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Int J Biol Sci. 2010 Jun 9;6(4):303-15.

Survival of Escherichia coli under lethal heat stress by L-form conversion.

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Institute of Microbiology, Bulgarian Academy of Sciences, Acad. G. Bonchev str. 26, 1113 Sofia, Bulgaria.


Transition of bacteria to cell wall deficient L-forms in response to stress factors has been assumed as a potential mechanism for survival of microbes under unfavorable conditions. In this article, we provide evidence of paradoxal survival through L-form conversion of E. coli high cell density population after lethal treatments (boiling or autoclaving). Light and transmission electron microscopy demonstrated conversion from classical rod to polymorphic L-form shape morphology and atypical growths of E. coli. Microcrystal formations observed at this stage were interpreted as being closely linked to the processes of L-form conversion and probably involved in the general phenomenon of protection against lethal environment. Identity of the morphologically modified L-forms as E. coli was verified by species specific DNA-based test. Our study might contribute to a better understanding of the L-form phenomenon and its importance for bacterial survival, as well as provoke reexamination of the traditional view of killing strategies against bacteria.


E. coli; L-form conversion; lethal heat stress

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