Nasal samples for the detection of MRSA and rectal samples (stoma sample in case of colostomy) for ESBL producing gram negative bacteria will be obtained at hospital admission and discharge. Patients starting antibiotic therapy per os and/or intravenously will be sampled at antibiotic start (t0, within one hour) and at the following intervals: day 3 (t1), 7 (t2), 15 (t3), 30 (t4). Patients colonized with MRSA and/or ESBL-producing gram negative bacteria before starting antibiotic therapy (t0 sample) will be excluded from follow-up cultures and analysis. All patients included in the study will be followed to determine whether they develop clinical infections with the target ARB. Patients will be followed during the hospitalization and afterwards for a total of 30-day from the inclusion in the study. Screening will be performed in outpatient clinics after patients' discharge from the hospital within 30 days of starting antibiotic (t0 sample).