Ancillary studies will be performed for evaluating the impact of S. aureus beta lactamases on the effectiveness, for determination of PKPD (pharmacokinetic and pharmacodynamic) parameters of cloxacillin and cefazolin predictive of therapeutic effectiveness, and for comparing cloxacillin and cefazolin impact on the emergence of 3rd generation resistant enterobacteria and on the intestinal microbiota (evaluated by 16 rRNA (ribosomal ribonucleic acid) gene profiling).