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FOOD MICROBIOLOGY MEDI 2371. Prepared BY PROF. MOHAMMAD EID SHUBAIR. Definition. Food Microbiology is the study of microorganisms that inhabit, make or contaminate food.

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food microbiology medi 2371


Prepared BY



Food Microbiology is the study of microorganisms that inhabit, make or contaminate food.

Many microorganisms are used to make food e.g the bacteria which converts liquid milk into yogurt or cheese, yeast for making bread, through fermentation.

Other microorganisms or the toxins they produce cause spoilage of food.

Foodborne diseases e.g gastroenteritis and salmonellosis results from eating contaminated food with various types of microorganisms.

Safety of food must be ensured to protect human health.

Food preservation is being used to avoid health hazards.

Preservation methods include drying, cooking, smoking, salting, sugaring, low temperature storage and radiation.

Following the discovery of microorganisms, scientists associated the role of these organisms with food spoilage, fermentation and foodborne diseases.

Historical Development of Food Microbiology

FOOD Fermentation

Louis Pasteur(1860) showed that fermentation of lactic acid and alcohol from sugar was the result of growth of specific bacteria and yeast.

He concluded that souring of milk was caused by the growth of organisms in it.

Theodor Schwan (1837) named the organism involved in sugar fermentation as Saccharomyces( sugar fungus).

Emil Chirstianhansen (1883) used pure culture of yeasts to ferment sugars.

food spoilage
FOOD Spoilage

Harry Russell (1895) showed that gaseous swelling with bad odors in canned peas was due to growth of heat-resistant bacteria(spores).

Tyndall(1876) observed that bacteria in decomposing substances were always traceable to air, substances or containers.

Bacillus coagulanswas first isolated from coagulated milk by B.W. Hammer(1915).

In 1902 the term psychrophile was first used by Schmidt-Nielsen for organisms that grow at 0C.

food poisoning
Food Poisoning

In 1857 milk was incriminated as a transmitter of typhoid fever by W. Taylor

In 1888 gaertner first isolated Salmonella enteritidis from meat that had caused 57 cases of food poisoning.

In 1894 T. Denys was the first to associate Staphylococci in food poisoning.

In 1896 Van Ermengen first discovered Clostridium botulinum.

In 1906 Bacillus cereus food poisoning was recognized.

In 1937 paralytic shellfish poisoning was recognized.

In 1939 gastroenteritis caused by Yersiniaenterocolitica was first recognized by Schleifstein and Coleman

In 1951 Vibrioparahaemolyticus was shown to be an agent of food poisoning.

In 1960 the production of aflatoxin by Aspergillusflavus was first reported.

In 1986 Bovine spongiform encephalopathy was first diagnosed in cattle in U.K.

food preservation
Food Preservation

In 1810 preservation of food by canning was developed by Appert in France.

Fish and Fruits were first canned in 1840.

In 1842 H- Benjamin in U.K. introduced freezing foods by immersion in an ice and salt brine.

In 1878 the first successful cargo of frozen meat went from Australia to England.

In 1890 pasteurization of milk began in USA.

In 1907, Metchnikoff isolated one of yogurt bacteria (Lactobacillus bulgaricus).\

In 1943, Procter in USA employed ionizing radiation to preserve hamburger meat.

groups of microorganisms in food
Groups of Microorganisms In Food

Enteric Viruses

Hepatitis A and Hepatitis E Viruses



Enteric Adenovirus

Human Caliciviruses(Noroviruses and Sapporo Viruses)

  • Ascaris
  • Trichinella
  • Taenia
  • Giardia
  • Entamoeba
  • Cyclospora
  • Toxoplasma
  • Produce Mycotoxins
  • Species of Aspergillus, Fusarium, Penicillium, Claviceps
  • Mycotoxins Include aflatoxin, Ochratoxin, Ergot Alkaloids








Escherichia coli


Bacillus cereus

important bacterial groups in food
Important Bacterial GroupsIn Food

These bacteria have a rapid growth rate, ability to grow under a wide range of temperature, aerobiosis, pH, water activity, and utilization of food nutrients

They better survive adverse conditions such as survival of spores at high temperatures.

1- Lactic Acid Bacteria

These bacteria produce large quantities of lactic acid from carbohydrates, species from genera Lactococcus, Leuconostoc, Pediococcus, Lactobacillus, and Streptococcus thermophilus.

2- Acetic Acid Bacteria

They are bacteria that produce acetic acid , such as Acetobacteraceti

3- Propionic Acid Bacteria

They produce propionic acid and are used in dairy fermentation e.gPropionibacteriumfreudenreichii

4- Butyric Acid Bacteria

They are the bacteria that produce butyric acid in relatively large amounts, Clostridium butyricum is included in this group.

5- Proteolytic Bacteria

They are bacteria that can hydrolyze proteins because they produce extracellular proteinases, they include species in the genera Micrococcus, Staphylococcus, Bacillus, Clostridium, Pseudomonas, Alteromonas, Flavobacterium, Alcaligenes, some Enterobacteriaceae and Brevibacterium.

6- Lipolytic Bacteria

They hydrolyze triglycerides as they produce lipases. Species in genera Micrococcus, Staphylococcus, Pseudomonas, Alteromonas and Flavobacterium are included in this group.

7- Saccharolytic bacteria

They hydrolyze complex carbohydrates, Species in the genera Bacillus, Clostridium, Pseudomonas, and Enterobacter are included in this group.

8 thermophilic bacteria
8- Thermophilic Bacteria

They are the bacteria that are able to grow at 50C and above. Species from genera Bacillus, Clostridium, Pediococcus, Streptococcus and Lactobacillus are included in this group.

9-Psychrotrophic Bacteria

They are the bacteria that are able to grow at refrigerated temperature. Some species from Pseudomonas, Alteromonas, listeria, Yersinia and Aeromonas are included in this group.

10- Thermoduric Bacteria

They are the bacteria that are able to survive Pasteurization temperature treatment. Some species from Micrococcus, enterococcus, Lactobacillus, Bacillus(spores), and Clostridium (spores) are included in this group.

11- Halotolerant Bacteria

Bacteria that are able to survive high salt concentration. Some species from Bacillus, Stahylococcus, Vibrio are included in this group.

12- Aciduric Bactria

They are the bacteria that can survive low pH. Some species from Lactobacillus, Lactococcus, Enterococcus, and Streptococcus are included in this group.

13- Osmophilic Bacteria

Bacteria that can grow at a relatively higher osmotic environment than that needed for other bacteria. Some species from genera Staphylococci, Leuconostic, and Lactobacillus are included in this group. They are much less osmotic than yeasts and molds.

14- Gas- Producing Bacteria

They are bacteria that produce gas(CO2, H2, H2S) during metabolism of nutrients. Species from genera Leuconostic, Lactobacillus, Propionibacterium, Escherichia, Enterobacter, Clostridium and Desulfotomaculum are included in this group.

15- Slime Producers

They are the bacteria that produce slime because they synthesise polysaccharides. Examples include Xanthomonas, Leuconostic, Alcaligenes, Enterobacterand lactobacillus.

16- Spore Formers

They are the bacteria that have the ability to produce spores. Species from Bacillus, Clostridium, and Desulfotomaculum are included in this group. They are further divided into:

Aerobic sporeformers

Anaerobic sporeformers

Flat sour sporeformers


Sulfide-producing sporeformers

17- Aerobes

They are the bacteria that require oxygen for growth and multiplication. Examples include Pseudomonas, Bacillus, and Flavobacterium

18- Anaerobes

They are the bacteria that cannot grow in the presence of oxygen. Species from Clostridium are included in this group.

19- Facultative Anaerobes

They are bacteria that are able to grow in both the presence and absence of oxygen. Lactobacillus, Pediococcus, Leuconostoc. Enteric pathogens, and some species of Bacillus, Serratia and Coliforms are included in this group.

20- Coliforms

Species from Escherichia, Enterobacter, Cirtbacter and Klebsiella are included in this group. They are used as index of sanitation.

21- Fecal Coliforms

Mainly Escherichia coli is included in this group. They are also used as an index of sanitation.

22- Enteric Pathogens

Pathogenic Salmonella, Shigella, Campylobacter, Yersinia, Escherichia, Vibrio, Listeria, Hepatitis A, Rotavirus and others that can cause gastrointestinal infection are included in this group

descriptive terms
Descriptive Terms

Indicator organism(s)

An indicator organism or group of organisms is one whose numbers in a product reflect the success or failure of (good manufacturing practices). Coliform group of microorganisms and Escherichia coli are commonly used as indicator organisms.

Index organism

An index organism is one whose presence implies the possible occurrence of a similar but pathogenic organism. E. coli is used as an index organism and its presence indicates possible presence of pathogenic enterobacteriaceae.g Salmonella sp.

Food Poisoning Organisms

1- Those which cause the disease by infection

2- Those which produce toxin in food

Those which cause infection must grow in food in large numbers and cause infection when consumed together with food. Common organisms in this category include Salmonella typhimurium, enteropathogenic E. coli, Vibrioparahaemolyticus and Yersiniaenterocolitica.

Toxin-Producing Organisms

These are microorganisms which cause intoxication, they must grow in food in large numbers and produce enough toxin and when consumed together with food cause intoxication.

Common microorganisms in this category include Clostridium botulinum, Staphylococcus aureusand toxigenic fungi e.g. Aspergillusflavus.

Infectious Microorganisms

These are the organisms whose presence in small numbers in food or water and when consumed can cause infection. In this case the food acts as a vector but not necessarily as a growth medium. Infectious organisms can be transmitted by various ways including man to man and are said to be contagious. Organisms in this group include; Vibriocholerae O1, Salmonella typhi, Shigellasonnei, Hepatitis A etc.

Spoilage Organisms

Spoilage organisms are the organisms whose growth in the food causes undesirable characteristics in that food. Any organism which is not intentionally added into food or intentionally allowed to grow in food so as to impart certain qualities in that food is considered a contaminant. Growth of the contaminant in that food will spoil the food making it unfit for human consumption. Some useful microorganisms e.g. lactic acid bacteria are considered as spoilage organisms when in alcohol but not in milk.