Antibacterial efficacy of tobramycin against anaerobic Escherichia coli cultures in the presence of electron acceptors

The antimicrobial activity of tobramycin against anaerobic cultures of Escherichia coli was tested in the presence of various electron carriers. The presence of 2,6-dichlorophenol 4-indophenol (DCIP) significantly enhanced the killing efficacy of tobramycin. Only 0.003% of the initial cell population (i.e. 10(6) cfu/mL) remained viable after exposure for 10 h to the mixture of antibiotic (20 x MIC, i.e. 40 mg/L) and electron acceptor (10(-3) M), as compared with 9% of surviving organisms in the presence of tobramycin alone. Less synergy was obtained with p-benzoquinone and 1,2-naphthoquinone. Fumarate did not affect the efficiency of the antibiotic. The mixture of tobramycin and DCIP was ineffective against agar-entrapped bacteria which, like biofilm organisms, are subject to oxygen limitation.