Whenever the state of a biological system is not determined solely by present conditions but depends on its past history, we can say that the system has memory. Bacteria and bacteriophage use a variety of memory mechanisms, some of which seem to convey adaptive value. A genetic type of heritable memory is the programmed inversion of specific DNA sequences, which causes switching between alternative patterns of gene expression. Heritable memory can also be based on epigenetic circuits, in which a system (...) with two possible steady states is locked in one or the other state by a positive feedback loop. Epigenetic states have been observed in a variety of cellular processes, and are maintained by diverse mechanisms. Some of these involve alternative DNA methylation patterns that are stably transmitted to daughter molecules and can affect DNA–protein interactions (e.g., gene transcription). Other mechanisms exploit autocatalytic loops whereby proteins establish the proper conditions for their continued synthesis. Template polymers other than nucleic acids (e.g., components of the cell wall) may also propagate epigenetic states. Non‐heritable memory is exemplified by parasitic organisms that bear a signature of their previous host, such as host‐controlled modification of phage DNA or porin hitchhiking in predatory bacteria. The heterogeneous nature of the examples known may be indicative of widespread occurrence of memory mechanisms in bacteria and phage. However, the actual extent, variety and potential selective value of prokaryotic memory devices remain open questions, still to be addressed experimentally. BioEssays 24:512–518, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

Cell‐wall‐less bacterial variants, or L‐forms, have been described in many bacterial species under laboratory conditions, in infected eukaryotic cell cultures and inside animals. Of special interest for human health is the formation of L‐forms as a consequence of specific antibiotic treatments, and the potential involvement of L‐forms in persistent and relapsing infections. An old enigma about L‐forms is how they can divide in the absence of cell wall synthesis, since septum formation is an essential requisite for cell division. However, the (...) classical definition of L‐forms as cell‐wall‐less bacterial variants may need a revision to accomodate recent observations by Richard d'Ari and coworkers:1 genetic and biochemical evidence indicates that E. coli L‐forms induced by β‐lactam antibiotics do contain small amounts of peptidoglycan, essential for their growth and probably required for septum formation. If these observations are extrapolated to all known L‐forms, the very concept of cell‐wall‐less bacteria may need revision, and be restricted to mycoplasmas and their relatives. BioEssays 29:1189–1191, 2007. © 2007 Wiley Periodicals, Inc. (shrink)