The study is the WP4 of the EU-funded (7th FW) project SATURN (Impact of Specific Antibiotic Therapies on the prevalence of hUman host ResistaNt bacteria). A total of 6 surgical and 6 medical wards will participate in the study. Sites of the study are located in 3 countries (Italy, Serbia, Romania). This WP will compare nosocomial acquisition rates of methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum beta-lactamase (ESBL)-producing gram-negative bacteria (E.coli, Klebsiella spp. and Proteus spp.) among different treatment groups and define the temporal relationship between the start of antibiotic therapy, the acquisition of new colonisation in patients previously not colonised, and the development of a bacterial infection caused by the same strain isolated in a screening sample. This goal will be achieved by completing the following primary objectives:

• To determine the rate of acquisition of target antibiotic-resistant bacteria by 1,000 antibiotic-days according to different classes of antibiotics, duration of therapy and antibiotic combination (monotherapy versus combination therapy);
• To determine genotypic relation between colonising and infecting strain in the same patient and patients' and hospital staff colonising strains (to be performed in collaboration with WP1 of the SATURN project);
• To study the virulence and fitness of the isolates (i.e. new colonising strains) causing subsequent nosocomial infections (to be performed in collaboration with WP1 of SATURN project);
• To predict the risk for nosocomial infections due to target bacteria after a single treatment therapy adjusted by length of hospitalisation and ward colonisation pressure.

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).

Nasal and rectal cultures will be also obtained from the ward staff at the beginning and at the end of the study. This group includes nurses and all staff including doctors having contacts with patients. These cultures will be handled in the same manner as the patients' cultures

Inclusion Criteria:

• All hospitalized non ICU patients at hospital admission/discharge;
• Age >18 years old;
• All patients starting intravenous and/or oral antibiotic treatments during hospitalization.

Exclusion Criteria:

• Pregnancy;
• Recent nose surgery (for nasal swabs).

All patients will be screened at hospital admission and hospital discharge during the study period. Patients colonised with MRSA and/or ESBL-producing gram-negative bacteria before starting antibiotic therapy will be excluded from follow-up cultures and analysis.

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). Screening will be performed in outpatient clinics after patients' discharge from the hospital.