Microcalorimetric study of the effects of cephalexin and cephaloridin on Escherichia coli and Staphylococcus aureus.
The kinetics of the antibacterial actions of cephalexin and cephaloridin against a strain of Escherichia coli and a strain of Staphylococcus aureus were studied by a flow microcalorimeter. The heat production was related to the number of viable organisms (CFU ml-1), the pH, the optical density of the culture medium (OD540), and the morphology of the antibiotic-exposed bacteria. No heat effects could be registered when the number of CFU was below 10(4) ml-1. The addition of cephalexin, 2.5 microgram ml-1 (5 x MIC), to cultures of S. aureus caused a decrease in the heat production which was only roughly correlated with the number of CFU ml-1. This was also the case when 9.0 microgram ml-1 (2 x MIC) of this drug were added to cultures of E. coli. Two to three hours after the drugs had been added, no heat effects could be registered for the following 6--8 hours, after which an increase in the heat production again occurred. The MIC and MBC of the organisms isolated during this late heat increase were 8--40 times higher than those of the parent test organisms. A direct relation between drug concentration and response, i.e. heat effects produced, was found when increasing concentrations of cephalexin, i.e. 1.0 up to 50 micrograms ml-1 (2--100 x MIC) were added in the logarithmic growth phase to cultures of S. aureus. In ampoule calorimetric experiments, E. coli was cultured in a non-aerated, sealed growth vessel in the presence of cephalexin or cephaloridin in concentrations corresponding to 1/2 x MIC. The thermograms did not differ in shape, although the heat effects occurred somewhat later in the culture containing cephaloridin.
Microcalorimetric study of the effects of cephalexin and cephaloridin on Escherichia coli and Staphylococcus aureus. The kinetics of the antibacterial actions of cephalexin and cephaloridin against a strain of Escherichia coli and a strain of Staphylococcus aureus were studied by a flow microcalorimeter. The heat production was related to the number of viable organisms (CFU ml-1), the pH, the optical density of the culture medium (OD540), and the morphology of the antibiotic-exposed bacteria. No heat effects could be registered when the number of CFU was below 10(4) ml-1. The addition of cephalexin, 2.5 microgram ml-1 (5 x MIC), to cultures of S. aureus caused a decrease in the heat production which was only roughly correlated with the number of CFU ml-1. This was also the case when 9.0 microgram ml-1 (2 x MIC) of this drug were added to cultures of E. coli. Two to three hours after the drugs had been added, no heat effects could be registered for the following 6--8 hours, after which an increase in the heat production again occurred. The MIC and MBC of the organisms isolated during this late heat increase were 8--40 times higher than those of the parent test organisms. A direct relation between drug concentration and response, i.e. heat effects produced, was found when increasing concentrations of cephalexin, i.e. 1.0 up to 50 micrograms ml-1 (2--100 x MIC) were added in the logarithmic growth phase to cultures of S. aureus. In ampoule calorimetric experiments, E. coli was cultured in a non-aerated, sealed growth vessel in the presence of cephalexin or cephaloridin in concentrations corresponding to 1/2 x MIC. The thermograms did not differ in shape, although the heat effects occurred somewhat later in the culture containing cephaloridin.