This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Antibiotic gramicidin S is derived from ? 
Select and explain. 
A. Bacillus brevis, B. Cl difficle, C. S epidermidis, D. Streptococcus bovis
A: Bacillus brevis
Answer: A: Bacillus brevis 
Explanation: Ans. is'a'i.e., Bacillus brevisGramicidin-s is an antibiotic effective against some Gram positive and Gram negative bacteria as well as some fungi.It is a derivative of gramicidin, produced by the Gram positive bacterium Bacillus brevis.