when did patient 16879 first get mrsa screen microbial tests until 09/2104?
select microbiologyevents.charttime from microbiologyevents where microbiologyevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 16879 ) and microbiologyevents.spec_type_desc = 'mrsa screen' and strftime('%y-%m',microbiologyevents.charttime) <= '2104-09' order by microbiologyevents.charttime asc limit 1