Legionella testing – Is Culture the gold standard?
Conventional wisdom holds that the gold standard method for the detection of Legionella is the culture method, which became the reference method when it was developed to isolate the organism after the first recorded outbreak of Legionnaires disease in Philadelphia in 1976. This method is adopted by most laboratories around the word and is better known as the standard ISO 11731: Water quality – Detection and enumeration of Legionella. The culture method allows the isolation and the quantification of Legionella in water samples and biofilms, however, it has limitations that should not be disregarded :
1. It requires selective media and prolonged incubation periods
2. The concentration stage followed by decontamination with heat or acid may involve bacteria loss, which results in recovery rates much lower than 100 %
3. The real number of bacteria may be underestimated due to background organisms interfering with Legionella making false negatives likely.
4. ISO 11731 requires subculture onto buffered charcoal yeast extract (BCYE) agar minus cysteine, but Legionella oakridgensis also grows in this medium , making false positives possible.
5. Legionella spp. may enter a latent state being viable but non culturable (VBNC), which means that it is very difficult to be cultured on agar plates from water samples, making false negatives possible – see also below.
Legionella pneumophila is able to survive in low-nutrient environments as a free organism (even for long periods), during which time it is at its most dangerous and virulent, but also non-culturable. The culture method does not detect Legionella pneumophila in this phase, which is a serious limitation.
Raphael Garduno comments that Legionella is an obligate intracellular bacterium which means it grows inside a host organism, in the case of Legionella pneumophila usually an amoeba. The culture method carried out in laboratories is totally artificial and DOES NOT REPRESENT THE REAL NATURE OF THE BACTERIUM. This leads to false negatives.
Download a technical note on the Technical paper Legionella Life Cycle and Culture Testing here.