Microcalorimetric method to determine ceftazidime and piperacillin–tazobactam susceptibility in Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa is a gram-negative bacterium, considered one of the most common causes of nosocomial infections. Bacterial cultures produce an ex- change of energy as a result of their metabolism. The rate of heat production is an adequate measure of the metabolic activity of the organisms and their constituent parts. Micro- organisms produce small amounts of heat, in the order of 1-3 pW per cell. Although the heat produced by bacteria is very small, their exponential reproduction in a culture medium permits heat detection through microcalorimetry. In this work, we analysed the microcalorimetric behaviour of P. aeruginosa and its susceptibility against two antibi- otics: ceftazidime and piperacillin-tazobactam. We per- formed this experiment five times using different concentrations of antibiotics (2 9 MIC, 1 9 MIC, 0.75 9 MIC, 0.5 9 MIC y 0.25 9 MIC). The effect of antibiotics on bacterial growth can be evaluated by the delay of heat flow and reduction of the heat flow peak compared to results with the growth control without the presence of antibiotics. The measures were carried out using a Calvet microcalorimeter that contains two stainless steel cells of approximately 10 cm 3 (reference and ex- perimental). A constant temperature of 309.65 K was maintained within the microcalorimeter. Recording the difference in calorific potential over time, we obtained the growth curves of P. aeruginosa.