Culture media used to grow microorganisms
The lower one is a conical flask filled with genetically modified bacteria grown in Lb media. Behind it is a laboratory centrifuge used to separate the bacteria from the media.

Microorganisms is everywhere in nature, but sometimes there is need to cultivate them in the laboratory, an example is the need to study their morphology or physiological and biochemical characteristics.

Other reasons that may lead to growing of microbes in the laboratory, includes need to extract component or convert whole cells to useful products such as antigen or vaccines or the need to propagate them for use in industries for the synthesis of essential molecule or to serve as bio catalyst in manufacturing processes, the need to study bacteria sensitivity and resistance to antibiotics and chemotherapeutic agents.

The need to grow them for foods such as single cell proteins or robotic remedies for human disorders, eg gastrointestinal disorder that requires decolonization of the gut with microorganisms like lactobacillus species, in which the organism is kept in a culture media.


Microbial cultivation is the process of transferring microorganisms into a medium with the subsequent provision of environmental conditions that supports its growth and survival in that particular medium.

Types of Culture Media

There are two types of culture media; synthetic and natural.

Natural Media

Natural media include plants and plant products and bacteria.

Synthetic Media

Synthetic media are chemically defined, made from a variety of substances with the aim of providing essential elements the organism would normally need if grown in its natural environment. The elements include oxygen, hydrogen carbon and nitrogen with phosphorus and sulphur needed in trace amounts (HCON, S.P). Others are sodium potassium magnesium, iron and manganese in very small amount. Water is essential for growth; it serves as a source of hydrogen and oxygen.

In synthetic media, the nutrients are specified and are added separately to give a chemically define product.

Simple Synthetic Media

Simple media contains a suitable carbon and energy source eg (glucose or lactate), an inorganic nitrogen source eg (ammonium salt) and various inorganic salts in a buffered aqueous solution, such a medium supports the growth of many nonparasitic organisms but will not support the growth of parasites which may require complex synthetic media containing amino acids, purines pyrimidines  Purine and pyrimidine and growth factors.

Culture media can be in liquid or solid form, Natural solid media include cassava, yam, potato tubers or other processed foods such as biscuit, bread, and secretion for example palm wine, animal body fluids such as sperm and urine.

It is possible to convert culture media from liquid to solid; this can be done by the addition of gelation factor such as starch, agar and gelatin, usually composed of hydrolysate of cheap proteins called peptones, which contains all essential amino acids, a source of growth factor and inorganic salt eg extracts of meat or of yeast and sodium chloride which regulates the osmotic pressure of the medium.

Media in this group are composed to,

  • Support the growth of a wide range of organisms
  • Save cost, this means that it is usually cheap

Special Media

Special media are those developed for specific purposes, examples are selective media which contains substances that inhibits the growth of certain microorganisms while supporting the growth of a few type, for example

  • Enrichment media: this favor the multiplication of particular specie of organism either by containing additional materials that specifically favor a particular organism or inhibiting substances that suppress competitors.

The addition of serum, blood, or extract to tryptic soya agar will support the growth of many fastidious bacteria. Enchrichment media is mainly used to isolate bacteria from cerebrospinal fluid, sputum, wound abscesses and pleural fluid.

Blood agar is a soya agar that contains 5% sheep blood. It is a general purpose medium. The additions of citrated blood to soya agar cause the formation of hemolysis patterns which makes it possible to differentiate some species of bacteria.

The followings are examples of the hemolysis patterns observed.

α-hemolysis: Greenish to brownish halo around the colony e,g, Streptococus gordonii, Streptococcus pneumoniae.

β-hemolysis: complete lysis of blood cells resulting in a clearing effect around growth of colonies e.g, Staphylococcus aureus and  Staphylococcus pyogenes

γ-hemolysis:Change in medium, eg Staphylococcus saprophyticus

  • Differential indicator media which contains some substance that changes visibility as a result of the metabolic activity of the said organism.

The introduction of certain chemicals into a medium may result in diagnostically useful growth visible change in the medium after incubation, example of differential media are

  • Eosin Methylene Blue agar (EMB): this differentiates between lactose fermenters and nonlactose fermenters , EBM, contains lactose salt and two dyes, esosin and methylene blue. coli, which is a lactose fermenter will produce a dark colony or one that has a metallic sheen while S.typhi will produce a colorless colony.

 Marconkey agar: Marconkey agar is used for the selection and recovery of entero bacteriaceae and related Gram negative rods The bile salt and crystal violet in this medium inhibits the growth of gram-positive bacteria and some fastidious gram-negative bacteria. Since lactose is sole carbohydrate, lactose fermenters produce colonies that are various shades or red whereas nonlactose fermenter prod.uce colorless colonies.

Selective Media:A selective media, is one which contains a substance that will allow the growth of some species of bacteria while inhibiting others for example,

Salmonella Shigella Agar(SS): this media is used to isolate Salmonella and Shegella species from coliform bacteria, the culture media contains bile salt which inhibits the growth of coliforms.

Mannitol Salt Agar(MS):This is used for isolation of pathogenic bacteria from nonpathogenic species, for example, pathogenic Staphylococci is differentiated from nonpathogenic specie using this media, the selectivity is brought about  by the high 7.5%  salt concentration.

Pathogenic staphylococci ferment mannitol to form acid while nonpathogen does not form any reaction with the salt.

Bismuth Sulfite Agar(BS):Bismuth sulfite agr is used for the isolation of Salmonela typhi, especially from stool and food specimens, S.typhi reduces the sulfite to suldide, resulting in black colonies with a metallic sheen.

Characteristic Media: Characteristics media are used to test bacteria for particular metabolic activity, products or requirement.

Types of Characteristics Media

Urea Broth:  This is used to detect the enzyme urease, some enteric bacteria are able to break down the enzyme urea, in the presence of urease to form Ammonia (NH3) and carbon dioxide (CO2).

Triple Sugar Iron (TSI) agar: TSL is used for identification organism, based on their ability to attack glucose, lactose or sucrose and also produce sulfides from ammonium sulfate and sodium thiosulfate. TSI contains lactose, sucrose, glucose, ferrous, ammonium sulfate and sodium thiosulfate.

Citric Agar: This contains sodium citrate which serves as the sole source of carbon and ammonium phosphate, the sole source of nitrogen citrate is used to differentiate enteric bacteria based on citrate utilization.

Lysine Iron Agar: Lysine Iron agar is used to differentiate bacteria that can either deaminate or decarboxylate the amino acid lysine.

LIA, contain lysine which permits enzyme detection and ferric ammonium citrate for the detection of H2S production.


Lactobacillus species:  A genius of microaerophilic or anaerobic nonsporing, nonmotile bacteria containing Gram positive rods ranging from the long and slender cells to short coccobacilli chains. They are pathogenic and often found in dairy products.

Culture media: A laboratory preparation that supports the growth and survival of microorganism.


Susceptibility and Resistance:CLICK HERE

Fastidious: Bacteria with complex nutritional requirement.


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