Indicator microorganisms

                                                 Indicator microorganisms

Indicator micro-organisms are used to suggest the presence of pathogens in water. The direct detection of pathogenic bacteria and viruses, and cysts of protozoan parasites requires costly and time-consuming procedures, and well-trained labor. These requirements led to the concept of indicator organisms of fecal contamination. 

The criteria for an ideal indicator organism are: 

1. It should be one of the intestinal microflora of warm-blooded animals.

 2. It should be present in samples when pathogens are present. 

3. It should be present in greater numbers than the pathogen. 

4. It should be at least equally resistant as the pathogen to environmental factors and to disinfection in water and wastewater treatment plants. 

5. It should not multiply in the environment. 

6. It should be detectable by means of easy, rapid, and inexpensive methods. 

7. The indicator organism should be nonpathogenic. 

8. It should be useful for all water types. 

Various microorganisms have been proposed and used for indicating the occurrence of fecal contamination, treatment efficiency in water and wastewater treatment plants, deterioration and post- contamination of drinking water in distribution systems. 

The main indicator groups are: 

 Coliforms 

 Fecal Streptococci 

 Sulphite Reducing Clostridia 

Faecal indicator: A group of organisms that indicates the presence of faecal contamination, such as the thermo tolerant coliforms (E. coli) or fecal streptococci. Their presence denotes that pathogens may be present. 

1. Coliforms

 The total coliform group belongs to the family Enterobacteriaceae. These are aerobic and facultative anaerobic, gram-negative, nonspore-forming, oxidase-negative, rod-shaped bacteria that ferment lactose (β-galactosidase positive) with gas and acid production within 24-48 hours at 35°C. They are not necessarily specific indicators of faecal pollution.

 eg. Escherichia coli, Enterobacter, Klebsiella, and Citrobacter. 

Coliforms are discharged in high numbers in human and animal feces, but not all of them are of fecal origin. These indicators are useful for determining the quality of potable water. They are less sensitive than viruses or protozoan cysts to environmental factors and to disinfection. Some members (e.g., Klebsiella) of this group may sometimes grow under environmental conditions in industrial and agricultural wastes. 

Fecal Coliforms 

Fecal coliforms or thermo tolerant coliforms include all coliforms that can ferment lactose with gas and acid production at 44.5°C within 24-48 hours, in addition to 35°C. The fecal coliform group comprises bacteria such as Escherichia coli. 

Thermotolerant coliforms produce indole from tryptophan, cannot utilize citrate as sole source of Carbon and produce βglucuronidase. They are the specific indicators of recent faecal pollution from warm-blooded animals. 

The presence of fecal coliforms indicates the presence of fecal material from warm-blooded animals. However, human and animal sources of contamination cannot be differentiated. They are much less resistant to disinfection than viruses or protozoan cysts. 

2. Fecal Streptococci 

Gram-positive, catalase-negative cocci. This group comprises Streptococcus faecalis, S. bovis, S. equinus, and S. avium. Streptococcus faecalis, S. faecium, S.durans are commonly isolated from humans, while S. bovis, S. equinus, and S. avium are isolated from animal sources, cattle, horse and birds, respectively. Because they commonly inhabit the intestinal tract of humans and warm-blooded animals, they are used to detect recent fecal contamination in water.

 All fecal streptococci that grow at pH 9.6, 10° and 45°C and in 6.5% NaCl are designated as Enterococci. They show resistance to 60°C for 30 min and ability to reduce 0.1% methylene blue. Faecal streptococci can also grow in presence of 40% bile, sodium azide and potassium tellurite concentrations which normally inhibit other organisms including coliforms. 

The fecal coliform to fecal streptococci ratio (FC/FS ratio) is an indicator of the origin of pollution of surface waters. A ratio of >4 indicates a contamination of human origin, whereas a ratio of 0.7 is indicative of animal pollution. This ratio is only valid, however, for recent (24 hours) fecal pollution and is unreliable for chlorinated effluents. 

3. Sulphite Reducing Clostridia - Clostridium perfringens. 

Clostridia are mostly opportunistic pathogens, but are also implicated in human diseases such as gas gangrene (C. perfringens). Clostridium perfringens are gram-positive, strictly anaerobic rods which are endospore-forming, non-motile and can reduce sulphite to H2S. 

Clostridium perfringens can ferment lactose, sucrose and inositol with the production of gas, produce a stormy clot fermentation with milk, reduce nitrate, hydrolyse gelatin and produce lecithinase and acid phosphatase. These sulfite-reducing bacterium found in the colon form approximately 0.5 percent of the fecal microflora. The spores are quite resistant to environmental stresses, and to disinfection by oxidizing agents and UV than bacterial and phage indicators. It is commonly found in human and animal feces and in wastewater-contaminated aquatic environments. 

The hardy spores make this bacterium too resistant to be useful as an indicator organism. It is an indicator of past pollution. 

4. Bacteriophages 

To evaluate the virological quality of water, the use of bacteriophages as indicators has been proposed. Three groups of phages have been suggested: somatic coliphages (infect mostly E. coli), F-specific RNA bacteriophages and phages infecting Bacteroides fragilis. 

The occurrence of specific pathogens is seasonal. Also, viruses and other pathogens not part of the normal faecal microbiota and are excreted by infected individuals. 

So, the idea of indicators of microbial water quality help in alerting and managing waterborne microbial risks, where the pathogens may escape chances of detection.