Most bacterial cells possess a cell wall, an outer layer surrounding the cell membrane. This structural component is fundamental for bacterial survival and interaction with their environment. Its composition and characteristics vary significantly among different types of bacteria.
Building Blocks of Bacterial Cell Walls
The primary structural component of bacterial cell walls is a unique molecule called peptidoglycan, also known as murein. This complex polymer is made of sugar derivatives and amino acids, forming a mesh-like layer that provides structural strength. The amount and arrangement of peptidoglycan vary, leading to a classification of bacteria into two main types: Gram-positive and Gram-negative.
Gram-positive bacteria are characterized by a thick peptidoglycan layer, which can constitute a significant portion of their cell wall, ranging from 20 to 80 nanometers thick. This thick layer often contains additional polymers such as teichoic acids, which are covalently linked to the peptidoglycan.
Gram-negative bacteria, in contrast, have a much thinner peptidoglycan layer, typically about 5 to 10 nanometers thick. This thin layer is situated in a space between an inner cytoplasmic membrane and an outer membrane. The outer membrane of Gram-negative bacteria also contains lipopolysaccharide (LPS), a molecule composed of lipids and sugars.
Purpose of Bacterial Cell Walls
The bacterial cell wall provides essential functions for the organism’s survival. It gives the bacterium its characteristic shape, such as a rod, sphere, or spiral, helping to maintain its cellular form. This rigid structure ensures that the cell maintains its integrity and proper morphology.
The cell wall acts as a protective barrier against external stresses. It shields the bacterium from physical damage and harmful substances in its environment. The cell wall also plays a significant role in preventing osmotic lysis, which is the bursting of the cell due to excessive water intake.
Bacteria often live in environments where the concentration of solutes outside the cell is lower than inside, causing water to move into the cell. The cell wall counteracts this internal pressure, known as turgor pressure, maintaining the cell’s stability. This allows the bacterium to survive in diverse conditions that would otherwise cause its delicate cell membrane to rupture.
Cell Walls and Medical Science
The distinct structure of bacterial cell walls has significant implications in medical science, particularly concerning antibiotic treatments and bacterial identification. Many antibiotics specifically target the bacterial cell wall, exploiting its presence in bacteria and absence in human cells. This selective targeting allows these drugs to disrupt bacterial growth without harming human cells.
Antibiotics such as penicillin, cephalosporins, and vancomycin work by interfering with the synthesis or integrity of the peptidoglycan layer. For example, beta-lactam antibiotics, which include penicillin, prevent the proper cross-linking of peptidoglycan chains during cell wall formation. This weakens the cell wall, making the bacterium vulnerable to osmotic lysis and leading to its death.
Another application of bacterial cell walls in medicine is Gram staining, a diagnostic tool developed by Hans Christian Gram in 1884. This technique differentiates bacteria into Gram-positive (which stain purple) and Gram-negative (which stain pink or red) based on their cell wall properties. Gram stain results provide rapid information about the type of bacteria present, guiding antibiotic selection.
When Bacteria Lack a Cell Wall
While most bacteria possess a cell wall, there are exceptions to this general rule. The genus Mycoplasma represents a unique group of bacteria that naturally lack a cell wall. This absence sets them apart from most other bacterial species.
The lack of a rigid cell wall means that Mycoplasma species do not have a fixed shape; they are pleomorphic, meaning they can vary in form. This characteristic distinguishes them from bacteria with defined rod, spherical, or spiral shapes. A significant consequence of lacking a cell wall is their natural resistance to antibiotics that specifically target cell wall synthesis, such as penicillin.
Mycoplasma species can cause various infections in humans. For instance, Mycoplasma pneumoniae is a known cause of respiratory infections, including a form of pneumonia often referred to as “atypical pneumonia.” Other species like Mycoplasma genitalium are associated with urogenital infections. Their unique structure necessitates different approaches for diagnosis and treatment compared to most other bacterial infections.