Turbidimetric study of the effect of inoculum density on the response of Escherichia coli to gentamicin under aerobic and anaerobic conditions.

The turbidimetric response to gentamicin of two strains of Escherichia coli was examined under aerobic and anaerobic conditions with 3 inocula. Turbidimetry showed that the minimum inhibitory concentration (MIC) was influenced by growth of partially inhibited cultures during the overnight incubation period. Continuous turbidimetric monitoring also allowed the minimum antibacterial concentration (MAC: the lowest concentration to cause an effect on normal growth) to be measured. As judged by the MAC, the effect of inoculum was small over the range 5 x 10(5) to 9 x 10(7) colony-forming units (CFU), but increased substantially as the inoculum was raised further. Exposure to gentamicin under anaerobic conditions caused an average 4-fold rise in the MAC with different inocula.