Cell Wall–Deficient Bacteria

Although most prokaryotes cannot survive in nature without their cell walls, some do so naturally. These include the mycoplasmas, a group of pathogenic bacteria that causes several infectious diseases of humans and other animals, and the Thermoplasma group, species of Archaea that naturally lack cell walls.

These bacteria are able to survive without cell walls because they either contain unusually tough cytoplasmic membranes or because they live in osmotically protected habitats such as the animal body.

Cell Wall-Deficient Bacteria

1. Mollicutes (often known as Mycoplasma species)
2. L-forms
3. Spheroplasts
4. Protoplasts

Why cell wall deficient bacteria matter clinically

This topic falls squarely into the category of "interesting but why should I care?" The honest answer is: cell wall deficient bacteria are one of the key explanations for why some bacterial infections never fully resolve, why some antibiotic treatments fail despite in vitro susceptibility, and why Mycoplasma infections are so commonly misdiagnosed.

Three clinical scenarios that make this topic immediately relevant:

1. The atypical pneumonia that won't respond to amoxicillin

Mycoplasma pneumoniae causes "walking pneumonia," and prescribing amoxicillin for it is one of the most common, entirely avoidable errors in primary care. Mycoplasma has no cell wall, so amoxicillin has zero mechanism of action against it. See the full clinical case in our dedicated Mycoplasma pneumoniae article.

2. The recurrent UTI that keeps coming back. Some "recurrent" UTIs may not be reinfections at all, but the same organism persisting in a wall-less form that standard culture cannot detect. The mechanism, and why it matters for treatment, is covered under L-forms below.

3. The Mycoplasma that looks like a virus Early in the history of microbiology, Mycoplasma was classified as a virus because it passed through bacteriological filters and could not be seen by light microscopy. This misclassification had real consequences — infections were treated with antiviral measures rather than antibiotics. Understanding what makes Mycoplasma genuinely unique (no cell wall, membrane stabilized by cholesterol, smallest free-living organism) explains not just its biology but why its diagnosis and treatment differ fundamentally from all other bacteria.

Mycoplasmas

The Mollicutes, often called mycoplasmas, are naturally occurring stable bacteria that lack cell walls (mollis is latin for “soft”). These pleomorphic bacteria are not stained by Gram-stain but evolved from gram-positive ancestors in the Firmicutes lineage by genome reduction and loss of the cell wall. This is why they are phylogenetically gram-positive even though they have no wall to take up the Gram stain.

Figure: Mycoplasma (cell wall-deficient bacteria)

Mycoplasma (cell wall-deficient bacteria) resemble protoplasts (bacteria treated to remove their cell walls) but are more resistant to osmotic lysis than protoplasts. Species of medical importance include Mycoplasma pneumoniae and Ureaplasma urealyticum.

Characteristics feature

1. They are among the smallest microorganisms capable of independent existence, roughly 0.2 to 0.3 μm, approaching the size of the largest viruses (the poxviruses). Their small size and deformable, wall-less bodies are why mycoplasmas can pass filters that retain other bacteria, which makes them a notorious contaminant of laboratory cell cultures.
2. Cytoplasmic membranes of mycoplasmas are more stable than that of other bacteria due to the presence of sterols. Sterols add strength and rigidity to the cytoplasmic membranes.
3. The genomes of mycoplasmas are between 500 and 1100 kilobase pairs of DNA in most cases. This is smaller than those of most bacteria, comparable to the genome size of the obligately parasitic chlamydia and rickettsia.
4. Mycoplasmas give a characteristic “fried-egg” appearance in solid culture media consisting of a dense central core that penetrates downward into the agar, surrounded by a circular spreading area that is lighter in color.
5. Growth of mycoplasmas is not inhibited by antibiotics such as penicillin that inhibit cell wall synthesis. However, mycoplasmas are as sensitive as most Bacteria to antibiotics whose targets are other than the cell wall.
6. Certain mycoplasmas contain lipoglycans, long-chain heteropolysaccharides covalently linked to membrane lipids. These are LPS-like but lack the lipid A backbone that makes true LPS toxic. Lipoglycans help stabilize the cytoplasmic membrane and facilitate attachment of mycoplasmas to receptors on animal cell surfaces.
7. Media for the culture of most mycoplasmas are typically quite complex requiring unsaturated fatty acids, sterols, vitamins, amino acids, purines, and pyrimidines as growth factors.
8. Oxygen requirements of mycoplasmas vary widely, some are strictly respiratory while others are facultative or even obligate anaerobes.

Figure: Fried egg colonies of Mycoplasma

Some mycoplasmas require sterols in their growth media. Based on sterol requirements, mycoplasmas can be differentiated into two groups.

### Why Mycoplasma is clinically unique

The critical clinical table from this: Beta-lactams have zero activity against Mycoplasma. No dose of amoxicillin, ceftriaxone, or meropenem has any effect. This is intrinsic, complete, and permanent resistance.

L-forms

L-forms (also called L-phase bacteria) are bacteria that have lost their cell wall, either in the laboratory or in a patient being treated with a wall-active drug. They are named after the Lister Institute in London, where they were first described. Because the cell wall is gone, L-forms are resistant to penicillins, cephalosporins, and other wall-active antibiotics.

Figure: L form of bacteria

How L-forms form in patients: When bacteria are exposed to cell wall-targeting antibiotics (penicillins, cephalosporins, vancomycin) or lysozyme (in tissues or body fluids), the peptidoglycan cell wall is damaged or prevented from being built. In standard conditions this kills the bacterium. But in certain environments — particularly within host cells (intracellular L-forms) or in osmotically protected niches (certain body cavities, urinary epithelium) — the bacterium can survive without its cell wall by relying on its plasma membrane alone.

These L-forms:

• Are completely resistant to all cell wall-active antibiotics (the target no longer exists)
• Are difficult or impossible to culture on standard media (they lyse in hypotonic conditions)
• Are not detected by standard clinical microbiology culture
• Can revert to normal walled bacteria when antibiotic pressure is removed
• May persist intracellularly in macrophages, uroepithelial cells, and renal tubular cells

The clinical consequence: a patient may appear to have responded to a wall-active antibiotic (symptoms improve, cultures turn negative), while L-forms persist intracellularly, invisible to culture and unaffected by the drug. When treatment stops, they revert and the infection relapses. This mechanism is increasingly recognized as contributing to:

• Recurrent UTIs
• Relapsing endocarditis
• Treatment-refractory chronic infections

This does not mean beta-lactams should be avoided; they remain essential. It simply helps explain why an infection can relapse with the same organism after an apparently successful course, and why such cases are sometimes treated with agents that act independently of the cell wall.

Protoplasts

A protoplast is a cell from which the entire peptidoglycan wall has been removed, produced artificially, classically by treating a gram-positive bacterium with lysozyme (which digests peptidoglycan) in an osmotically protected medium. Because gram-positives have no outer membrane, removing the peptidoglycan leaves only the plasma membrane. Protoplasts are metabolically active but osmotically fragile and lyse readily unless the medium is isotonic.

If bacteria are incubated with penicillin in an isotonic solution, gram-positive bacteria are converted to protoplasts and continue to grow normally when isotonicity is maintained.

Spheroplasts

A spherical, osmotically sensitive cell derived from a bacterium by loss of some but not all of the rigid wall layer.

Spheroplasts are spherical, osmotically sensitive cells derived from Gram-negative bacteria by loss of some but not all of the rigid wall layer. The damage in the wall is caused by a toxic chemical or antibiotic such as penicillin (gram-negative bacteria retain their outer membrane after penicillin treatment). They are able to change back to their normal form when the toxic agent is removed.

Archaea lacking cell walls

Figure: Thermoplasma Photo by Dr. William Hixon

Cells of some Archaea, such as Thermoplasma and Ferroplasma, lack cell walls. Thermoplasma is a chemoorganotroph that grows optimally at around 60°C and pH 2 in complex media. The cytoplasmic membrane of Thermoplasma contains a lipopolysaccharide-like material called lipoglycan. This substance consists of a tetraether lipid monolayer membrane with mannose and glucose. The membrane also contains glycoproteins but not sterols. These molecules render the Thermoplasma membrane stable to hot, acidic conditions.

Ferroplasma is a chemolithotrophic relative of Thermoplasma and is a strong acidophile.

How to Learn and Remember Cell Wall Deficient Bacteria

"Cell wall deficient bacteria are the bacteria that beta-lactam antibiotics cannot touch either because they were born without a wall (Mycoplasma) or because they shed their wall during treatment (L-forms)."

Key exam facts in one table

A clinical story that makes this unforgettable

The infection that survived inside the bladder wall

A microbiologist studying recurrent UTI takes bladder biopsies from women who have had more than 3 UTIs per year with the same E. coli strain. Using fluorescence microscopy and specialized culture techniques, she finds E. coli L-forms within uroepithelial cells — invisible to standard urine culture, invulnerable to the amoxicillin courses the women have been receiving. The cells show them as faint, wall-less shadows deep within the epithelium, waiting for antibiotic pressure to subside. This is not a hypothetical — L-form persistence in the bladder epithelium has been demonstrated in mouse models and increasingly in human studies. It changes our understanding of why some women have truly recurrent rather than reinfection-based UTIs.

References and further readings

1. Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock Biology of Microorganisms (15th ed.). Pearson.
2. Tille, P. M. (2017). Bailey & Scott's Diagnostic Microbiology (14th ed.). Mosby Elsevier.
3. Errington, J., Mickiewicz, K., Kawai, Y., & Wu, L. J. (2016). L-form bacteria, chronic diseases and the origins of life. Philosophical Transactions of the Royal Society B, 371(1707). https://doi.org/10.1098/rstb.2015.0494
4. Hayward, R. J., & Schneewind, O. (2019). Atypical bacterial development: Mycoplasma. Nature Reviews Microbiology, 17(6), 381–391.