive form. These transmissive bacteria look physically different: short, stubby rods with smooth, thickened cell walls and internal granules of a carbon-storage polymer called poly-3-hydroxybutyrate, essentially energy reserves for survival outside the cell.
Transmissive bacteria reactivate motility via flagella, become cytotoxic, and express the full suite of traits needed to burst out of the spent host cell, survive in the extracellular environment, and invade a new macrophage. Once inside a fresh host cell, the cycle restarts: the Dot/Icm system fires, effectors flood the cytoplasm, lysosome fusion is blocked, ER vesicles are hijacked, and a new LCV is built from the ground up.