In the history…

• Robert Koch- described his culture techniques in 1881.

• Fanny/Frau Hesse- suggested the use of agar.

• Richard Julius Petri- invented the glass Petri dishes.

• Joseph Lister- the first person to obtain a pre culture of bacterium (Streptococcus lactis) in a liquid medium.

Classification of Culture Media Based on Whether the Exact Contents are Known

• Chemically defined media- exact chemical composition is known

• Complex media- exact contents are not known, from extracts and digests of yeasts, meat, or plants

Liquid and Solid Media

• Liquid media- or broths are contained in tubes, referred to as tubed media.

• Solid media- prepared by adding agar to liquid media and then poured into test tubes or Petri dishes, where the media solidifies.

1. Agar plate - one grown on a medium, usually agar or gelatin, on a Petri dish

2. Agar slant - one made on a slanting surface of a solidified medium in a tube, the tube being tilted to provide a greater surface area for growth.

3. Agar butt/deep - one in which a tube of solid medium is inoculated by a needle thrust deep into the contents.

Bacterial Media

• Selective

• Differential

• Enriched

Selective Medium

• Has added inhibitors that discourage the growth of certain organisms without inhibiting growth of the organism being sought.

• Solid medium is employed with selective medium so that individual colonies may be isolated.

Examples:

• MacConkey agar- screen for S. aureus and is selective for Gram (-) bacteria.

• Phenylethyl alcohol agar (PEA) and colistin-nalidixic acid agar (CNA)- inhibit growth of Gram (-) bacteria.

• Thayer-Martin agar and Martin-Lewis agar- selective for N. gonorrhoeae.

• Mannitol salt agar (MSA)- only for salt-tolerant (haloduric) bacteria

• Eosin methylene blue agar (EMB) – selective against gram-positives

MacConkey agar E. coli on EMB

Differential Medium

• Permits the differentiation of organisms that grow on the medium.

• Reveals the presence of 2 or more similar microorganisms by differences in the appearance of their colonies.

Examples:

• MacConkey agar- used to differentiate various Gram (-) bacilli that are isolated from fecal spcimens.

  • Gram (-) bacteria are able to ferment lactose produces pink colonies, those are unable to ferment lactose produce colorless colonies.

  • Differentiates between LF and NLF Gram (-) bacteria.

• Mannitol salt agar- used to screen for S. aureus, pink to yellow.

• Centrimide agar - used for the differentiation of strains of Pseudomonas spp.

P. aeruginosa on centrimide agar

Two different species of Staphylococcus growing on mannitol salt agar (MSA).

Enriched Medium

• Broth or solid medium containing rich supply of special nutrients that promotes the growth of fastidious organisms.

• Prepared by adding extra nutrients to a medium called nutrient agar.

Liquid and Solid Media

• Liquid media- or broths are contained in tubes, referred to as tubed media.

• Solid media- prepared by adding agar to liquid media and then poured into test tubes or Petri dishes, where the media solidifies.

4. Agar plate - one grown on a medium, usually agar or gelatin, on a Petri dish

5. Agar slant - one made on a slanting surface of a solidified medium in a tube, the tube being tilted to provide a greater surface area for growth.

6. Agar butt/deep - one in which a tube of solid medium is inoculated by a needle thrust deep into the contents.

Bacterial Media

• Selective

• Differential

• Enriched

Selective Medium

• Has added inhibitors that discourage the growth of certain organisms without inhibiting growth of the organism being sought.

• Solid medium is employed with selective medium so that individual colonies may be isolated.

Examples:

• MacConkey agar- screen for S. aureus and is selective for Gram (-) bacteria.

• Phenylethyl alcohol agar (PEA) and colistin-nalidixic acid agar (CNA)- inhibit growth of Gram (-) bacteria.

• Thayer-Martin agar and Martin-Lewis agar- selective for N. gonorrhoeae.

• Mannitol salt agar (MSA)- only for salt-tolerant (haloduric) bacteria

• Eosin methylene blue agar (EMB) – selective against gram-positives

MacConkey agar E. coli on EMB

Differential Medium

• Permits the differentiation of organisms that grow on the medium.

• Reveals the presence of 2 or more similar microorganisms by differences in the appearance of their colonies.

Examples:

• MacConkey agar- used to differentiate various Gram (-) bacilli that are isolated from fecal spcimens.

  • Gram (-) bacteria are able to ferment lactose produces pink colonies, those are unable to ferment lactose produce colorless colonies.

  • Differentiates between LF and NLF Gram (-) bacteria.

• Mannitol salt agar- used to screen for S. aureus, pink to yellow.

• Centrimide agar - used for the differentiation of strains of Pseudomonas spp.

P. aeruginosa on centrimide agar

Two different species of Staphylococcus growing on mannitol salt agar (MSA).

Enriched Medium

• Broth or solid medium containing rich supply of special nutrients that promotes the growth of fastidious organisms.

• Prepared by adding extra nutrients to a medium called nutrient agar.

Blood Agar Types