when did patient 31088 receive his or her tissue microbiology test for the first time during their current hospital visit?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 31088 and admissions.dischtime is null ) and microbiologyevents.spec_type_desc = 'tissue' order by microbiologyevents.charttime asc limit 1