3 Microbial Cell Structure
Explore bacterial shapes, cell envelopes, surface structures, and internal components, then compare bacterial cells with other microbial groups.
Shapes and arrangements
Bacterial cells share a basic plan, but their visible forms and groupings vary. Shape describes the form of an individual cell; arrangement describes how cells remain grouped after division, partly depending on their planes of division.
Common shapes include:
Cocci: spherical or nearly spherical cells.
Bacilli: rod-shaped cells; short rods may be called coccobacilli.
Vibrios: curved rods, often comma-shaped.
Spirilla: rigid spiral-shaped cells.
Spirochetes: flexible, helical cells.
Cocci may form pairs called diplococci, chains called streptococci, clusters called staphylococci, or groups of four called tetrads. Rods may occur singly, in pairs, in chains, or in side-by-side palisades. These names describe appearance; shape and arrangement alone do not identify a species.
Takeaway: Shape and arrangement are separate features, and both can vary among bacteria.
The
The surrounds the cytoplasm. Its core is the plasma membrane, and most bacteria also have a cell wall outside it. The wall helps maintain cell shape and protects the cell from bursting when water enters by osmosis.
In most bacteria, the wall's structural mesh is , a polymer of sugar chains cross-linked by short peptides. The mesh provides strength while allowing growth. Two common envelope patterns explain the characteristic results of the Gram stain:
have a thick layer outside the plasma membrane. Their walls commonly contain teichoic acids; lipoteichoic acids connect the wall to the membrane.
have a thin layer in the periplasm, between the plasma membrane and an outer membrane. The outer membrane contains lipopolysaccharide (LPS), and proteins such as porins allow certain small molecules to pass.
These structural differences affect how substances interact with bacterial cells. Some bacteria, including mycobacteria, have waxy, mycolic-acid-rich envelopes and are identified using acid-fast staining rather than relying only on the standard Gram distinction. A few bacteria lack a cell wall, so antibiotics that target do not act on them in the usual way.
The bacterial plasma membrane is generally a phospholipid bilayer containing proteins. It is selectively permeable: some substances cross directly, while channels, carriers, or energy-dependent pumps move others. The membrane also hosts important processes such as energy conversion. Some photosynthetic bacteria have specialized internal membrane structures containing pigments.
Takeaway: Wall composition and membrane organization shape how bacteria maintain their structure and interact with their environment.
Surface structures and their roles
Bacterial structures outside or extending from the envelope help cells attach, move, and interact with their surroundings. Not all bacteria have the same set of surface features.
A is a surface coat. A firmly attached and organized is often called a capsule; a loose coat that is easily removed is often called a slime layer. These coats can promote attachment and biofilm formation. A capsule can also make it harder for host immune cells to engulf a bacterium.
are typically short, numerous protein structures that help bacteria attach to host cells or other surfaces. are often longer and fewer; some participate in DNA transfer between bacterial cells.
Flagella are rotating, protein-based appendages that propel many bacteria through liquid. They may be located at one end, at both ends, or around the cell. An S-layer, where present, is an outer layer made of structural proteins or glycoproteins.
Takeaway: Surface structures have distinct roles: coats can aid protection and attachment, and can support attachment or DNA transfer, and flagella can enable movement.
Inside the bacterial cell
Bacteria lack a membrane-bound nucleus. Their chromosome occupies a region of the cytoplasm called the . Many bacteria also carry smaller DNA molecules called plasmids. The cytoplasm contains 70S ribosomes, which synthesize proteins, as well as enzymes and molecules needed for cellular activity.
Bacteria generally lack the extensive membrane-bound organelles found in eukaryotic cells, but some have specialized internal structures. Inclusions can store materials such as carbon or phosphate. Other inclusions have specific functions: gas vesicles and magnetosomes are examples. Some bacteria form an when conditions become unfavorable. It is a dormant, highly resistant survival structure—not a means of reproduction—and can germinate when conditions improve.
Takeaway: Bacterial cells organize DNA and protein synthesis without a membrane-bound nucleus, and some have specialized storage or survival structures.
Comparing microbial cell types
Bacteria and archaea are : their cells lack a membrane-bound nucleus. Their envelopes are not identical. Bacterial walls typically contain , whereas archaeal walls do not; archaeal membranes also have distinctive lipids with ether linkages and branched chains.
Eukaryotic microorganisms, including yeasts and protozoa, have a nucleus and membrane-bound organelles. Their cell walls, when present, contain materials such as chitin in fungi or cellulose in many algae rather than bacterial .
Viruses are acellular: they are not cells and do not have the cellular organization described here. They depend on host cells to make new viruses.
Takeaway: The presence or absence of a nucleus, membrane-bound organelles, and particular envelope materials helps distinguish major microbial groups.