Is it possible for pathogens to be present when indicators are not?

Pathogens can indeed be present in water even when indicator organisms are not detected. This is primarily because indicator organisms are used as a proxy to indicate potential contamination and the presence of pathogens, but they do not directly indicate the presence of these harmful microorganisms.

Indicator organisms, such as E. coli or coliform bacteria, are often easier to detect and are used because their presence suggests fecal contamination and the possible presence of pathogens, which can pose a risk to human health. However, there are situations where pathogens might exist without a corresponding detection of these indicators. For instance, in water that has been treated or filtered well, pathogens might be present due to contamination at a different source or through biofilm formation, while the indicators remain undetected.

Additionally, certain pathogens can survive in environments where indicators are not, such as specific microbiological conditions in water bodies. Therefore, the presence of pathogens without the presence of microbial indicators can pose significant risks, which is why water testing often includes both indicator and pathogen tests for a more comprehensive analysis. This understanding highlights the limitations of relying solely on indicator organisms for assessing water quality.