ANTIBIOTIC MEDIUM NO. 1

SEED AGAR

Code: CM0327

A medium recommended for the seed layer in the preparation of plates for the microbiological assay of antibiotics.

Typical Formula*	gm/litre
Peptone	6.0
Tryptone	4.0
Yeast extract	3.0
`Lab-Lemco’ powder	1.5
Glucose	1.0
Agar No.1	11.5
pH 6.5 ± 0.2 @ 25°C

* Adjusted as required to meet performance standards 

Directions
Suspend 27g in 1 litre of distilled water and bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes.

Description
This is perhaps the most important medium in antibiotic assay work and it is in a specially modified form to take advantage of the properties of Oxoid Agar No.1 (LP0011).

Hanus, Sands and Bennett¹ drew attention to the inhibitory properties which certain agars have towards some antibiotics, particularly streptomycin, kanamycin, polymyxin B and neomycin. Because Oxoid Agar No.1 (LP0011) does not share these inhibitory properties it is especially suited to antibiotic assay work. In addition, this agar, with its superior diffusion properties, produces more clearly defined inhibition zones.

NB   In the Oxoid formulation, only 11.5g of Agar No. 1 (LP0011) are used to give the equivalent gel strength of 15g ordinary agar.

This medium is used as a seed agar with Micrococcus flavus for the plate assay of bacitracin; with Sarcina lutea for the plate assay of chloramphenicol and with Staphylococcus aureus for the assay of kanamycin sulphate, penicillin G, sodium methicillin and sodium oxacillin.
It is also employed as a base agar in the assay of the following drugs: chloramphenicol, kanamycin sulphate, colistin sulphate, sodium methicillin, sodium oxacillin and vancomycin hydrochloride.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared plates of medium at 2-8°C.

Appearance
Dehydrated medium: Straw coloured, free-flowing powder
Prepared medium: Straw coloured gel