**Abstract**

Netilmicin is a potent and safe antibiotic with a very low incidence of resistance used as a topical ophthalmic medication in bacterial ocular infections. The aim of this study was to compare netilmicin's Quotient of Inhibitions (QIs) and killing kinetics vs Staphylococci with other ophthalmic antimicrobials. Conjunctival and corneal QIs of netilmicin formulations, in single and multiple doses of administration, were compared with those of tobramycin, ofloxacin, levofloxacin and azithromycin preparations. The same analysis was performed in human tears, comparing netilmicin eye drops solution with tobramycin ofloxacin and levofloxacin. Furthermore, killing kinetics against Staphylococci (ATCC strains and ocular isolates) of the above-cited antibiotics, as well as chloramphenicol, were compared at different time points. QI results showed that in the conjunctiva, netilmicin, in both single and multiple doses of administration, is highly effective against all staphylococcal strains tested, while in the cornea it was particularly active against methicillin-resistant Staphylococci strains. Moreover, in human tears, netilmicin eye drops solution showed a more favourable QI against Staphylococci than tobramycin, ofloxacin and levofloxacin all in single-dose administration regimen. Killing kinetic results showed that netilmicin has a great bactericidal activity vs all the microbe strains tested as netilmicin showed to be almost the most active antibiotic. Results suggest that netilmicin has one of the most favourable killing kinetic and tissue inhibitory effects against Staphylococci than the principal ophthalmic antibiotics on the market.

**Keywords:** netilmicin, quotient of inhibition, killing curves, Staphylococci, methicillin resistance
