Abstract: A method wherein predetermined amounts of a microorganism of interest or the DNA or RNA thereof are used as the external standard; a fraction of the genome of said microorganism is extracted, revealed using a specific method or subjected to reverse transcription and/or amplification; and the DNA or RNA concentrations of the standard and target microorganisms or the target microorganism amplification products are compared with those of the external standard to determine values for the DNA or RNA concentration or the total microorganism concentration in each target microorganism sample. The method is useful for the quantitation and detection of all microorganisms.