在筛选抗青霉素菌株时，须在培养基中加入青霉素，其作用是： 

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='48019' and id < 3101279 and id<>'3101279' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='48019' and id > 3101279 and id<>'3101279' order by id asc limit 5