Different media used in

Food microbiology

Arun Kr. Gupta

MSc. Food Science & Technology

Ist yr.

15396012

Introduction

The study of microorganisms requires techniques for isolating cells

from natural sources and growing them in the laboratory on

synthetic media. Thus, development of synthetic culture media and

culture techniques have played important roles in the advancement

of this field. Microbiologists use bacterial culture media for many

purposes and applications.

Media :The nutrient preparation on or which a culture is grown in the

laboratory is called culture medium. Media are used to isolate and

identify bacteria, reveal their metabolic properties, and allow long-

term storage of pure cultures.

Key terms

Culture medium: Nutrients prepared for microbial growth

Sterile: No living microbes

Inoculum: Introduction of microbes into medium

Culture: Microbes growing in/on culture medium

Requirements for Growth

Physical requirements

Temperature

pH

Osmotic pressure

Chemical requirements

Carbon

Nitrogen, sulfur, and phosphorous

Trace elements

Oxygen

Organic growth factor

Bacteria nutritional requirements Generalists (which are able to use a wide range of nutrients) and/or nutrients

that are commonly available.

Auxotroph's because, even given a carbohydrate carbon source, they cannot

synthesis one or more organic molecules required for their growth – these

molecules must be also provided in growth media.

Fastidious, meaning that they have very specific and/or complex nutritional

requirements. These species do not grow because they cannot use one or more

nutrients in the form provided in the medium (e.g., they might require H2S

rather than SO4 as a sulphur source), have requirements for very specific types

of nutrients

Purpose of culture media:

Culture media serve a number of purposes in practical microbiology. They can be

used to:

Grow a wide range of organisms in a particular group, e.g. bacteria

Maintain organisms in culture collections;

Distinguish b/w different types of microorganisms;

Select specific groups or types of microorganisms from an environment, e.g. a

food material;

Help identify microorganisms;

Assay nutrient or antimicrobial compounds.

Classification of media

• Liquid ( broth)

• Solid (Agar)

Ingredients.

•Simple (2-5 ingredients)

•Complex (>5 ingredients)

Origin

•Natural

•Synthetic

Functionality

•Selective

•Differential

State

The types of culture media recognized are:

1. General purpose (non-selective)

2. Selective

3. Enrichment

4. Differential

5. Selective and differential

6. Chemically defined

7. Elective

8. Living

Note: There is not always a clear distinction between one type and another, e.g. a

medium can be both selective and chemically defined.

Cont.

General purpose: general purpose media have a nutritional content that will allow

the growth of a wide range of either bacteria or yeasts and mold. Media of this type

is often complex, prepared from natural products such as meat, yeast and vegetable

extracts and hydrolysis products of meat.

Ex: For the bacterial growth are nutrient agar/broth, plate count agar & for molds/

yeasts malt extract agar/ broth, potato dextrose agar.

Selective media: selective media are those which permit the growth of some

specific group or type of organism while preventing or retard or inhibiting the

growth of others, thus facilitating bacterial isolation.

Selection can be operate in a number of ways:

1. Selection on the basis of pH e.g. citric acid in citric acid agar for the isolation of

fungi.

2. Selection on the basis of water activity- high salt or sugar concentration for

halophilic or osmophilic organisms.

Ex: EMB (Eosin methylene Blue): E.coli and related species produce colonies with a

brilliant green metallic sheen. On the same medium Aerobacter aerogenes and related

species produces pink colonies with a dark centres.

Enrichment media:

Enrichment media are broths contain selective ingredients and are designed to shift the growth of a

mixed population of bacteria in the direction of a specific organism or group of organisms so that

these organisms become dominant, increase in number and are therefore easier to isolate.

Ex: Selenite broth for the isolation of Salmonella from mixed population.

Differential media:

Differential media cause certain colonies to develop differentially from other organism present by

producing a characteristics change in the bacterial growth and or the medium surrounding the

colonies. These media are used for distinguishing among morphologically and biochemically

related groups of organism.

Ex: Blood agar is a differential media used to differentiate haemolytic properties of Streptococci

by their action on the medium.

Selective/ differential media: selective/ differential media used extensively for the isolation of

specific organism. It is designed to isolate a group of closely related organisms and

differentiate b/w them. Ideally, media used for the analysis of foods for specific organisms

should be totally specific for the organisms.

Ex: Salmonella Shigella Agar: it is used for the isolation of salmonella & some shigella species

from the pathological specimens, suspected foods etc.

Chemically defined media: chemically defined media are those in which the

detailed composition of the medium is known in terms of the chemical in nature of

each individual component and the quantity present. Chemically defined media can

be general purpose or selective and/or differential.

Ex: Minerals modified glutamate medium, which is used for the enumeration of

coliforms in water is both selective and differential.

Elective medium: Elective media are designed to promote the growth of specific

organisms that have special nutritional requirements by adding a particular

ingredient to the medium so that their growth is improved.

Ex: tomato juice agar is used for culturing lactobacilli, it contain some ingredients but

they are not selective agents and the medium will grow a wide range of other

microorganism, including bacteria, yeasts, molds.

Living media: Some microorganism, e.g. viruses, will only grow in the living cells

of their host. In order to culture them in the laboratory a living culture of host cells

needs to be provided.

e.g. chick embryos or tissue cultures. Bacteriophages viruses are grown on bacterial

cultures.

Basic method for the production

of culture media

Weigh out the ingredients

Dissolve in water (this may require boiling if agar is present)

Adjust pH

Dispense in suitable containers (conical flask, tubes)

Sterilize by autoclaving (121 C for 15-20 min. for neutral media, 115 C

for 10 min for acid media) or boil for some selective media

Thank you for your kind attention.