Selection and characterization of cyanophage resistance in marine Synechococcus strains

Abstract

Marine viruses are an important component of the microbial food web, influencing microbial diversity and contributing to bacterial mortality rates. Resistance to cooccurring cyanophages has been reported for natural communities of Synechococcus spp.; however, little is known about the nature of this resistance. This study examined the patterns of infectivity among cyanophage isolates and unicellular marine cyanobacteria (Synechococcus spp.). We selected for phage-resistant Synechococcus mutants, examined the mechanisms of phage resistance, and determined the extent of cross-resistance to other phages. Four strains of Synechococcus spp. (WH7803, WH8018, WH8012, and WH8101) and 32 previously isolated cyanomyophages were used to select for phage resistance. Phage-resistant Synechococcus mutants were recovered from 50 of the 101 susceptible phage-host pairs, and 23 of these strains were further characterized. Adsorption kinetic assays indicate that resistance is likely due to changes in host receptor sites that limit viral attachment. Our results also suggest that receptor mutations conferring this resistance are diverse. Nevertheless, selection for resistance to one phage frequently resulted in cross-resistance to other phages. On average, phage-resistant Synechococcus strains became resistant to eight other cyanophages; however, there was no significant correlation between the genetic similarity of the phages (based on g20 sequences) and cross-resistance. Likewise, host Synechococcus DNA-dependent RNA polymerase (rpoC1) genotypes could not be used to predict sensitivities to phages. The potential for the rapid evolution of multiple phage resistance may influence the population dynamics and diversity of both Synechococcus and cyanophages in marine waters.

FIG. 1.

Cluster diagram of BOR profiles for the 4 ancestral Synechococcus strains (in boxes), for 20 strains that were selected for resistance to one phage, for 2 strains selected for resistance to two phages, and for 1 strain selected for resistance to three phages. Phenotypic similarity between two Synechococcus strains is defined as the percentage of phages to which the strains are either both sensitive or both resistant. See the text for details.

FIG. 2.

Comparison of the genetic and phenotypic similarities among the 32 cyanomyophages in this study. A neighbor-joining tree (left) illustrates the genetic similarities among phage isolates based on pairwise distances between g20 amino acid sequences (15). A cluster diagram (right) illustrates phenotypic similarities between phage isolates; phenotypic similarity is defined as the percentage of Synechococcus strains that the paired phages could either both infect or both not infect. See the text for further details. The bottom arrows illustrate a case where genotypic and phenotypic similarities are both high, whereas the top arrows highlight two phages that have a high phenotypic similarity but a low genetic similarity.