RESUMO
The effect of three pH values [5.8, 7.4, and 8.0] and human serum on the activity of ampicillin, cefazolin, and cefotaxime alone and in combination with four aminoglycoside antibiotics [streptomycin, tobramycin, gentamicin, and amikacin] against sensitive and resistant strains of Staphylococcus aureus and Pseudomonas aeruginosa was evaluated. The change of pH from 5.8 to 8 noticeably affected the activity of all studied combinations. In general, the five B-lactam aminoglycoside combinations were more active at alkaline pH [pH 8]. However, pH shift towards the acidic side [pH 5.8] decreased their activity and increased their FIC indices. Addition of human serum decreased the in vitro antibacterial activity of B-lactam and aminoglycoside antibiotics, as well as their combinations against the used strains. The degree of influence was related to the ability of each antibiotic to bind with serum albumin. Cefazolin/streptomycin and cefazolin/tobramycin combinations showed the highest degree of protein binding effect