Yes, all bacteria are prokaryotes. Every known bacterial species lacks a membrane-bound nucleus, which is the defining feature that separates prokaryotic cells from eukaryotic cells like those in plants, animals, and fungi. However, the reverse isn’t true: not all prokaryotes are bacteria. The prokaryotic world includes two entirely separate domains of life, Bacteria and Archaea, which are as genetically different from each other as either is from eukaryotes.
What Makes a Cell Prokaryotic
Prokaryotic cells are defined by what they lack. They have no nucleus surrounded by a nuclear membrane. Instead, their DNA sits in an open region of the cell called the nucleoid. They also lack the membrane-bound organelles that eukaryotic cells use to compartmentalize tasks, like mitochondria for energy production or an endoplasmic reticulum for protein processing.
Beyond these absences, prokaryotic cells share several positive traits. Their genetic material is typically organized as a single, circular chromosome rather than the multiple linear chromosomes found in eukaryotic cells. They’re also dramatically smaller. Most prokaryotic cells measure 0.2 to 2 micrometers across, while eukaryotic cells range from 10 to 100 micrometers. That size difference means you could fit dozens of bacteria inside a single human cell.
Bacteria reproduce through binary fission, a simpler process than the mitosis eukaryotic cells use. The cell copies its circular chromosome, a protein ring assembles at the middle of the cell, and the cell splits into two daughter cells. There’s no elaborate spindle apparatus pulling chromosomes apart the way there is in eukaryotic division.
The Bacterial Cell Wall
One feature that distinguishes bacteria from other prokaryotes is their cell wall. Nearly all bacteria are surrounded by a rigid layer made of peptidoglycan, a mesh-like molecule that acts as a protective exoskeleton. This layer is essential for bacterial survival, which is why antibiotics like penicillin work by disrupting its construction. The only major exception is a group called mycoplasmas, which lack a cell wall entirely and are among the smallest self-replicating organisms known.
Bacteria and Archaea: Two Kinds of Prokaryote
In 1977, microbiologist Carl Woese analyzed the genetic sequences of ribosomal RNA across different organisms and discovered something startling. The organisms we’d been lumping together as “bacteria” actually belonged to two fundamentally different lineages. Methane-producing microbes and their relatives were no more closely related to typical bacteria than they were to eukaryotic cells. This led to the three-domain system of classification: Bacteria, Archaea, and Eukarya.
Both bacteria and archaea are prokaryotic, meaning both lack nuclei and membrane-bound organelles. But the similarities can be misleading. Their cell membranes are built from completely different chemistry. Bacterial membranes use fatty acid chains connected to a glycerol backbone by ester bonds. Archaeal membranes use branched chains called phytanyl groups connected by ether bonds, and some archaea build single-layer membranes instead of the double-layer structure found in bacteria and eukaryotes. These differences in basic cell architecture reflect billions of years of independent evolution.
Bacteria That Blur the Lines
While all bacteria are classified as prokaryotes, a few species have features that challenge the clean division between prokaryotic and eukaryotic cells. The freshwater bacterium Gemmata obscuriglobus, for example, has its DNA enclosed within two membranes that resemble a eukaryotic nucleus. Researchers confirmed this using multiple microscopy techniques and antibody labeling of double-stranded DNA within the membrane boundary. This was described as “a significant exception to the prokaryote/eukaryote dichotomous classification of cell structure” in the Proceedings of the National Academy of Sciences.
An even more dramatic example is Thiomargarita magnifica, a bacterium discovered in marine environments that grows over 9,000 micrometers long, roughly one centimeter. That’s visible to the naked eye. Most bacteria are around 2 micrometers long, so this single cell is about 50 times larger than any previously known giant bacterium. Its DNA and protein-building machinery are compartmentalized inside membrane-bound structures throughout the cell, something no typical prokaryote does. The organism also carries over half a million copies of its genome, a level of genetic redundancy unheard of in bacteria.
These organisms don’t reclassify bacteria as eukaryotes. They remain firmly within the domain Bacteria based on their evolutionary lineage and core genetics. But they show that prokaryotic cells are capable of more structural complexity than the textbook definition suggests.
Why the Distinction Matters
Understanding that bacteria are prokaryotes isn’t just a classification exercise. It has real consequences for medicine and biology. Antibiotics target features specific to prokaryotic cells, like peptidoglycan cell walls and prokaryotic ribosomes, which differ enough from eukaryotic ribosomes that drugs can disable one without harming the other. That’s the entire basis for why antibiotics can kill bacteria in your body without destroying your own cells.
The prokaryotic nature of bacteria also connects to one of the biggest events in evolutionary history. Mitochondria, the energy-producing structures inside every animal and plant cell, descended from an ancient bacterium. Specifically, the ancestor was related to a group called alphaproteobacteria, which was engulfed by a host cell related to Asgard archaea. Over time, that bacterial passenger became a permanent, essential part of eukaryotic life. So while all bacteria are prokaryotes today, a bacterium’s integration into another cell is what made complex eukaryotic life possible in the first place.