Colourimetric sensor for monitoring ammonia-generating infectious bacteria

Abstract

A simple, inexpensive bromophenol blue (BPB), colourimetric ammonia (NH₃) sensor is used to monitor the growth of different wound associated bacteria in three different, simple wound models which utilise a typical, readily available, commercial occlusive wound dressing. The 25 % colour change point of the sensor is ca. 0.0032 %NH₃ and its 90 % response and recovery times are, 8 and 230 s, respectively. Using P. stuartii as the test species, in a model wound with an occlusive dressing, the increase in the sensor’s colour up to the 25 % point correlates with that of the exponential growth of the bacterial species (units: CFU/mL) and a simple mathematical model is used to show that a linear relationship should exist between log(CFU/mL) of the initial inoculum and the time it takes for the sensor to reach its 25 % colour change point, the threshold time, TT. This prediction is confirmed using the NH₃ producing bacteria, E. coli, E. cloacae, P. stuartii, and K. aerogenes using three different wound models, namely ones in which the ‘wound’ was a non-urea containing agar plug, a urea-containing agar plug and a damaged porcine skin model, respectively. These results suggest that the NH₃ sensor has promise as a non-invasive wound monitoring indicator and the additional work required to progress the sensor in this role is discussed briefly.