Ploidy is not a term that has much currency in bacteriology, but it does make an appearance once in a while. Ploidy, as per the dictionary, is the number of chromosomes per cell. It's a term widely used for cells that are generally uninucleated, such as our gametes and our somatic cells. The problem here is that bacteria are not consistently uninucleated. In some bacterial species, the cells always contain more than one nucleoid, while in others the number is variable, depending on their circumstances. Due to such vagaries, there are several ways to define ploidy in bacteria.
- Ploidy sensu stricto refers to the number of copies of a chromosome per nucleoid. (When there are two or more different chromosomes, as in Vibrio cholerae, what matters is still the number of copies of each.) By nucleoid, I mean a discrete physical body. In this sense, determining the degree of ploidy requires knowledge of the structure of the replicating chromosome. In fast growing bacteria, a new round of DNA replication is initiated before the ongoing round has been completed, so you may have one complete chromosome and one or more partial chromosomes. Consequently, the amount of DNA per chromosome can vary by a factor of four or more, depending on the rate of growth. The other thing one needs to know is the number of nucleoids per cell. So defined, I am not aware of any report of stable ploidy in this sense in bacteria (but I could be wrong).
- A looser definition (sensu lato) is that ploidy refers to the number of copies of a chromosome per cell, regardless of how they are partitioned into nucleoids. In the case of E. coli growing unperturbed in the laboratory, the number of (incompletely replicated) nucleoids per cell can vary between one and four or more. Other cells habitually grow as multinucleated filaments. I suggest that such cells should not be called polyploid but rather multinucleated, or, to coin an awkward term, multinucleoided.
- An even looser definition (sensu latissimo) is that ploidy refers to the number of copies of a chromosome per particle, regardless of the number of cells in one particle. For example, measurements by flow cytometry do not distinguish between single cells and aggregates of several cells. A case in point is Deinococcus radiodurans, which usually comes as a packet of four cells. The term here should be multicellular.
I recognize that for many purposes, such as when considering the radiation resistance of an organism, these definitions make little difference. It comes down to simply a matter of words. Still, can you guess which definition I favor?