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: Microbiology
For phage typing, how many phages of staphylococcus aureus are used ? 
Select and explain. 
A. 12, B. 15, C. 20, D. 23
D: 23
Answer: D: 23 
Explanation: Ans. is 'd' i.e., 23 Bacteriophage typing of staphylococcus is based on susceptibility of cocci to bacteriphages. This is carried out by pattern method where a set of 23 standard typing phages of S. aureus is used to type staphylococcal isolates and distinguish them from one another by their patterns of susceptibility to lysis. o The phage type of a strain is known by designation of the phages that lyse it. For example, if a strain is lysed by phages 83A, 84 and 85, it is called type 83A/84/85.