Spontaneous zygogenesis (Z-mating) in mecillinam-rounded bacteria

In Escherichia coli the process of spontaneous zygogenesis (Z-mating), i.e. complete genetic mixing in the absence of a conjugative plasmid, was investigated further. Spontaneous-zygogenesis-promoting (Szp⁺) cells displayed strong clustering with each other and with ordinary F⁻ cells in the optimal cell density range for Z-mating. When induced to rounding by the drug mecillinam, they aggregated into large, dense, stainable syncytium-like cells leaving giant ghosts upon lysis. In Z-mating mixtures of mecillinam-treated cells, these giant cells co-purified with mating products as other cells died. Giant cells recovering from mecillinam treatment yielded monstrous, branching forms, whereas non-Szp⁺ coccal cells reverted to rods in one step, and some 29% of the colonies formed were identified as deriving from entities possessing two distinct genomes. Z-mating was examined between E. coli and a distantly related Serratia marcescens strain. In the presence of calcium, mecillinam-rounded cells of a stable non-complementing diploid hybrid with the E. coli phenotype segregated normally dividing cells of the Serratia form.