Bacteria, Forms of

the external shapes of bacterial cells. The most common forms of bacteria are spherical (cocci), rod-shaped (bacilli), and spiral-shaped (vibrios and spirilla).

A distinct type of bacterium, called an L form, or L variant, is produced when a bacterium’s cell wall is partly or almost destroyed, or when the cell loses its ability to form a wall. Unlike spheroplasts and protoplasts, L forms retain their ability to grow and reproduce. They were discovered in 1935 by the English biologist E. Klieneberger-Nobel and were named L (Listeria) forms after the Lister Institute of Preventive Medicine in London. L transformation can occur in most species of bacteria, including cocci, Escherichia coli, and Pasteurella.

L forms of different species of bacteria may be identical morphologically; they are generally spherical, vacuolated bodies varying in size from 1 micron to 250 nanometers. L forms are produced under the influence of such substances as penicillins, which prevent the biosynthesis of the bacterium’s cell wall. L forms are also produced when bacterial cell division is inhibited while cell growth is maintained. Under certain conditions, L forms can revert to the original bacteria. L forms often occur during the course of such prolonged diseases as brucellosis.