when did patient 19144 first undergo a microbiology test in 08/2105?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 19144 ) and strftime('%y-%m',microbiologyevents.charttime) = '2105-08' order by microbiologyevents.charttime asc limit 1