inhibit the growth of Gram-negative bacteria. Media supplemented with penicillin (5-50 units/ml) or crystal violet (2 mg/l) will inhibit the growth of Gram-positive bacteria. Tellurite agar, therefore, is used to select for Gram-positive organisms, and nutrient agar supplemented with penicillin can be used to select for Gram-negative organisms. Jul 10, 2017 · Complete lesson for Required Practical 2 on Investigating the effect of Antiseptic on Bacterial growth for new AQA Biology 2016 Trilogy syllabus. Practical sheet, results sheet and relevant exam questions included.