when did patient 64519 receive his or her last microbiology test until 09/2105?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 64519 ) and strftime('%y-%m',microbiologyevents.charttime) <= '2105-09' order by microbiologyevents.charttime desc limit 1