Moreover, because many of these properties are the product of one or a small set of genes, a vast array of plasmids have been engineered to have a specific set of useful properties, including a number of unique cloning sites for in vitro manipulation. Specialized manuals describe the general choices and new versions are constantly being described in the literature. The above sections are in no way meant to be an all-inclusive summary of plasmids in prokaryotic organisms. Clearly, molecular biology is only scratching the surface of the various forms and properties that these elements can possess. There are, however, several other classes of plasmids which deserve mention.
Many bacteria have plasmids with no obvious function.
Some are extremely large and this property makes them even harder to detect physically. In general, if you isolate a new organism and it has an unstable phenotype, it would be prudent to consider the possibility that a plasmid is harbored in the strain. Their low copy number increases the likelihood that the analysis is physiologically relevant. Multicopy plasmids can cause higher level of gene products to be made under some conditions. Such plasmids are also a good source of DNA for cloning.