As a further control, we evaluated two bacterial strains for cross-reaction with antibodies. Pseudomonas aeruginosa PAO1 strain [30] and Staphylococcus aureus ATTC strain 29213 were thawed and bacteria were recovered on agar-blood plates. One colony of P. aeruginosa and S. aureus were allowed to grow in 1 ml of Trypticase Soy Broth (TSB) (Difco, Becton Dickinson, Sparks, MD) or BBL™ brain heart infusion (BD Diagnostic Systems, Sparks, MD) respectively, for 1 hour at + 37°C. Bacteria were then incubated with 400 μg/ml gentamicin for 2 hours at + 37°C, and subsequently with anti-granulocyte antibodies under the same conditions of MPs, then finally analyzed by flow cytometry with the same settings used for MPs.