Chapter 23
Indicator Microorganisms
Charles P. Gerba
1. What are some of the criteria for indicator bacteria?
These are listed in Table 23.1.
2. What is the difference between standards and criteria?
Standards are required and enforceable by law. Criteria and guidelines are terms used
to describe recommendations for acceptable levels of indicator microorganisms.
3. Why are geometric means used to report average concentrations of indicator
organisms?
Geometric averages prevent one or two high values from giving overestimates of high
levels of contamination.
4. Calculate the arithmetic and geometric averages from the following data set: fecal
coliforms/100 ml on different days on a bathing beach were reported as 2, 3, 1000, 15,
150, and 4000.
Arithmetic average = 861.6
Geometric average = 61.44
5. Define coliform and fecal coliform bacteria. Why are they not ideal indicators?
Gram-negative aerobic and facultatively anaerobic, non-spore-forming, rod-shaped
bacteria that produce gas upon lactose fermentation in prescribed culture media within 48
hours at 35oC. Fecal coliform bacteria are described as the same as coliform bacteria, but
ferment lactose at 44.5oC within 24 hours. They are not ideal indicators because under
some conditions they can grow in the environment and do not always reflect the
occurrence of enteric viruses and protozoa.
6. Why have coliphage been suggested as indicator organisms?
Because of their constant presence in sewage and polluted waters. The organisms have
also been suggested as indicators of viral pollution.
7. What are two methods that can be used to detect indicator bacteria in water?
The most probable number method (MPN) and the membrane filter method.
8. What is the difference between library-independent and –dependent source tracking
methods? What are the major disadvantages of both methods?
These are listed in Information Box 23.1.
9. What is the difference between a fecal indicator organism and process indicator? Give
an example of each.
These are listed in Table 23.4.