Coliform bacteria are a group of microorganisms widely distributed in nature, including in soil, vegetation, and the intestinal tracts of humans and warm-blooded animals. While most strains are harmless, their presence in drinking water is a significant public health concern. They serve as a reliable indicator of potential contamination, signaling that conditions are present for disease-causing pathogens to also exist in the water supply. Understanding the traits, spread, and elimination methods of these bacteria is necessary for ensuring water safety.
Defining Coliform Bacteria and Their Indicator Role
Coliforms are characterized biologically as Gram-negative, rod-shaped bacteria that do not form spores. A defining trait is their ability to ferment the sugar lactose, producing acid and gas when incubated at 35 to 37 degrees Celsius. They are also classified as facultative anaerobes, meaning they can thrive both with and without oxygen, which contributes to their survival in diverse environments.
The coliform group is divided into three categories: Total Coliforms (TC), Fecal Coliforms (FC), and Escherichia coli (E. coli). Total Coliforms are widespread and can originate from the environment, while Fecal Coliforms and E. coli are specifically associated with the feces of warm-blooded animals. Because testing for every disease-causing microorganism is impractical, coliforms are used as “indicator organisms.” Their presence suggests a breach in water quality that could allow more dangerous bacteria, viruses, or protozoa to enter the system.
E. coli is considered the most specific indicator of fecal contamination, as it does not typically grow and multiply in the environment like other coliform species. The detection of E. coli indicates recent contamination by human or animal waste. This finding is treated with urgency due to the risk of pathogens like Salmonella or Giardia being present alongside the E. coli.
Primary Sources of Coliform Contamination
The most common pathway for coliforms to enter a water supply is through fecal matter from humans and animals. Failing septic systems are a frequent source, as untreated wastewater can seep through the soil and into groundwater, contaminating nearby wells. Aging or cracked sanitary sewer lines and poor well construction also create direct channels for raw sewage to infiltrate the water system.
Agricultural runoff poses a threat, carrying large quantities of coliforms and E. coli from livestock manure into surface waters and shallow groundwater. High-intensity farming areas with dense water drainage networks are susceptible to this type of non-point source pollution. Stormwater runoff from urban areas can also wash contaminants from streets and land surfaces into water bodies.
Total Coliforms often enter water systems from environmental sources such as soil and decaying vegetation. Surface water contamination, caused by heavy rains or flooding, can introduce these bacteria through poorly sealed well casings or inadequate well caps. The presence of organic carbon from decaying matter can also fuel the growth of coliforms already present in biofilms within the water distribution pipes.
Practical Steps for Testing and Result Interpretation
Testing for coliforms is performed by collecting a water sample in a sterile container and sending it to a certified laboratory for analysis. The test determines the presence or absence of bacteria in a 100-milliliter sample, often using methods involving color change. For public water systems and private wells, the Maximum Contaminant Level (MCL) for E. coli in drinking water is set at zero, meaning any detectable presence is a violation.
A positive result for E. coli requires immediate action, as it confirms fecal contamination and signifies an acute health risk. Water should be stopped for consumption unless it is boiled for at least one minute. This finding necessitates investigation into the source of the fecal matter, such as a faulty septic system or surface water breach.
If a sample tests positive for Total Coliforms but negative for E. coli, it indicates a potential problem with the system’s integrity or the presence of environmental bacteria. While this scenario presents a lower health risk than E. coli contamination, it shows a pathway exists for contamination to occur. Corrective action involves inspecting the wellhead, re-sampling to confirm the result, and investigating potential entry points for surface water.
Techniques for Elimination and Water Safety
In an emergency, the most reliable method for neutralizing coliform bacteria is by boiling the water for one minute. This high temperature destroys the microorganisms, rendering the water safe for drinking and cooking. However, this is only a temporary solution while a permanent treatment method is implemented.
For a one-time contamination event, such as after well maintenance or flooding, shock chlorination is the recommended procedure for private wells. This involves pouring a high concentration of unscented household bleach into the well and circulating the chlorinated water throughout the entire plumbing system. The highly chlorinated water must be left to stand in the system for several hours before being completely flushed out through an outdoor faucet.
For persistent or recurring contamination, continuous disinfection systems provide a permanent solution. Ultraviolet (UV) disinfection systems are highly effective, using UV-C light to disrupt the bacteria’s DNA, preventing them from reproducing and causing illness. UV systems can inactivate nearly all coliform bacteria without adding chemicals to the water. Other permanent treatments include chemical feed pumps that inject chlorine or other oxidants, and filtration like reverse osmosis, which can physically remove bacteria from the water.