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Bacterial cells are prokaryotic and lack a nucleus, but possess a highly organized cytoplasm with ribosomes, nucleoids, and specialized compartments. The cell wall and cell membrane are critical for structure and selective transport. Understanding bacterial ultrastructure is essential for classifying bacteria and predicting their interactions with the environment.
Compare prokaryotic and eukaryotic cell structures side-by-side. Use electron micrographs to identify structures like mesosomes and inclusion bodies.
Bacteria are not simply 'blobs of cytoplasm'—they have complex internal organization. The cell wall is not a rigid container but a dynamic, permeable structure that can remodel.
From your prerequisite study of basic bacterial cell structure and cell membrane biology, you know that bacteria are prokaryotes — cells without a membrane-bound nucleus. But "no nucleus" does not mean "no organization." Bacterial cells are far more structured than they appear under a light microscope, and understanding their ultrastructure — the fine architectural details visible only by electron microscopy — is essential for understanding how bacteria grow, divide, resist antibiotics, and interact with hosts.
The most prominent internal structure is the nucleoid, a concentrated region where the bacterial chromosome (a single circular DNA molecule, typically 1–5 million base pairs) is compacted by supercoiling and nucleoid-associated proteins. Unlike eukaryotic chromatin wrapped around histones, the nucleoid has no surrounding membrane, meaning transcription and translation happen simultaneously — ribosomes begin translating mRNA while it is still being transcribed from DNA. The cytoplasm is packed with 70S ribosomes (smaller than the 80S ribosomes of eukaryotes, which is why certain antibiotics can selectively target bacterial protein synthesis without harming human cells). Some bacteria also contain inclusion bodies — storage granules of glycogen, polyphosphate, or polyhydroxybutyrate that serve as nutrient reserves.
Surrounding the cytoplasm, the cell membrane functions as the selective permeability barrier you studied previously, but in bacteria it also houses the electron transport chain (since bacteria lack mitochondria) and many biosynthetic enzymes. Outside the membrane lies the cell wall, whose structure is the basis for one of microbiology's most fundamental classification tools. Gram-positive bacteria have a thick peptidoglycan layer (20–80 nm) studded with teichoic acids that project outward. Gram-negative bacteria have a thin peptidoglycan layer (1–3 nm) sandwiched between an inner membrane and an outer membrane containing lipopolysaccharide (LPS) — a potent immunostimulatory molecule. This outer membrane creates a periplasmic space where enzymes involved in nutrient processing and antibiotic degradation (like beta-lactamases) reside.
Beyond the cell wall, many bacteria possess additional surface structures that are critical for survival. Capsules — thick polysaccharide layers — shield bacteria from phagocytosis and desiccation. S-layers — crystalline protein arrays — provide mechanical protection. Flagella enable motility, while pili and fimbriae mediate attachment to surfaces and other cells. Each of these structures represents a potential drug target and a diagnostic feature. The reason Gram staining works, the reason penicillin kills some bacteria but not others, and the reason certain bacteria can evade the immune system all trace back to specific ultrastructural differences that are invisible without understanding bacterial architecture at this level of detail.