Wall-Defective Microbial Variants: Terminology and Experimental Design

Bacteria with damaged or deficient cell walls have increasingly attracted the interest of microbiologists and clinicians.Research with these "walldefective microbial variants" has been hampered, however, by lack of accepted definitions for the different wall-defective phases of bacteria and by lack of a critical evaluation of what constitutes adequate experimental design in the study of these microorganisms.An attempt has been made in the.following discussion to formulate terminology and experimental guidelines that will meet these needs.Most bacteria with damaged cell walls can undergo marked morphological and physiological changes and can exist in at least four phases, the protoplast phase, spheroplast phase, transitional phase, and L-phase, each differing from the others and from the classical bacterial phase (figure 1).Continued use of the terms for the different variant phases as if the terms were interchangeable obscures the recognizable differences summarized in the definitions below.These definitions represent a synthesis of previous definitions and pertinent information from in-vivo and in-vitro studies.Wall-defective microbial variants.This is a general term denoting microorganisms that have undergone changes in morphology, physiology, and/or cultural characteristics as a result of damage to or deficiency of their cells walls.These changes are more profound than those normally associated with the species in its vegetative bacterial phase.The term variant, however, does not imply a change in genetic constitution.Protoplasts.Protoplasts are wall-defective microbial variants with external surfaces free of all cell-wall constituents (figure 1 b); they are typically gram-negative, spherical, osmotically fragile, and penicillin-indifferent [1-3].Their size is generally uniform and equal to or greater than that of the bacteria from which they were derived [2][3][4][5].For limited periods they may maintain metabolic activity, increase in size, increase in dry weight [2,5,6], and undergo a few divisions [7].However,