Liquid and Solid Media

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 agarandMartin-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

A) Blood agar plates (BAP)

> Contains mammalian blood, typically at a concentration of 5–10%

> Used to isolate fastidious organisms and detect hemolytic activity (Neisseria and Streptococcus).