Just a doubt on my previous post. Isn't "bacteria" plural? Can I use bacterium for the singular?

1) In particular, we prepared (have prepared)a genetically modified bacterium (?), the ..., which can survive using arsenic instead of phosphorous in building its DNA.
3) Actually, it contains a recombined plasmid in which we inserted the necessary sequences to interrupt the pathogenic action of the ... itself and of the main bacteria present in the (?) water.
4) Using plasmid is the most convenient way to transfer very quickly, and without any additional expenditure, the factor we want all the bacteria to have in order to mute their pathogenicity. 5) Moreover, this particular bacterium (?) can survive in presence of arsenic and, therefore, won't die until its action of purification is completed.

Yes -- 'bacterium' is singular; 'bacteria' is plural.

1. OK (either verb tense will work)

3. 'bacterium' -- ??

4. OK

5. OK

Yes, you are correct. "Bacteria" is the plural form of the word, while "bacterium" is used for the singular form.

In the sentences you provided, you can use "bacterium" to refer to a single organism. Here is a revised version of your sentences:

1) In particular, we prepared a genetically modified bacterium, the ..., which can survive using arsenic instead of phosphorous in building its DNA.
3) Actually, it contains a recombined plasmid in which we inserted the necessary sequences to interrupt the pathogenic action of the ... itself and of the main bacterium present in the water.
4) Using plasmids is the most convenient way to transfer the factor we want all the bacteria to have in order to mute their pathogenicity, without any additional expenditure.
5) Moreover, this particular bacterium can survive in the presence of arsenic and, therefore, won't die until its action of purification is completed.

In these revised sentences, "bacterium" is used correctly to represent a single organism, while "bacteria" is correctly used for multiple organisms.