How should words emphasized by the writer be indicated in a direct quotation?
Use italics to add emphasis to a specific word or words in a direct quotation that were not originally emphasized by the author. Additionally, type the phrase emphasis added and enclose it in brackets directly after the emphasized words to indicate to the reader that the emphasis is not present in the original text.
Let's look at an example:
Consider this excerpt from Katherine Cullen’s book, Biology: The People Behind the Science:
“Nature selects variations that are advantageous for survival and reproduction in a particular environment [emphasis added], just as farmers artificially select for economically desirable characteristics” (Cullen, 2006, pp. 52-53). 
Note: The phrase emphasis added is placed inside brackets and is not italicized.
 Cullen, K. E. (2006). Biology: The people behind the science. New York, NY: Chelsea House.