there are several ways to determine this and they range from difficult to hard.
1) you can easily get the genomic DNA sequence of the bacteria you are studying from pubmed DNA.
Then simply analyze the DNA in a bioinformatics software to detect G / C / A / T percentage.
2) If this is a new bacteria strain and its DNA is unknown there are many ways to determine GC content. You can clone the DNA into a plasmid, or extract the genomic DNA then PCR it then insert into a BAC (bacterial artificial chromosome). Either way, you can then sequence the DNA to determine G/C percentage.
Let me know if you need more help
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