Objectives:

Antibiotic combinations are used to enhance the antibacterial effect and to prevent the development of bacterial resistance. We have tested possible synergistic effect of several antibacterial combinations in Bacillus anthracis.

Methods:

The in vitro activity of antibiotic combinations against two strains of B. anthracis, the Sterne strain and the Russian anthrax Vaccine strain ST-1 were tested by calculating the fractional inhibitory concentration derived from the MIC's of the different agents and by measuring the rate of bacterial kill by antibiotic combinations.

Results:

The FIC calculations rendered the results shown in the table:

The results of the bacterial time-kill demonstrated the same trend, i.e combinations that exhibited antagonism had slower kill rate than any of the members of the pair.

Conclusions:

These in vitro results demonstrate the difficulties in obtaining synergistic combinations of antibiotics against B. anthracis and suggest that the recommendations for therapy with antibiotic combinations are unfounded.