Of Terms in Biology: Bacterial Ploidy

Deinococcus radiodurans. Source.

by Elio

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.

1. 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).
    
   
2. 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.
    
   
3. 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?

Comments

[anti snore]

When the cell is ready to
divide, there are four copies per cell, and they are segregated into pairs
immediately prior to division.This link might be more helpful.

[christoph weigel]

How boring life would be without... Epulopiscium

maybe not really multinucleoided, Elio, the study of bresler et al (1998) suggests a binuleoided organisation of the genetic material, rather. this bug seems to hold for a couple of more surprises...

[christoph weigel]

How boring life would be without exceptions to every rule... :-) cont.

e.g. the case of Epulopiscium spp. (Firmicutes) where single cigar-shaped cells have lengths exceeding 600 µm and contain ~1.000 genome copies arranged around the periphery of the cytoplasm. and have apparently invented some sort of 'germ line' [Mendell et al.(2008) PNAS 105:6730–6734].

Elio replies:

I stand second to noone in admiration for Epulopiscium, but this is a case of being "multinucleoided", not multiploid. But thanks for bringing this up.

[Abe Eisenstark]

Abe Eisenstark writes:
The Seifert lab at Northwestern discovered using a variety of genetic, molecular, and
microscopy techniques that Neisseria gonorrhoeae is diploid. Most likely it is homozygous
diploid. So-while the entire genome is on a single chromosome, there are two copies of
that chromosome in each non-dividing cell. When the cell is ready to
divide, there are four copies per cell, and they are segregated into pairs
immediately prior to division.

See: Tobiason DM and Seifert HS.
The Obligate Human Pathogen,Neisseria gonorrhoeae, Is Polyploid.
PLOS Biology 4:1070, 2006.

[Helen Hunter (Coldtoes)]

How boring life would be without exceptions to every rule... :-)
(And, yes, multinucleoided is an awkward term. But, heck, since I've started reading microbiology I've come across PLENTY of awkward terms. A microbiology dictionary may be in my Christmas stocking this year but I'll have to scrawl in the margins every time I get a better definition or update here on STC.)