Isolation And Culturing Of Microbes From Food


  • Microorganisms are abundant and found in food stuff as well.
  • The microbes found on food can be pathogenic in nature and may cause various diseases.
  • The identification of such microbes is necessary to study and research on infectious agent.
  • Primary culture from food stuff is a mix culture of various microbes which has to be isolate form each other and identify for research.
  • The procedure below is used to isolate and cultivate pure culture form food stuffs.


  • Nutrient agar plate
  • Food sample
  • Spreader
  • Micropipette
  • Test tube


  • Take nutrient agar plate and well label them.
  • In one test tube take 4.5 ml autoclaved water and add small quantity of food sample in it
  • Mix thoroughly, after that with the help of micropipette take 100 microliter of the mixture and spread onto the petri plate with a spreader.
  • Finally put the petri plates in the incubator for 18-24 hours at 37o C.
  • After the incubation period mark the different colonies with marker.
  • Now pick each single colony with an inoculating loop and streak on nutrient agar plate.
  • Put the petri plates inside the incubator for 18- 24 hours at 37o C.


  • After 18-24 hours examine the plates for bacterial growth.


  • Record the result of isolated colonies in tabular form.



Food Spoilage:

It refers to change in Physical and Chemical property of food, making food unfit for Consumption. Invasion of microorganisms like bacteria and fungi usually causes spoilage of food.


Food spoilage generally occurs due to Physical, Chemical or Biological agents that changes colour, flavour, appearance, odour and other properties of food. Shelf life of most of the natural foods is very less and is perishable, for example, meat, fish and bread can spoil easily. Decomposition of food generally involves 3 processes: Putrefaction (chemical breakdown of food or decay of organic matter), Fermentation (chemical breakdown of substances by action of microorganisms, yeast), Rancidity ( refers to oxidation of fats).

Natural Contamination:

It refers to contamination of food when microorganisms themselves attaches to food in its growing stages and this kind of contact is essential for certain kinds of food. For example, Yeasts contaminates fruit for carbohydrates fermentation.

Artificial Contamination:

This type of contamination occurs during handling of food when food is under various stages of production like, packaging, storage, etc. Improper handling of food during this stages results in contamination of food by microorganisms.

Intrinsic factors of food like pH, redox potential, H2o activity determines the type of microflora growing on the food. This final composition of microflora is responsible for food spoilage.

Types of Food Spoilage:

1.Microbial Spoilage:

Microorganisms associated with food are:

Bacteria, Filamentous Fungi, Viruses, Yeasts, and animal parasites.


They are associated with both plant and animal foods. Bacteria are associated with food intoxication and spreading of food borne diseases.


Acenatobacter Gram negative- present in raw and prepared foods like beef and poultry carcasses.

Aero monas: gram negative, responsible for spoilage of fish.

Alkaligans: gram negative, responsible for spoilage of egg and dairy products.

Citrobacter: gram negative, it is responsible for spoilage of vegetables and fresh meat.

Corynebacterium: gram positive, involved in spoilage of vegetables and  meat.

Filamentous Fungi:

When food is left for one or more day covered, tangled mass of furry growth appears on food which is called fungi or mould. Fungi are responsible for spoilage of Grains, nuts, and fruits as they have low pH and H20 activity.


Mucor: Zygomycotena-common contaminant of fruits, berries and nuts.

Rhizopus: Zygomycotena- known commonly as bread mould. It is more prevalent in fermented and stored foods.

Claviceps: Ascomycotena-  produces toxic alkaloids in cereals, when consumed can cause Hallucinations.


Contamination by yeasts results in Souring of milk.


Candida: most common contaminant of dairy products, fresh fruits, and alcoholic beverages.

Saccharomyces: spoilage of fruits and fruit products.

Torulopsis: responsible for spoilage of beef, creamed butter, condensed milk, etc.


viruses found in food are termed as enteric or intrinsic viruses.


Enterovirus, Adenovirus, Reovirus, Hepatitis A virus.

Animal Parasites:

They belong to 3 distinct groups:

Protozoa: Giardia, Entamoeba Hystolytica

Flatworms: Taenia, Fasciola

Roundworms: Ascaris

2. Physical Spoilage:

Physical Spoilage refers to damaging of food during Harvesting, Processing or distribution of food. During such processes there are high chances of food spoilage if proper measures are not followed. The damage increases the chance of spoilage as the outer layer is completely broken or bruised. For example- Canned foods gets spoiler easily if the cans are not properly packed with lid or are contaminated during processing.

3. Chemical Spoilage:

Chemical reactions in food are responsible for change of colour, texture and taste of the food products. Generally foods are fresh especially vegetables and animal food, but after harvesting and slaughtering, chemical changes begin automatically in the food and the quality of food becomes deteriorated.

4. Enzymic Spoilage:

Enzymes acts as biological catalyst to carry out biological reactions in cell and play an important role in biochemical reactions. After death of cells or tissues, enzymes play a role in its decomposition by a process called Autolysis( self destruction )

Example: In tomatoes, some enzymes helps it for ripening, but at the same time there are certain enzymes which are responsible for its decay. Once enzymic Spoilage is underway, it damages the outer skin of tomato and exposes it to mould growth and decay.

Factors Affecting Food Spoilage:

  1. Water Content: Amount of water holding capacity in foods is referred to as it’s water activity.(WA). Water activity of most of the fresh fruits is approximately 0.99, which makes them more susceptible to microbial growth.
  2. Environmental Conditions: Environmental influence on food is the major concern. When food is exposed to intrinsic conditions like temperature, air, or even small amount of moisture, can result in growth of Micro-organisms. Changing environmental conditions can help to prevent spoilage. For example- storing food at lower temperature can prevent it from spoiling.
  • Packaging and storage: Packaging of foods is after processing is very vital as it protects food from harmful contaminants and also from various other factors like environment, temperature, etc. The type of packaging plays a key factor in ensuring the safety and preventing spoilage. Food packed in jars, cans ensures safety and prevents food from dust, moisture, air and harmful microbes.

Sources of Micro-organisms for Food Spoilage:

Micro-organisms are present everywhere. General source of Micro-organisms include air, water, sewage, soil and animal wastes. Foods grown in ground have higher risk of spoilage due to micro-organisms.  Foods like fish, meat are contaminated by presence of bacteria in their  internal organs like skin and feet. Meat has higher tendency of contamination as raw meat attracts lot of microbes, so it is advisable to store raw meet immediately after chopping.

Ways to Prevent Food Spoilage:

  • Ensure proper packaging is available to the food cans and jars after processing.
  •  Don’t leave the food in open air for more than 15min, to avoid contact with microbes.
  •  Ensure that your refrigerators are operating at correct temperatures.
  •  Food must be protected from light and must be stored in amber colour or transparent containers.
  • Low temperature is a key as it retards microbial growth.
  • Avoid placing food where there is more humidity, as high humidity attracts more growths of microbes and moulds. Placing food in dry places is most appropriate.


Scope and appliance of microbiology


  • Microbiology is a discipline of biology which deals with the study of microscopic organism, their interaction with other organisms and with environment.
  • It includes microscopic level organisms like bacteria, algae, fungi, protozoa and the infectious agent viruses too.
  • Microorganisms present all over the globe form high altitude to the deep seas include hot springs, extremely cold climates, and high pressure even in the salty lakes.
  • Microorganisms are both beneficial and harmful to human. I.e. required in the industrial production of food stuff (bread, yogurt, beer, wine, etc), antibiotics(penicillin, chloromycetin, streptomycin) vaccine, enzymes, vitamins and many more products along with it is harmful in the way by causing fatal disease like small pox, plague, malaria, cholera, HIV, influenza and more.
  • They plays important role in maintaining the stability of ecosystem by recycling the organic and inorganic substance in carbon, nitrogen, sulphur and phosphorus cycle.
  • There were many events in history that tells us how infectious microbes put the human population in danger. Like black death(1346),  yellow fever(1793), Spanish flu(1918), SARS(2002), H1N1 flu(2009), MERS(2014), ebola(2014), etc.
  • In addition to the disease outbreaks microorganism plays major role in food spoilage, detoriation on materials like paper, wood, metal and plastics.
  • In agriculture nowadays genetically improved crops are used to get more yields and disease resistant crop which can be obtained by the involvement of microorganism.
  • As microbes are present everywhere it enhances the scope and contribute in many fields like pharma, agriculture, dairy, food industries, research, nanotechnology, water industry, chemical industry.
  • Microbiologists are the person who studies these microorganisms and their morphology, behaviour, metabolic activity, habitat, reproduction, nutritional requirement, their application and pathogenicity, improvemnet and modifications which leads to the high demand of microbiologist globally.


Dairy and food industry:

  • Deals with the microbial production for the food stuff, prevention of spoilage of food and transmission of food borne disease.  

Agricultural microbiology:

  • It include the study of microbial strains which are used to obtain genetically modified crops which are resistant to many diseases and higher yields. Production of bio-fertilizers and maintenance of the rhizo-flora.

Medical microbiology:

  • Study of disease their causative agents, prevention, diagnosis and treatment. In addition in includes various clinical appliance of microbes oh human health.  

Environmental microbiology:

  • The study of microbes and their interaction with environment, role in geochemical cycles, microbial diversity, bio-remediation

Genetic engineering:

  • It deals with the study of modifying microorganism at gene level and engineered microbes are used to produce hormones, enzymes, vaccines, vitamins, antibiotic and other products.

Microbial physiology:

  • Includes the study of microbial morphological structure, metabolism and growth.

Industrial microbiology:

  • Deals with the production of antibiotic, fermented food, aminoacids, vitamins, steroids, enzymes, alcohol. In addition with the strain improvement and process of enhancing product quantity.

Soil microbiology:

  • The study of soil flora and role of microorganism in soil fertility

Water microbiology:

  • Major part of this field is the waste water management as is it a challenging condition for the world with industrial waste discharged in the water bodies and leading to the pollution causing threat for aquatic life.


The diversity of microbes on the globe makes microbiology the most complex and largest discipline.


  • There are a majority of microbes used in the food and dairy industries for the production of food from wine, beer through the cheese, yogurt to manufacturing of bread.
  • Include Process of fermentation, pasteurization, industrial production, processing of food its packaging, food preservation and storage.
  • Microbial spoilage of food production and their prevention.

Environmental microbiology:

  • working of biogeocycle(carbon, nitrogen, sulphur and phosphorus) done by microorganism
  • microorganism are present in free living state and in association with plants in symbiotic relationship.
  • Maintaining the soil fertility without exhausting soil nutrients.
  • Responsible for cleaning toxic substance from the environment. 
  • Some are pathogenic to the plant but there are few strain which act as biological control agents and protect plant against this diseases.

Medical microbiology:

  • Disease causing microbes i.e. bacteria, algae, fungi, protozoa and virus responsible for causing numerous types of diseases ranging from  acute to severe life threatening.
  • Examples are cholera, influenza, malaria, HIV, tuberculosis, plague, etc
  • Their diagnosis, transmission, prevention and cure are the major part of the medical microbiology
  • In contrast to the pathogenicity there are some strains inhibit the growth of other diseases causing microbes by producing antibiotic, hence, used for the production of antibiotics.


  • Genetically engineered strain used for the production of therapeutic substance like human growth hormone, insulin, etc
  • Also contribute in the commercial production of  acetone, alcohol, drugs etc


  • Diversity and unicellular structure of microbes make them easy to study and research over multicellular structure.
  • In addition they can produce millions of copies from a single cell rapidly with very low cost which is good for experiments performed.
  • Short generation time leads to quick result analysis.

Future aspects of microbiology:

  • Due to population explosion in the world there could be scarcity of food in near future in that condition single cell protein can be an alternative.
  • Newly and highly resistant species of diseases causing microbes is a challenge for the present and in the future so r DNA technology is useful to overcome this problem.
  • Treatment of cancer and HIV like diseases
  • Food preservation methods for highly perishable food items