Microbiological Quality of Water
Microbial Indicators of Fecal
Contamination
• Approach used to evaluate
the quality of water for
human use
• Quantification of bacteria
normally found within the
intestines of warm
blooded animals
• High numbers is an
indication of the presence
of pathogens
Criteria for an Ideal Indicator of Fecal
Contamination
• Applicable to all water sources
• Present in stools, sewage water, and water
samples that contain pathogens
Criteria for an Ideal Indicator of Fecal
Contamination
• Number correlated to level of fecal
contamination and to number of pathogens
• No growth in the environment
• Survival/persistence that is longer than that of
pathogens
• Easily detected and quantified
• Numbers associated to the risk of consumers
of contracting enteric diseases
Ideal Indicator
• Ex: Mooney’s Bay 
water quality warnings
based on E.coli levels
• Representative of ALL
potential sources
• To date no single
indicator meets all the
criteria
Indicators of Fecal Contamination
• Coliform bacteria :
– Bacteria of the Enterobacteriaceae family
– Includes 4 genera
• Enterobacter, Klebsiella, Citrobacter, and Escherichia
– Present in the intestine of most warm blooded
animals (humans)
– Gram negative rods, non-sporulating
• Ferment lactose with acid and gas production in 48
hours at 37°C
Fecal Coliforms
• Subset of total
coliforms
• Escherichia coli (E. coli)
• Thermotolerant
coliforms
• Grow and ferment
lactose at 44oC
• Source: Fecal material
only
– Exception the tropics
Fecal Streptococci
• Gram positive
• Mainly associated with animal intestines
– S.faecalis, S.faecium
• Longer survival time than coliforms
• Survive broad range of environmental
conditions ex: temperature, pH, salt
• The ratio between coliforms and Streptococci
is used to confirm a contamination from
human sources
– Ratio Escherichia/Streptococci greater than 0.7
Tests for Fecal Contamination
• Selective media
– Allows the growth of certain genera
• Differential media
– Allows the visual discrimination of different
genera
Methods
• Presumptive tests
– MPN
• Viable counts
Presumptive Tests
• Makes use of differential/selective media to
detect characteristics common to indicator
microorganisms
– Positive test= presence suspected
– Negative test = absence
• Most probable number (MPN)
– Probable number based on a statistical point of
view
Lactose Broth
A
B
C
A. Negative test; No acid or gas
B. Negative test; Acid no gas
C. Positive test; Acid + gas
Gas
• Presumptive test for total coliforms
– Differential medium
• Allows to detect lactose fermentation
• pH and gas indicators
Presumptive Test for S. faecalis
(Enterococcus)
• Selective medium
– Contains azide
– Selective for Enterococcus
A
B
• Differential
– Fermentation of glucose
A. Negative; no acid
B. Positive; presence of acid
Food Microbiology
Concerns
• Provide clean, healthy, and
safe foods to consumers
• Prevent and retard spoiling
• Prevent food associated
diseases
• Preservation and production
of foods
– 1/3 of all foods are lost
because of spoilage
Sources of Bacteria in Foods
Sources of Bacteria in Foods
• Soil and water
• Plants
• Equipment for the treatment and
manipulation of food products
• Humans and Animals (Natural flora)
• Compost (fertilizer)
• Air/dust
Spoilage
• Considerations:
– Organoleptic properties
• Taste
• Smell
• Texture
• Appearance
Bacteria Associated with Food Spoilage
• Meats
– Enteric bacteria
• Escherichia coli
• Salmonella
• Milk products
– Lactic bacteria
Gram (-),
short rods,
facultative anaerobes
Gram (+),
short rods,
non-spore forming
• Lactobacillus
• Fruits and vegetables
– Pseudomonas
Gram (-),
short rods,
strict aerobes
Types of Food Borne Diseases
• Food infections
–
–
–
–
–
Usually associated with endotoxins
Requires ingestion of pathogen
Requires growth of pathogen in host
Appearance of symptoms between 24-72 hours
Often aggravated by the use of antibiotics
• Bacteriolytic antibiotics are not recommended
• Bacteriostatic antibiotics can help
Types of Food Borne Diseases (Cont’d)
• Food intoxications
–
–
–
–
–
Usually associated with exotoxins
No infection
Appearance of symptoms between 1-3 hours
Antibiotherapy useless in the absence of infection
Use of anti-toxins recommended
Bacteria Associated with Food
Infections
• E. coli O157:H7
– Mostly associated with meats from beef and pork
– Normal inhabitant of the natural flora of the
intestine
• Absent in muscles
• Contamination is usually the result of inappropriate
manipulations
– Utensils
– Opening and spilling of intestines
– Skin of employees
– Endotoxin A (lipid A of LPS layer)
E. coli O157:H7 (Cont’d)
• Can be associated with
fresh vegetables
• Can be associated to
products said to be
“organic ”
– Contamination by animal
and human compost
• Can be associated with
water ex: Walkerton, ON
Salmonella sp.
• Mainly associated
with the intestines of
poultry and pork
• Can also be found on
fresh vegetables just
like E.coli
• Very low infectious
dose
– < 10 cells
Bacteria Associated with Food Borne
Intoxications
• Clostridium perfringens
– Gram Positive rods
– Spore former  sometimes used as indicator
– Natural constituent of the natural flora of
mammals (including humans) and birds
– Also found in soil and water
– Intoxications associated with foods prepared a
few days in advance
– Intoxication caused by an enterotoxin
• Stomach aches and diarrhea
Bacteria Associated with Food Borne
Intoxications
• Clostridium Botulinum
– Gram Positive rods
– Spore former found in soil and water
– Intoxications associated with poorly sterilized cans
of fruits and vegetables
– Also associated with food prepared a few days
earlier
– Intoxication caused by a neurotoxin
• Flaccid paralysis, double vision, speech impediment,
difficulty to swallow
• Death due to paralysis of respiratory system