Salmonella is a bacterium recognized for causing foodborne illnesses, from mild gastroenteritis to severe systemic infections. Its survival and disease-causing mechanisms are tied to its physical and genetic makeup. The bacterium’s structure is linked to its ability to navigate the host environment, defend against immune responses, and manipulate host cells.
Unveiling Salmonella: A Microscopic Snapshot
Salmonella are prokaryotic microorganisms, so their cells lack a nucleus and other membrane-bound organelles. They are classified as Gram-negative bacteria due to their cell wall architecture. This classification means they do not retain a crystal violet stain in laboratory tests, appearing red or pink under a microscope.
These bacteria are rod-shaped bacilli, measuring 2 to 5 micrometers in length and 0.5 to 1.5 micrometers in diameter. Their cell wall has a thin inner layer of peptidoglycan outside the cytoplasmic membrane. Beyond this is an outer membrane, a defining feature of Gram-negative bacteria that is important for interacting with its environment.
Salmonella’s Protective Armor and Toxic Coat
The cell envelope of Salmonella is a multi-layered barrier that protects the bacterium and interacts with its host. The outer membrane is an asymmetrical bilayer whose outer leaflet is dominated by a molecule called lipopolysaccharide (LPS). LPS is a component for the bacterium’s survival and ability to cause disease. It acts as a barrier, shielding the bacterium from harsh environments like the stomach’s acid.
LPS is composed of three parts: Lipid A, a core oligosaccharide, and the O-antigen. Lipid A anchors the LPS molecule in the outer membrane and is recognized by the host immune system as an endotoxin, triggering inflammatory responses like fever. The core oligosaccharide links Lipid A to the O-antigen, a long chain of repeating sugar units that extends from the bacterial surface. The composition of this O-antigen varies between different strains, and it is used to classify Salmonella into more than 2,600 serotypes.
Navigating and Anchoring: Salmonella’s External Tools
Salmonella has external appendages for movement and attachment within a host. Motility is achieved through flagella, long, whip-like structures protruding from the cell surface. Each flagellum consists of a filament, a hook, and a basal body that anchors it to the cell envelope. The basal body acts as a rotary motor, spinning the filament to propel the bacterium toward the cells lining the intestine.
For attachment, Salmonella uses pili (also known as fimbriae), which are shorter and thinner appendages than flagella. Their main function is adhesion, allowing the bacterium to bind to host cells in the gut. This attachment is a prerequisite for colonization and invasion, preventing the bacteria from being flushed out of the digestive system.
Inside Salmonella: The Command Center and Building Blocks
The cytoplasm is where the life processes of a Salmonella cell occur. This gel-like substance contains ribosomes, the machinery for synthesizing proteins. These proteins form the cell’s structural components and carry out the chemical reactions for survival and replication. The instructions for building these proteins are encoded in the bacterium’s genetic material.
Salmonella’s genetic material is not in a nucleus but in a region of the cytoplasm called the nucleoid. Here, a single, circular chromosome contains most of the bacterium’s genes. Salmonella can also carry plasmids, which are small, independent circles of DNA. Plasmids often contain genes that provide an advantage, like antibiotic resistance or specific virulence factors.
Structural Adaptations for Causing Illness
Salmonella has structures designed to manipulate host cells and cause disease. A key part of this are protein complexes known as secretion systems. The Type III Secretion System (T3SS) is often described as a “molecular syringe.” This apparatus is built from many different proteins and forms a needle-like structure extending from the bacterial surface.
The T3SS spans both the inner and outer membranes and can make direct contact with a host cell. Upon contact, it injects effector proteins directly into the host cell’s cytoplasm. These proteins are molecular tools that hijack the host’s cellular machinery for the bacterium’s benefit. They can trigger the host cell to engulf the bacterium, facilitating invasion into the intestinal lining.
Once inside the host cell, Salmonella resides within a compartment called the Salmonella-containing vacuole (SCV). The bacterium uses its T3SS to inject more effector proteins that modify this vacuole. These modifications prevent the SCV from fusing with lysosomes, which would normally destroy the bacterium. This allows Salmonella to survive and replicate within the host cell, hidden from the immune system, leading to infection.