Electron microscopy of the anti-bacterial agent produced by Escherichia coli 15.

When it was first observed that the strain 15 of Escherichia coli, after u.v.-irradiation and further incubation, would eventually lyse and release an anti-bacterial agent, this agent was considered to be a ‘colicine’ (Mukai, 1960; Ryan, Fried & Mukai, 1955). The possibility that, in addition, a defective phage was produced, was left open. But, since in an u.v.-induced lysate of this strain all anti-bacterial activity can be spun down by applying centrifugal forces of only 20,OOOg for 2 hr (Mukai, 1960) and since those colicinogenic strains, which do lyse after u.v.irradiation, were found to be also lysogenic (Kellenberger & Kellenberger, 1956), we concluded (Mennigmann, 1964b) that the anti-bacterial agent must be a large particle, probably a phage. In this communication we wish to verify these conclusions by presenting some electronmicroscopical pictures of such lysates, which show a defective phage to be present. Thus, regarding the antibacterial agent produced by E. coli 15, we rather favour the opposite of the above picture: the antibacterial agent is a defective phage and the possibility of the additional presence of a colicine has to be left open.