Composition
Ingredients | Gms/Ltr. |
Agar | 15.00 |
Lactose | 12.00 |
Proteose peptone | 12.00 |
Sucrose | 12.00 |
Bile salts | 9.00 |
Sodium chloride | 5.00 |
Sodium thiosulphate | 5.00 |
Yeast extract | 3.00 |
Salicin | 2.00 |
Ferric ammonium citrate | 1.50 |
Acid fuschin | 0.10 |
Bromo thymol blue | 0.065 |
* Dehydrated powder, hygroscopic in nature, store in a dry place, in tightly -sealed containers below 25°C and protect from direct Sunlight.
Instructions for use
Dissolve 76.6gms in 100ml distilled water. Gently heat to boiling with gentle swirling and dissolve the medium completely . DO NOT AUTOCLAVE OR OVERHEAT . Cool to 45- 50°C & use immediately.
Appearance: Light to dark green colour, trace to slightly hazy solution
pH: 7.5 ± 0.2
Principle
HEKTOEN ENTERIC AGAR is formulated by King and Metzger to isolate Shigella and Salmonella species. The medium contains Bile salt which is inhibitory to Gram -positive bacteria and many Gram – negative bacteria. The medium contains P roteose peptone as an essential amino acid and nitrogen supplement. Yeast extract is added as the source of vitamins and other growth factors. Sodium chloride helps maintaining the osmotic balance of the cells in the medium. Lactose present in the medium make s helps to differentiate lactose f ermenters from non – lactose fer menter s like Salmonella and late -lactose fermenters like Shigella. The medium cont ains other carbon sources like Sucrose and S alicin for optimal differentiation of enteric pathogens based on their ability to ferment different sugars. Acid fuschi n and Bromo thymo l blue acts as a pH indicator in the medium. E. coli is partially inhibited wherein they grow to a limited extent utilizing all the three sugars like lactose, Sucrose and S alicin; forming salmon or orange coloured colonies in the medium. Sodium thiosulphat e and Ferric ammonium citrate helps in differentiation of H 2S producing coliforms. The H 2S producing coliforms like Proteus and Salmonella grow in the medium forming clear colony with a black center. Agar is added in the medium as a solidifying agent.
Interpretation
Cultural characteristics observed after inoculating (103 – 105CFU/ ml ), on incubat ion at 35 ± 20C for 18 – 24 hours.
Microo rganisms | ATCC | Inoculum (CFU/ml) | Recovery rate (%) | Appearance of colon y |
Escherichia coli | 25922 | 103 – 105 | Not limited | Orange colour colony with bile ppt., partially inhibited |
Enterococcus faecalis | 29212 | 103 – 105 | = 0.01% | Partially inhibited |
Salmonella typhimurium | 14028 | 103 – 105 | = 20% | Black centered colony |
Shigella flexneri | 12022 | 103 – 105 | = 5% | Greenish blue colour colony |
References
- King and Metzger. Abstr. M 99, p. 77. Bacteriol. Proc. Am. Soc. Microbiol. (1967).
- King and Metzger. Appl. Microbiol . 16:577. (1968).
- King and Metzger. Appl. Microbiol. 16:579. (1968).
- MacFaddin. Media for isolation -cultivation -identification -maintenance of medical bacteria, vol. l. Williams & Wilkins, Baltimore, Md. (1985).
- Chapin and Murray . In Murray, Baron, Pfaller, Tenover and Yolken (ed.), Manual of clinical microbiology, 7th ed. American Society for Microbiology, Washington, D.C. (1999).
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