L-form bacteria

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L-form bacteria, also known as L-phase bacteria, L-phase variants or cell wall-deficient bacteria (CWDB), are growth forms derived from different

spheroplasts
that are capable of dividing, but can revert to the original morphology, and stable L-forms, L-forms that are unable to revert to the original bacteria.

Discovery and early studies

L-form bacteria were first isolated in 1935 by

Lister Institute in London where she was working.[2]

She first interpreted these growth forms as symbionts related to pleuropneumonia-like organisms (PPLOs, later commonly called mycoplasmas).[3] Mycoplasmas (now in scientific classification called Mollicutes), parasitic or saprotrophic species of bacteria, also lack a cell wall (peptidoglycan/murein is absent).[4][5] Morphologically, they resemble L-form bacteria. Therefore, mycoplasmas formerly were sometimes considered stable L-forms or, because of their small size, even viruses, but phylogenetic analysis has identified them as bacteria that have lost their cell walls in the course of evolution.[6] Both, mycoplasmas and L-form bacteria are resistant against penicillin.

After the discovery of PPLOs (mycoplasmas/Mollicutes) and L-form bacteria, their mode of reproduction (proliferation) became a major subject of discussion. In 1954, using phase-contrast microscopy, continual observations of live cells have shown that L-form bacteria (previously also called L-phase bacteria) and pleuropneumonia-like organisms (PPLOs, now mycoplasmas/Mollicutes) ) do not proliferate by binary fission, but by a uni- or multi-polar budding mechanism. Microphotograph series of growing microcultures of different strains of L-form bacteria, PPLOs and, as a control, a Micrococcus species (dividing by binary fission) have been presented.[3] Additionally, electron microscopic studies have been performed.[7]

Appearance and cell division

Transmission electron micrograph of a population of L-form Bacillus subtilis, showing a range of sizes. Scale bar is 10 micrometers.

phase contrast microscopy or by transmission electron microscopy.[8]

Although L-forms can develop from

Gram stain test
, the L-forms always colour Gram-negative, due to the lack of a cell wall.

The cell wall is important for

bacterial cytoskeleton such as FtsZ. The ability of L-form bacteria and mycoplasmas to grow and divide in the absence of both of these structures is highly unusual, and may represent a form of cell division that was important in early forms of life. This mode of division seems to involve the extension of thin protrusions from the cell's surface and these protrusions then pinching off to form new cells. The lack of cell wall in L-forms means that division is disorganised, giving rise to a variety of cell sizes, from very tiny to very big.[1]

Phase contrast image of L-form cells from Bacillus subtilis showing a range of sizes. Scale bar is 5 micrometers.

Generation in cultures

L-forms can be generated in the laboratory from many bacterial species that usually have cell walls, such as

osmolarity as the bacterial cytosol (an isotonic solution), which prevents cell lysis by osmotic shock.[2] L-form strains can be unstable, tending to revert to the normal form of the bacteria by regrowing a cell wall, but this can be prevented by long-term culture of the cells under the same conditions that were used to produce them – letting the wall-disabling mutations to accumulate by genetic drift.[9]

Some studies have identified mutations that occur, as these strains are derived from normal bacteria.[1][2] One such point mutation D92E is in an enzyme yqiD/ispA (P54383) involved in the mevalonate pathway of lipid metabolism that increased the frequency of L-form formation 1,000-fold.[1] The reason for this effect is not known, but it is presumed that the increase is related to this enzyme's role in making a lipid important in peptidoglycan synthesis.

Another methodology of induction relies on

biological corridor connecting adjacent micro habitat patches, L-form-like cells can be derived[10] using a microfluifics-based (synthetic) ecosystem implementing an adaptive landscape[11]
selecting for shape-shifting phenotypes similar to L-forms.

Significance and applications

Some publications have suggested that L-form bacteria might cause diseases in humans,

antibiotic resistance.[19][20]

L-form bacteria may be useful in research on early forms of life, and in

periplasmic space of bacteria.[24][25]

L-form bacteria are seen as a persister cells, and a source of recurrent infection that has become of medical interest.[26]

See also

References

  1. ^
    S2CID 4413852
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  2. ^
    PMID 17586646
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  3. ^
    S2CID 21257985
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  4. PMID 9841667
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  5. ^
    PMID 9105757. Full PDF
  6. PMID 6928642
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  7. S2CID 45546531
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  8. PMID 4631836
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  9. PMID 1905284
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  10. PMID 19706420
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  11. PMID 17090676
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  12. PMID 8501224
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  14. S2CID 24781904. Archived from the original
    (PDF) on 24 August 2009.
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  18. S2CID 1295920
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  21. OCLC 246350676
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  24. PMID 9835575
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  25. S2CID 9923116
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  26. PMID 37082179
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Further reading

External links

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