What is the difference between a bacterium and a plasmid?

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The other day during a discussion on microbial genetics, someone asked me how a bacterium differs from a plasmid. I had seen a breakdown in a textbook that clarified it really well. Bacteria are full organisms with all the machinery to live and reproduce, while plasmids are small DNA circles that just carry extra genes and hitch a ride in the bacterial cell. Here’s a clear comparison.

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Markko 2025-06-13T18:40:35+00:00 1 Answer 2 views
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  1. fleming
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    2025-06-13T18:43:51+00:00
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    Definition and Basic Nature

    • Bacterium: A bacterium is a complete, living, single-celled prokaryotic organism. It is a self-contained biological entity capable of independent existence, metabolism, and reproduction.
    • Plasmid: A plasmid is a small, circular, double-stranded DNA molecule that exists and replicates independently of the bacterial chromosome but is not capable of independent existence outside a host cell. Plasmids are genetic elements, not organisms.

    Size and Complexity

    • Bacterium: A typical bacterial cell is about 0.5–5 micrometers in size and contains thousands of genes (typically 1,000–10,000) encoding all the proteins and RNAs necessary for life.
    • Plasmid: Plasmids are much smaller, ranging from about 1 to 1,000 kilobase pairs (kbp) and containing anywhere from a few to several hundred genes. Most commonly used plasmids in molecular biology are 2–10 kbp.

    Structure and Components

    • Bacterium: A complete cell with a cell membrane, cytoplasm, ribosomes, and various other cellular components. Most bacteria also have a cell wall, and some have additional structures like capsules, flagella, or pili.
    • Plasmid: Simply a circular DNA molecule, sometimes referred to as “naked DNA.” It has no cellular components or structures.

    Genetic Content

    • Bacterium: Contains a complete genome, typically consisting of a single, circular chromosome of double-stranded DNA that encodes all essential genes for survival and reproduction. The bacterial chromosome is much larger than plasmids (typically 1–10 million base pairs).
    • Plasmid: Contains a limited set of genes, often encoding non-essential functions that may provide advantages under specific conditions. Common plasmid genes include:
      • Antibiotic resistance genes
      • Toxin production genes
      • Metabolic genes for utilizing specific nutrients
      • Conjugation genes (for self-transfer between bacteria)
      • Origin of replication (essential for plasmid replication)

    Essentiality

    • Bacterium: The bacterial chromosome contains essential genes without which the bacterium cannot survive.
    • Plasmid: Plasmids typically carry non-essential genes. A bacterium can generally survive without its plasmids, though it may lose certain advantageous traits.

    Replication

    • Bacterium: Reproduces through binary fission, a process where the entire cell divides into two daughter cells, each receiving a copy of the chromosome and other cellular components.
    • Plasmid: Replicates within the bacterial cell using the host’s enzymes and machinery. Plasmid replication is controlled by its origin of replication (ori) and can occur independently of chromosomal replication.

    Transmission

    • Bacterium: Primarily transmitted vertically from parent to offspring during cell division.
    • Plasmid: Can be transmitted both vertically (to daughter cells during bacterial division) and horizontally (between different bacterial cells, even of different species) through processes like:
      • Conjugation: Direct transfer between bacterial cells
      • Transformation: Uptake of naked plasmid DNA from the environment
      • Transduction: Transfer via bacteriophages (viruses that infect bacteria)

    Copy Number

    • Bacterium: Each bacterial cell typically contains only one chromosome (though some bacteria have multiple chromosomes).
    • Plasmid: Can exist in varying copy numbers within a single bacterial cell, from just one copy (low-copy plasmids) to hundreds of copies (high-copy plasmids) per cell, depending on the plasmid type and its origin of replication.

    Stability

    • Bacterium: The bacterial chromosome is highly stable and rarely lost during cell division.
    • Plasmid: Plasmids can be lost during cell division, particularly if they don’t provide a selective advantage under current conditions. This phenomenon is called plasmid curing.

    Practical Applications

    • Bacterium: Used in various applications including food production (yogurt, cheese), bioremediation, and as research model organisms.
    • Plasmid: Widely used as vectors in molecular biology and genetic engineering for:
      • Cloning genes of interest
      • Expressing proteins
      • Creating recombinant organisms
      • Gene therapy
      • Vaccine development

    Examples

    • Bacteria: Escherichia coli, Bacillus subtilis, Staphylococcus aureus
    • Plasmids:
      • F plasmid (fertility factor, enables bacterial conjugation)
      • R plasmids (carry antibiotic resistance genes)
      • Col plasmids (encode colicins, proteins that kill other bacteria)
      • Ti plasmid (from Agrobacterium tumefaciens, used in plant genetic engineering)
      • pBR322, pUC19 (common cloning vectors in molecular biology)

    In summary, a bacterium is a complete living organism, while a plasmid is a small, circular DNA molecule that exists within bacteria as an independent genetic element. Plasmids provide additional genetic information that may benefit the host bacterium under certain conditions, but they are not essential for bacterial survival in most cases. Their ability to move between bacteria makes them important vectors for horizontal gene transfer in nature and valuable tools in genetic engineering and biotechnology.

    Source: Snyder, L., et al. Molecular Genetics of Bacteria; Madigan, M.T., et al. Brock Biology of Microorganisms

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