By Momina Shehzadi | BS Zoology Graduate, Pakistan In this article, we will learn Fungi - Morphology, Reproduction, Physiology and Importance - in simple words with diagram. This topic is very important for BS Zoology and Microbiology students. Fungi are found everywhere around us. They are present in soil, in water, in air, on dead plants, on dead animals, on our food, on our skin and even inside our body. When bread becomes old, green or black spots appear on it. These spots are fungi. When we see mushroom growing after rain, that is also a fungus. When yeast is used to make bread soft, that is also a fungus. Fungi are very different from bacteria and plants. They are eukaryotic organisms. It means their cells have a true nucleus and other membrane bound organelles. They do not have chlorophyll, so they cannot make their own food like plants. They get their food from other living or dead things. That is why they are called heterotrophic organisms. The study of fungi is called mycolog...
By Momina Shehzadi | BS Zoology Graduate, Pakistan
In this article, we will learn Bacteriological Media - Types and Preparation - in simple words with diagram. This topic is very important for BS Zoology and Microbiology students.
INTRODUCTION
Bacteriological media or culture media is artificial food material which is used to grow microorganisms in laboratory. Bacteria need nutrients for growth like carbon source, nitrogen source, energy, water and minerals. Culture media provides all these nutrients in laboratory.
The study of culture media is very important in microbiology because we cannot study bacteria without growing them. Different bacteria need different types of media. By growing bacteria on different media we can identify them.
The culture media are prepared in laboratory and sterilized before use to avoid contamination. After sterilization, bacteria are inoculated on media and incubated at suitable temperature for growth.
WHAT IS BACTERIOLOGICAL MEDIA
Bacteriological media is defined as any material or medium which contains nutrients and supports growth and multiplication of microorganisms in laboratory. It may be liquid, semi solid or solid.
Basic requirements of media are water, energy source like glucose, carbon source, nitrogen source, minerals like phosphorus, potassium and growth factors like vitamins.
Media also contain agar which is solidifying agent obtained from seaweeds. Agar melts at 98 degree Celsius and solidifies at 42 degree Celsius. It is not used as nutrient by bacteria so it is ideal solidifying agent. 1.5 to 2 percent agar is used to make solid media.
TYPES OF BACTERIOLOGICAL MEDIA
On basis of physical state, media are divided into three types.
1. Liquid Media or Broth: These media do not contain agar and remain as liquid. Example Nutrient broth, Peptone water. Bacteria grow throughout liquid and make it turbid.
2. Solid Media: These media contain 1.5 to 2 percent agar and are solid. Example Nutrient agar, Blood agar. Bacteria grow as colonies on surface.
3. Semi Solid Media: These contain 0.2 to 0.5 percent agar and are soft. Example Motility medium. They are used to check motility of bacteria.
On basis of composition and function, media are divided into following important types.
1. Simple or Basal or General Purpose Media
These are simple media which support growth of many non fastidious bacteria. They contain peptone, beef extract, sodium chloride and water. Examples are Nutrient broth and Nutrient agar. Nutrient agar is most commonly used general purpose medium in laboratory.
Composition of Nutrient Agar: Peptone 5 gram, Beef extract 3 gram, Sodium chloride 5 gram, Agar 15 gram, Distilled water 1000 ml, pH 7.2 to 7.4.
2. Enriched Media
Enriched media are basal media to which extra nutrients like blood, serum, egg yolk or chocolate are added to support growth of fastidious bacteria which need extra growth factors.
Examples: Blood agar which contains 5 to 10 percent sheep or human blood in nutrient agar. It is used to grow Streptococcus, Staphylococcus and to observe hemolysis. Chocolate agar which contains heated blood is used to grow Haemophilus influenzae and Neisseria gonorrhoeae.
3. Selective Media
Selective media are media which contain substances that inhibit growth of unwanted bacteria and allow growth of wanted bacteria only. So they select specific bacteria.
Examples: Mannitol Salt Agar MSA contains 7.5 percent sodium chloride which inhibits most bacteria except Staphylococcus aureus. MacConkey agar contains bile salts and crystal violet which inhibit Gram positive bacteria and allow Gram negative bacteria only. Lowenstein Jensen medium is selective for Mycobacterium tuberculosis.
4. Differential or Indicator Media
Differential media contain indicators which differentiate between different bacteria on basis of their biochemical reactions. They show visible changes like color change.
Examples: Blood agar is differential because it differentiates bacteria on basis of hemolysis. MacConkey agar is differential because it contains lactose and neutral red indicator. Lactose fermenting bacteria like E. coli produce pink colonies while non lactose fermenters like Salmonella produce colorless colonies. Eosin Methylene Blue agar EMB is differential for E. coli which produces metallic sheen.
5. Transport Media
Transport media are used to transport clinical specimens from collection site to laboratory without letting bacteria die or overgrow. They preserve viability of bacteria but do not allow multiplication.
Examples: Stuart transport medium for Neisseria gonorrhoeae, Cary Blair medium for Vibrio cholerae and fecal specimens.
6. Anaerobic Media
These media are used to grow anaerobic bacteria which cannot grow in presence of oxygen. They contain reducing agents like thioglycolate which remove oxygen.
Examples: Robertson Cooked Meat medium RCM, Thioglycollate broth.
7. Assay Media
These media are used to test antibiotic sensitivity. Example Mueller Hinton Agar is used for antibiotic sensitivity testing by disc diffusion method.
8. Synthetic or Chemically Defined Media
These media are prepared from pure chemicals of known composition. All constituents and their amounts are known. They are used for research purposes.
PREPARATION OF BACTERIOLOGICAL MEDIA
Preparation involves following steps.
1. Weighing Ingredients: All ingredients are weighed accurately.
2. Dissolving: Ingredients are dissolved in distilled water by heating and stirring.
3. pH Adjustment: pH is checked and adjusted to 7.2 to 7.4.
4. Dispensing: Media is dispensed into test tubes or flasks.
5. Sterilization: Media is sterilized by autoclaving at 121 degree Celsius at 15 psi pressure for 15 to 20 minutes.
6. Pouring Plates: After sterilization, media is cooled to 45 to 50 degree Celsius and poured into sterile petri plates under aseptic conditions. About 15 to 20 ml media is poured in each plate.
7. Incubation Check: Few plates are incubated overnight to check sterility.
USES OF CULTURE MEDIA
1. To isolate pure cultures of bacteria from mixed populations.
2. To identify bacteria on basis of colony characteristics.
3. To maintain stock cultures of bacteria.
4. To test antibiotic sensitivity.
5. To produce vaccines, enzymes and other industrial products.
CONCLUSION
Bacteriological media are essential tools of microbiology laboratory. Without media we cannot cultivate, isolate or identify bacteria. Different types of media like general purpose, enriched, selective and differential have different purposes.
FAQs for Students - Bacteriological Media
Q1: What is difference between selective and differential media?
Ans: Selective media inhibit growth of unwanted bacteria and allow growth of wanted bacteria only. Differential media differentiate between different bacteria on basis of biochemical reactions and show color change. Some media like MacConkey agar are both selective and differential.
Q2: Why agar is used in culture media?
Ans: Agar is used as solidifying agent because it melts at 98 degree and solidifies at 42 degree, it is not used as nutrient by most bacteria, it is transparent and it gives firm surface for colony formation.
Q3: What is enriched media and give examples?
Ans: Enriched media are basal media to which extra nutrients like blood, serum are added to support growth of fastidious bacteria. Examples are Blood agar and Chocolate agar.
Q4: What is transport media?
Ans: Transport media are media used to transport clinical specimens from collection site to laboratory without loss of viability. Examples are Stuart medium and Cary Blair medium.
Q5: How culture media are sterilized?
Ans: Most culture media are sterilized by autoclaving at 121 degree Celsius at 15 psi pressure for 15 to 20 minutes.
Q6: What is Nutrient Agar composition?
Ans: Nutrient agar contains Peptone 5g, Beef extract 3g, Sodium chloride 5g, Agar 15g, Distilled water 1000ml, pH 7.2 to 7.4.
Also Read: Flagella Staining - Principle, Procedure and Types
Written by Momina Shehzadi, BS Zoology Graduate from Pakistan.
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