For many years it was believed that all bacteria have a single chromosome. However, more recently analysis of Rhodobacterium spherodes indicated that this bacterium has two circular chromosomes. [Reference: Suwanto, A., and S. Kaplan. 1992. Physical and genetic mapping of the Rhodobacter sphaeroides 2.4.1 genome: Genome size, fragment identification, and gene localization. J. Bacteriol. 171: 5840-5849.]
ANSWER: To function as a replicon each DNA molecule must have an origin of replication that can function in that particular organism. Often a replicon will also encode proteins or RNAs required for the initiation of replication or regulation of replication, but sometimes these factors are provided by a second DNA molecule in the cell.
ANSWER: Plasmids carry genes that are not needed under all growth conditions -- that is, plasmids are dispensible under certain growth conditions. (For example, if a plasmid encodes antibiotic resistance it may be needed when cells are exposed to that antibiotic but not needed when cells are growing in the absense of the antibiotic.) In contrast, chromosomes carry genes that are essential for growth of the organism under all conditions. Thus, if both DNA molecules carry essential genes (such as ribosomal RNA or ribosomal protein genes), then both DNA molecules would be considered chromosomes. [Note a replicon requires sites for initiation of replication. The reference shows evidence that essential genes are located on each of the two replicons.]
The dogma that all bacteria have a single, circular chromosome died with the development of pulsed field gel electrophoresis (PFGE). Analysis of bacterial genomes using PFGE indicated that some bacteria have multiple chromosomes and some bacteria have linear chromosomes. Based upon PFGE, Allardet-Servent and colleagues demonstrated that Agrobacterium tumefaciens has four replicons: a 3000 Kb circular chromosome, a 2100 Kb linear chromosome, a 450 Kb cryptic plasmid, and the 200 Kb Ti plasmid. [Reference: Allardet-Servent, A., S. Michaux-Charachon, E. Jumas-Bilak, L. Karayan, and M. Ramuz. 1993. Presence of one linear and one circular chromosome in the Agrobacterium tumefaciens C58 genome. J. Bacteriol. 175: 7869-7874.]