Question
L-forms
Answer
L-forms (L-phase variants) are bacterial variants that have partially or completely lost their rigid cell wall (deficient in peptidoglycan), yet remain viable and capable of growth and (in some cases) division under appropriate osmotically-protective conditions — named after the Lister Institute, where they were first described.
Formation: L-forms can arise spontaneously, but are most often induced experimentally or clinically by exposure to agents that interfere with cell-wall synthesis — beta-lactam antibiotics (penicillin), lysozyme, or antibody plus complement acting on the cell wall — in a hypertonic (osmotically stabilizing) environment that prevents the wall-deficient cell from lysing due to osmotic pressure.
Morphology and behaviour: highly pleomorphic (variable in size and shape, from large spherical bodies to filamentous forms), since the cell wall normally maintains characteristic bacterial shape; grow slowly, often producing characteristic “fried-egg” colonies on solid media, superficially resembling Mycoplasma colonies.
Types: stable L-forms — permanently unable to revert to the normal walled bacterial form even after the inducing agent is removed; unstable L-forms — can revert back to typical walled bacteria once the inducing stimulus (e.g., antibiotic) is withdrawn.
Clinical significance: because they lack a cell wall, L-forms are intrinsically resistant to cell-wall-active antibiotics (penicillins, cephalosporins) — this is postulated as one possible mechanism for relapse/persistence of infection despite apparently adequate antibiotic therapy directed against the wall-form organism, and for the difficulty of eradicating certain chronic/recurrent infections (e.g., some cases of chronic UTI, endocarditis). Mycoplasma species are naturally wall-deficient organisms and are sometimes discussed alongside L-forms, though they are a genetically distinct, permanently wall-less genus rather than a variant form of a walled species.