Susceptibility of multidrug-resistant strains of Mycobacterium tuberculosis to amoxycillin in combination with clavulanic acid and ethambutol

Thirty clinical isolates of Mycobacterium tuberculosis, 20 of which were mu ltidrug-resistant (MDR), were tested for susceptibility to different combin ations of amoxycillin, clavulanic acid and subinhibitory concentrations of ethambutol. beta-Lactamase production was assessed semiquantitatively with the nitrocefin method and susceptibility testing was performed with the BAC TEC method. All isolates were beta-lactamase positive and were resistant to 16 mg/L amoxycillin. The MIC of amoxycillin in combination with clavulanic acid was greater than or equal to 2 mg/L for 27/30 (90%) isolates. Additio n of subinhibitory concentrations of ethambutol significantly reduced the M IC of amoxycillin for all tested isolates. Twenty-nine (97%) isolates had a n MIC of amoxycillin of less than or equal to 0.5 mg/L when subinhibitory c oncentrations of ethambutol were added; this is well below the concentratio ns achievable in serum and tissue.