The mechanism of the chewed up sectored colony formation

Abstract

When the mixture of E. coli B and T3 phage are plated using the double agar layer method described by Adams, the T3 phage-resistant bacteria form the colonies on and in the upper layer after incubation of 24hrs, at 37°C. Among the resistant colonies formed, the chewed up sectored colonies and the so-called thin colonies were frequently observed. These colonies have been reported by Demerec and Fano (1954) and by Fraser (1954). However, the mechanism of the sectored colonies formation have not been studied precisely. The present paper concerns the experiments which were undertaken to reveal the mechanism.The sectored colonies (i.e., the bacteria and the phages lysing them) could be maintained by the transferring the colonies on the neutral agar slant medium. Hence, in the experiments these colonies transferred several times on the slant agar were used.The bacteria obtained from the sectored colonies were B/3, 4, 7 and the phages lysing the colonies T3h mutants. The former were designated as Bs or Bs/3 and the latter T3s or T3hs for convenience, because the bacteria and the phages seemed to be specific strains.When the Bs/3 strains were mixed with T3h phage in the appropriate ratio and plated by the the double agar layer method, the sectored and thin colonies could be reconstructed. Then, it was concluded that the sectored and thin colonies were formed by the interaction between B/3 and T3h phage: during the course of the B/3 colonies on the plate, they were lysed by T3h mutants which appeared among the great number of T3 wild type phages grown on E. coli B wild type strain.Furthermore, the sectored colonies were also obtained by mixing E. coli E wild type strain with T3 wild type phage or T3hs phage. When the sectored colonies reconstructed by mixing E. coli B and T3 phage, were incubated at 37°C for 48hrs., the papillae-like secondary colonies appeared on the thin colonies. However, when the T3hs phage instead of the T3 wild type phage was used, the secondary colonies were not observed. Therefore, the colonies would be T3 phage resistant colonies which appeared in the thin colonies.