What is an indicator organism?
Because enteric pathogens, such as Salmonella and E. coli O157:H7, are both difficult and expensive to reliably detect, it is often not practical to test for pathogens. Indicator organisms have been historically used to assess the microbiological quality of water and foods. Generic (or non-pathogenic) E. coli occur naturally in large numbers in the intestines (i.e., enteric) and feces of humans, mammals, and warm- blooded animals. The presence of E. coli in water directly indicates fecal contamination by animals or humans, and indirectly, the potential presence of enteric pathogens, such as bacteria, viruses, or parasites.
Chlorine chemistry and its mode of action
When chlorine is dissolved in water, the chlorine molecules combine with water in a reaction called hydrolysis, forming hypochlorous acid (HOCl) and hydrochloric acid (HCl-). Cl2 + H2O → HOCl + H+ + Cl-
Depending on the pH, hypochlorous acid can further dissociate into hypochlorite ions (OCl-).
HOCl → H+ + OCl-
Hypochlorous acid (HOCl) and hypochlorite (OCl- ) are together referred to as “free available chlorine” or “free chlorine”. These molecules coexist in an equilibrium relationship that is influenced by pH and temperature. Other factors will impact chlorine’s effectiveness including chlorine concentration, contact time, and organic matter in the water.
Chlorine’s Mode of Action
As previously stated, when chlorine is dissolved in water, it forms hypochlorous acid (HOCl) and hypochlorite (-OCl), both strong oxidizing agents reacting with various biological molecules including proteins, amino acids, peptides, lipids, and DNA as mechanisms for cell destruction. Hypochlorous acid is 40 to 80 times more effective at killing microorganisms than hypochlorite. The mechanism of germicidal activity is based on the ability of HOCl to penetrate the membrane of microbial cells (Figure 1). The neutral charge of hypochlorous acid, along with the negatively charged bacterial cell walls, allows HOCl to more easily penetrate into bacteria, oxidizing cellular structures both outside and inside the cell. Once inside the cell, the natural pH of the cell causes HOCl to dissociate and oxidize various components within the cell – such as the mitochondrion, which is the powerhouse of the cell.
Hypochlorite (-OCl) has a reduced germicidal effect on cells due to its negative charge and its inability to pass through the cell wall, exerting oxidizing action only from the outside of the cell.
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