Frequency, size and distribution of bacteriophages in different marine bacterial morphotypes

The frequencies of cells containing mature phages, the burst sizes, the phage head sizes and the distribution of phages inside cells of different bacterial morphotypes were investigated in the northern Adriatic Sea. Coccoid bacteria more frequently (2 .5%) contained mature phages than rodshaped bacteria (1.2%) and spirillae (1.4 %). Including an estimation of non-v~sible infection we found that up to 27 % of rods were infected with viruses, up to 79% of cocci and up to 100% of spirillae. The highest overall infection frequency of the entire bacterial community was 30 %. The percentage of rods with mature phages was significantly correlated to increasing rod densities. It is suggested that a threshold density of about 2 X 10' rods ml-' exists that is necessary for infection with phages. No threshold densities could be determined for cocci and spirillae. Burst sizes varied strongly between different host morphotypes. The burst sizes of rods increased significantly with the frequency of rods containing mature phages, probably as a result of superinfection of bacteria with phages. The volume of the host cells seemed to influence the number of phages produced per cell. Most of the phages within rods and all phages within spirillae were smaller than 60 nm, whereas the majority of phages within cocci were larger than 60 nm. Analyses of the distribution of phages inside the cells showed that phages were frequently concentrated in 2 defined areas at the 2 opposite ends of both rods and spirillae. Our results from an in situ study suggest that the production of bacteriophages is strongly influenced by the structure of the bacterial community, i.e. by the relative abundances of the various morphotypes.