Microbial response to disinfectants

© 2015 Elsevier Ltd All rights reserved. Risks associated with contaminated drinking water include bacterial, viral and protozoan diseases. The chemical structure and microbiological range are the most usual criteria to classify disinfectants used in water treatment. Disinfectants can be used in combination, to optimize their advantages and decrease their liabilities. The Gram-negative bacteria are generally less sensitive to biocides than Gram-positive bacteria because of their outer membrane. However, Gram-negative bacteria are more resistant to chlorine. Membrane permeability loss is the first lethal effect of chlorine on E. coli, due to damage to the external membrane, producing an imbalance in the transmembrane ionic gradient. When sub-lethal chlorine levels are present, the respiration inhibition is the first effect. Selective pressures of water treatment can produce microorganisms with resistance mechanisms favoring survival in an otherwise restrictive environment. Bacteria from the chlorinated systems are more resistant than those from the unchlorinated systems, implying that there may be selection for more disinfectant-tolerant microorganisms in chlorinated waters. Bacteria may differ considerably in their responses to biocides because the Gram-negative cell wall presents a more significant barrier to entry.