Course Code: FST 215

Course Title: Food Microbiology

Course Duration: Two hours per week

Taxonomy, ecology and technology of Food Microorganisms

Foods originate from plant and animal sources, all foods will have microorganisms
associated with them that are involved in reducing the food to inorganic compounds
in order to perpetuate the gas and mineral cycles on earth.

If the kinds of organisms associated with plant and animal foods in their natural
states are known, then the general types microbes that will occur on these foods at
some later stage in processing and the types of spoilage that may occur can usually
be predicted.

In the past, classification of organisms into related groups was primarily based upon
numerical taxonomy, a system that arranged organisms on the basis of phenotypical
similarities and differences among them. Today, taxonomists try to group organisms
phylogenetically, a classification scheme that mirrors evolutionary (genetic)
relationships.

Foods are ecosystems which composed of the environment and the organisms that
live in it.
* The food environment is composed of intrinsic factors inherent to the food (e.g. pH,
water activity and the nutrients) and the extrinsic factors external to it (e.g.
temperature, gaseous factors and the presence of other bacteria) .
* The extrinsic and the intrinsic factors could be manipulated to preserve foods

• Ecology was defined by International Commission on Microbial Specifications for

foods as the study of the interactions between the chemical, physical and structural
aspects of a niche and the composition of its specific microbial population.

*Interaction emphasizes the dynamic complexity of food ecosystems.

Factors that affect microbial growth are:

Intrinsic factors: Factors inherent to the food. They are chemical and physical
characteristics of food.

Extrinsic factors: Storage conditions of the food i.e. properties of the environment in
which the food is stored.

Intrinsic factors include:

 Acidity (pH)
 Water activity (aw)
 Oxidation- reduction potential/ redox potential (Eh)
 Nutrient content
 Presence of antimicrobials
 Biological structures
 Presence and activities of other microorganisms
pH
• Microorganisms are able to grow in an environment with a specific pH,
as shown in table 1:

Microorganisms Min. pH value Opt. pH value Max. pH value

Gram +ve bacteria 4.0 7.0 8.5
Gram –ve bacteria 4.5 7.0 9.0
Yeasts 2.0 4.0 – 6.0 8.5 – 9.0
Molds 1.5 7.0 11.0

• Some bacteria are:

 Acidophilic bacteria e.g. Lactic acid bacteria (pH 3.3 – 7.2) and acetic acid
bacteria (pH 2.8 – 4.3).
 Basophilic bacteria e.g. Vibrio parahaemolyticus (pH 4.8- 11.0) and
Enterococcus spp (pH 4.8- 10.6).
• Increasing the acidity of foods either through fermentation or the addition of weak
acids could be used as a preservative method.

Water activity (aW)

• Water activity is a measure of the water available for microorganisms to grow or
reactions to take place i.e. measure of the amount of water disposable for the
microorganisms.
• It is a ratio of water vapour pressure of the food substance to the vapour pressure
of pure water at the same temperature.
• Water activity is expressed as:
• Water activity (aw) = P/ Pw where P= water vapour pressure of the food substance
and Pw= water vapour pressure of pure water (Pw = 1.00).
• The growth of microorganisms is limited due to minimum water activity
values (Table 2):
Microorganisms Minimum water activity (aW)
Gram +ve bacteria 0.95
Gram -ve bacteria 0.91
Yeasts 0.88
Molds 0.80

• No growth of any microbe below aw = 0.60

• Exceptions are : Halophilic bacteria (min. aw = 0.75 e.g. Halobacter spp), Xerophilic
molds (min. aw = 0.60 e.g. Xeromyces bisporus) and Osmophilic yeasts (min. aw =
0.60 e.g. Zygosaccharomyces rouxii).
• The water activity of a food ranges from 0.00 – 1.00
• Water activity of a completely dehydrated food is 0.00
• Technologies to control water activity in foods are:
 Drying
 Addition of salts, sugars and glycols
 Concentrate

Oxidation- Reduction potential (O/R or Eh)

This is the ratio of the total oxidizing (electron accepting) power to the
total reducing (electron donating) power of a substance.
• Eh is a measurement of the ease by which a substance gains or losses
electrons.
• Eh is measured in millivolts (mV)
• The more oxidized substances, the higher the Eh; the more reduced substances,
the lower the Eh.
• Microorganisms that grow at:
 High Eh or +ve Eh (require oxygen) – Aerobes
 Low Eh or –ve Eh (oxygen is toxic)- Anaerobes
 High and low Eh (+ve /-ve Eh) – Facultative anaerobes
 Relatively low Eh values – Micro-aerophilic
• Technologies to control O/R in foods:
 Vacuum packaging
 Skin tight packaging
 Gas flushing
 Canning
 Antioxidants

Nutrient content
Microorganisms require
a. Energy source such as carbohydrates, amino acids, proteins, organic acids and
alcohol.
b. Nitrogen source such as amino acids, peptides, nucleotides, urea, proteins and
ammonia.
c. Carbon source
d. Minerals such as phosphorus, iron, manganese, magnesium, calcium and
potassium.
e. Vitamins and other growth factors

Presence of antimicrobials
1.Natural constituents of foods which affect microbial growth are:
 Lysozyme e.g. Eggs
 Lactoferrin e.g. Milk
 Lactoperoxidase e.g. Cow’s milk
 Conglutinin e.g. Cow’s milk
 Essential oils e.g. Spices and vegetables
2. Preservatives such as benzoic acid, sorbic acid and nisin

Biological structures
• Natural physical barriers of foods are:
 Cell walls e.g. Fruits and vegetables
 Shells e.g. Eggs
 Skin e.g. Fish

Microbiology of local foodstuffs

Fermented foods: Fermented legumes, fermented animal protein, fermented milk,
cereal, alcoholic beverages, etc.
Benefits of fermentation:
• Preservation
• Variety in flavor, types
• Inedible food becomes edible
• Improved digestibility
• Enhanced nutrient level
• Anti-viricidal, anti-tumour, anti-microbial etc.

Principles of food preservation

*Asepsis
*Removal of microorganisms e.g. filtration
* Maintenance of anaerobic condition
* Use of high temperature
*Use of low temperature
* Drying
* Chemical preservation
* Irradiation
* Combination of 2 or more methods

Food-borne illness
Definition
Food-borne illness has been defined by WHO as a disease of an infectious or toxic
nature caused by or thought to be caused by the consumption of food or water. Most
of foo d-borne diseases are microbial in origin and an important cause of reduced
economic productivity.

Types of food-borne diseases

1. Food intoxication: an illness caused by a toxin or poison in food. The toxin is an
exotoxin, extracellular, gram +ve bacteria and occasionally gram –ve. It is protein in
nature and present in filtrates of growing cell
2. Food infection: An illness caused by infection produced by organisms present in
food when consumed. The toxin is an endotoxin and is cell-associated. It is a
complex lipopolysaccharide of gram -ve bacteria. Toxin is intracytoplasmic and only
released on autolysis or extraction of cell. The endotoxin is less potent and more
specific in their action than the exotoxins

Possible causes of foodborne illness: Bacteria, Fungi, Chemicals, Viruses, Protozoa,

Helminths & Algae.

Microbiological agents of foodborne illness:

BACTERIA:
Aeromonas, Bacillus cereus, Brucella species, Campylobacter jejuni, Clostridium
botulinum, Cl. Perfringens, Escherichia coli, listeria monocytogenes, Mycobacterium
bovis, Salmonella Typhi, Salmonella (non-Typhi), Shigella, Staphylococcus aureus,
Vibrio cholera, Vibrio parahaemolyticus, Yersinia enterocoalitica

VIRUSES:
Hepatitis A virus, Norovirus, Rotavirus

PROTOZOA:
Cryptosporidium parvum, Entamoeba histolytica, Giardia lamblia

HELMINTHS:
Ascaris lumbricoides, Taenia saginata and T. solium, Trichinella spiralis,
Trichuris trichiura

Factors contributing to outbreaks of food poisoning:

Preparation too far in advance, Storage at ambient temperature, inadequate cooling,
contaminated processed food, undercooking, contaminated canned food,
inadequate thawing, cross contamination, food consumed raw, improper warm
handling, infected food handlers, use of left over, extra large quantities prepared.

The pathogenesis of diarrhoeal disease:

Causative agents confined to gut and its immediate vicinity.
Patient presents acute gastroenteritis characterized by diarrhea and vomiting.

Common features of mechanisms involved in diarrhea:

 Excessive evacuation of too-fluid faeces
 Gut unable to absorb 8-10 litres of fluid it receives daily
 Illness due to damage to host by microorganisms
 Toxins are the direct cause of diarrhea (Exotoxins & Endotoxins)
 Typical example is cholera toxin produed by Vibrio cholerae.
 Toxin (MW84,000) comprises five B subunits and a single A subunit
 B subunits bind to specific ganglioside receptors and creates hydrophilic
channel
 A subunit passes through the hydrophilic channel thus stimulating several
biochemical reactions.
 Na+ and Cl- inhibited
 Cl-, HCO3 and Na+ stimulated
 Creates osmotic imbalance
 Balancing osmosis requires massive outflow of water into intestinal lumen
 Results in profuse watery diarrhoea

Investigation of food-borne disease

 Necessary information obtained from the following:
 Persons who ingested suspected food
 Surveys of food preparation
 Survey of food storage
 Survey of serving operations
 Samples taken and processed rapidly in the laboratory
 Complete questionnaires from infected persons
 Collect remnant of suspected food aseptically
 Identify origin of sample, date, time of collection, name of Scientist
 Brief description of symptoms of patients & suspected organism or chemical
 Water samples should be taken
 All sewage and plumbing be evaluated
 Presence of insects and rodents must be determined
 Determine any ill food handler(6-8 weeks) prior to outbreak
  Any diarrhea or vomiting from food handlers, boils, carbuncles and respiratory
 infections

Mycotoxins: mostly food-borne, natural toxic metabolites of fungi, potent

carcinogens, mutagens, teratogens, immunosuppresants

Common mycotoxins and corresponding mycoflora

Mycotoxin Mycoflora
Aflatoxin Aspergillus flavus, A. parasiticus, A. nomius
Fumonisin Fusarium verticilloides
Zearalenone F. graminearum
Ochratoxin A. Ochraceus, Penicillium verrucosum
Patulin Penicillium expansum
Ergot alkaloids Claviceps purpurea

Toxicity of aflatoxins:
High doses---------------lethal, affects lungs, myocardial and kidney tissues
Sub-lethal doses-------- causes chronic toxicity e.g liver cirrhosis
Low dose---------------- human hepatocellular carcinoma

Mutagenicity: Aflatoxin B1 binds to DNA, induces G to T transversions (p53

mutations)
Teratogenicity:
 Embryonic abnormalities
 Forms synergy with hepatitis B virus to cause liver cancer
 Stunted growth in children
 Immunosuppression

Hazard Analysis Critical Control Point

Principles:
 Detailed surveillance of a production process
 Control of known hazards at specific critical stages
 Objective assessment of hazards using a flow-chart
 Identification of critical control points(CCPs)
 Monitoring at CCPs to ensure process continues within pre-determined
tolerance limit