Begin the entry in the works-cited list with the author's real name and, in parentheses, user name, if both are known and they differ. If only the user name is known, give it alone.
Next provide the entire text of the tweet in quotation marks, without changing the capitalization. Conclude the entry with the date and time of the message and the medium of publication (Tweet).
So, for example: in the future, when humanity's last epidemiologists are using tweets to model the spread of the zombie plague that laid waste to 99.9% of the world's population, their citations will read as follows*:
Gonzalez, Robert (rtg0nzalez). "Can anyone recommend a good doctor for a bite wound? This thing is starting to look pretty gruUUBRAINS." 3 June 2017, 6:17 p.m. Tweet.**
*Assuming, of course, that scientific journals of the future haven't come up with Twitter-referencing rules for their own proprietary style guidelines, of which there are many.
**Yes, the tweet would actually read "BRAINS." Didn't you know? In the future, your twitter account will be wired directly into your central nervous system, allowing you to update your feed with your thoughts and/or latent cannibalistic urges.