The sterility test is based on the principle that when microorganisms are supplied with nutrient medium and incubated at favorable temperature conditions, the microorganisms will grow and multiply. The presence of microorganisms can be identified by the turbidity in the clear medium.

Preparation of Culture Media

The culture media used for the sterility test must be capable of promoting the growth of a wide range of microorganisms such as aerobic and anaerobic bacteria and fungi. Two types of media are available:

Fluid Thioglycolate Medium: This medium is used primarily to support the growth of anaerobic bacteria. It also supports the growth of aerobic bacteria to some extent. Table 8.6 lists the ingredients required and their quantities for this medium.

 

Table 8.6 Composition of Fluid Thioglycolate Medium

Ingredients	Quantity (for 1000 ml)
L-cystine	0.5 g
Sodium chloride (NaCl)	2.5 g
Dextrose	5.5 g
Agar	0.75 g
Yeast extract	5.0 g
Pancreatic digest of casein	15.0 g
Sodium thioglycolate	0.5 g
Resazurin (0.1% fresh solution)	1.0 ml
Distilled water (q.s.)	1000 ml

Soybean–Casein Digest Medium: It is used to support the growth of aerobic bacteria and fungi. Table 8.7 lists the ingredients required and their quantities for this medium.

 

Table 8.7 Composition of Soybean–Casein Digest Medium

Ingredients	Quantity (for 1000 ml)
Pancreatic digest of casein	17.0 g
Peptic digest of soybean meal	3.0 g
Sodium chloride	5.0 g
Dibasic potassium phosphate (K2HPO4)	2.5 g
Dextrose	2.5 g
Distilled water (q.s.)	1000 ml