Water Bacteriology

Source: Alba’s Medical Technology

Water-Borne Diseases

• Salmonella – typhoid & parathypoid fevers
• Shigella – bacterial dysentery
• E. histolytica – amebic dysentery
• Vibrio – cholera

The bacteriological examination of water usually consists of

1. Estimating the number of bacteria present by TOTAL PLATE COUNT
2. Detecting the presence or absence of coliforms & estimation of the number of coliforms present by the “MOST PROBABLE NUMBER” (MPN) method

DRINKING WATER STANDARDS

U.S. Public Health Service Drinking Water Standards publication

• standard for public water supplies
• states that NO MORE THAN 10% OF ALL THE STANDARD 10 ml PORTIONS OF WATER EXAMINED IN A MONTH SHALL SHOW THE PRESENCE OF COLIFORM BACTERIA

Sampling

• collected in sterile glass containers with ground glass stoppers
• should be packed in ice or stored at 6-10°C
• examination of contents should be done within
  • 6 hours – IMPURE WATER
  • 12 hours – PURE WATER
• 20-50 mg of sodium thiosulfate is added to sample bottle when testing water from swimming pools to NEUTRALIZE RESIDUAL CHLORINE and prevent chlorine from killing bacteria in the interval between collection and testing

Two (2) Procedures carried out routinely:

1. TOTAL BACTERIAL PLATE COUNT – report as number of bacteria (each colony is counted as one bacterium) per mL of undiluted H2O. When bacterial total plate count exceeds 100 organisms/mL at 37°C, the water is usually CONSIDERED UNSAFE FOR HUMAN CONSUMPTION.
2. TESTS FOR COLIFORM GROUP OF BACTERIA

Coliform group – all AEROBIC & FACULTATIVE ANAEROBIC GRAM NEGATIVE NON-SPOREFORMING BACILLI which FERMENT LACTOSE WITH GAS FORMTION WITHIN 48 HOURS at 35°C.

PRESUMPTIVE TEST

• LACTOSE BROTH or LAURYL TRYPTOSE BROTH
• POSITIVE: Formation within 48± 3hours of GAS in ANY AMOUNT IN THE FERMENTATION TUBE
• NEGATIVE: Absence of gas formation at the end of 48± 4hours incubation

CONFIRMED TEST

• BRILLIANT GREEN LACTOSE BILE BROTH FERMENTATION TUBES
• POSITIVE: Formation and presence of gas in any amount within 48± 3hours
• ENDO or LEVINE’S EMB AGAR PLATES
  • Results on Endo or Levine’s EMB
    • Typical nucleated with or without metallic green sheen
    • Atypical, opaque, unnucleated after 24hour incubation. PINK.
    • Negative (others)
• POSITIVE: Growth of TYPICAL COLIFORM COLONIES with 24± 2 hours
• NEGATIVE: Growth of NEGATIVE COLONIES

COMPLETED TEST

• BRILLIANT GREEN LACTOSE BILE BROTH FERMENTATION TUBES showing gas used for confirmed test
• POSITIVE: Formation of acid and gas in Lactose broth; demonstration of Gram Negative non sporeforming bacilli in the agar
• NEGATIVE: Absence of gas formation or failure to demonstrate Gram Negative non sporeforming bacilli in a gas forming culture
• THE detection of the coliform group in significant number is takes as evidence of FECAL CONTAMINATION.

Fecal origin: E. coli IMViC (++–)

Non Fecal origin: E. aerogenes IMViC (–++)