Water Purity and Microbial Contamination

Water Purity and Microbiological Contamination

• Pure water does not exist naturally.
• Water pollution is a significant environmental issue due to chemical and microbiological contaminants.
• Microbial contamination is a major concern for water purity because it can transmit diseases like dysentery, typhoid fever, and cholera through contaminated water sources.
• Contaminated irrigation water can lead to produce contamination, resulting in human and animal disease.

Detection of Microorganisms in Water

• It is impractical to identify every organism in a water sample.
• Pathogen numbers are typically low in water, making them difficult to detect during routine testing.
• By the time individual pathogens are identified, it may be too late to prevent their spread.
• Water samples are examined for a limited number of indicator organisms to suggest the presence of potentially pathogenic fecal organisms.

E. coli as an Indicator Organism

• Escherichia coli (E. coli) is a common indicator organism found in the intestinal tracts of humans and animals.
• Its presence signifies potential fecal contamination and an increased risk of other pathogenic organisms.
• Any member of the coliform bacteria (Enterobacteriaceae) family, which are characterized by their ability to ferment lactose with acid and gas production, can serve as indicator organisms.
• Coliforms include genera like Enterobacter, Klebsiella, Citrobacter, and Escherichia.
• A significant number of coliforms in a water sample indicates fecal contamination, potentially making the water unsafe for drinking or recreational purposes.

Public Health Standards for Water Quality

• Public health standards set maximum coliform limits based on water usage.
• Drinking water has a strict limit of one coliform per 100 ml of water, with an action limit of four coliforms per 100 ml.
• Water for recreation, irrigation, and discharge into bays or rivers has higher allowable limits.

Membrane Filter Technique

• The membrane filter method is commonly used to determine coliform counts in water.
• It provides results quickly, allowing the processing of large water volumes while maintaining high reproducibility.
• The method involves filtering water through a nitrocellulose acetate or polycarbonate filter that retains particles, including bacteria, larger than 0.45 micrometers.
• The filter is incubated on a nutrient pad, allowing bacteria to grow into visible colonies.
• By counting colonies and calculating CFU/ml, the number of bacteria in the water sample can be determined.

Nutrient Pads and Media

• Nutrient pads used in the membrane filter technique are selective and differential media to encourage the growth of specific organisms while inhibiting others.
• m-Endo medium is a selective and differential medium commonly used for coliform detection.
• It inhibits the growth of gram-positive organisms and differentiates between coliforms and non-coliforms based on lactose fermentation, resulting in distinctive colony appearances (red with a metallic sheen for coliforms).

KF Streptococcus Agar

• KF Streptococcus agar is another type of nutrient pad used to detect fecal streptococci, which are also indicative of fecal contamination.
• The FC/FS ratio (number of fecal coliforms divided by the number of fecal streptococci) can help determine the source of fecal contamination:
  • Human fecal contamination: Higher fecal coliform count than fecal streptococci count.
  • Animal fecal contamination: Higher fecal streptococci count than fecal coliform count.

Spread Plate Technique

• The spread plate technique is another method for determining coliform counts and obtaining isolated colonies.
• It involves spreading a diluted broth culture of microbes evenly on the surface of an agar medium.
• A sterilized glass spreader or hockey stick is used to spread the culture.
• Single colonies can be picked and grown in broth to generate a pure culture.
• The spread plate technique can be used to estimate the total number of bacteria in a broth culture.