Printed in Great Britain 3rI Cryptic Lysogeny in Proteus mirabilis

Proteus mirabilis strains 13 and 5oo6 on rare occasions liberate temperate phages, I3M and 5oo6M, which may reinfect parent strains. These prophages are not inducible but the probability of phage liberation may be increased by growth in nitrosoguanidine. P. mirabilis strain 34 liberates a phage, 034, spontaneously. Strain 34 is doubly lysogenic for this phage. Phages I3M, 5oo6M and 34 are serologically identical and convert their hosts. The cryptic lysogeny of these strains may be due to the mode of integration of the prophages in the bacterial chromosome