Degree Name

Doctor of Philosophy


School of Civil, Mining, Environmental and Architectural Engineering


Colistin and carbapenems are two categories of antibiotics utilized as a last-resort treatment for infections caused by multi-drug resistant (MDR) and extensively drug resistant (XDR) bacteria. Resistance to these last lines of defence can eventuate total antibiotic resistance. At present, there is scarcity of research present to estimate the current load and impact of the residual concentrations of colistin and carbapenems on environmental microbiome. However, there have been several reports demonstrating resistance development to colistin and carbapenems in a range of environmental compartments in the past few years. Most of their dosage is discharged as waste in its original form, thereby making its way into the urban wastewater. Expected low environmental concentrations, complex structures, stability, and interfering substances make quantification of colistin and carbapenems from environmental samples extremely challenging. There are a few major knowledge gaps addressed in this PhD project to gain some knowledge towards predicting the effects of their residual concentrations on the environment and environmental microbiome: development of UHPLC-MS/MS methods of detection and quantification of colistin and carbapenems from raw domestic wastewater; study of their stability in water and wastewater under varied pH and temperature conditions; study of their degradability in wastewater in the presence of sewer biofilms during the transportation from domestic sewers to WWTPs; and development of a solid phase extraction (SPE) method for extraction and enrichment of their low soluble concentrations from wastewater.

The UHPLC-MS/MS methods were developed for colistin (i.e., major components colistin A and B) and four carbapenems: meropenem, doripenem, biapenem and ertapenem, and validation was performed based on EMA Guideline on Bioanalytical Method Validation 2011. SPE methods were developed for colistin and carbapenem extraction from wastewater at soluble concentrations of 0.01 µg/mL for carbapenems and 0.1 µg/mL for colistin. SPE can be a very effective tool to analyse contaminants like last resort antibiotics from wastewater. It can help extract and enrich such compounds that are estimated to be found in very low soluble ranges in environmental water matrices. Furthermore, an integral property that contributes to a greater understanding of their persistence in environmental water is their chemical stability, which has not been evaluated yet. This PhD project identified the stability of colistin (colistin A and colistin B) and carbapenems (meropenem, doripenem, biapenem, and ertapenem) in water and wastewater under different pH (5 and 7) and temperature (4 and 25°C) conditions to establish the variation in their stability between the two matrices. Lastly, to study their degradability in sewer environment, laboratory scale rising main (RM) and gravity sewer (GS) bioreactors were employed to culture mature biofilms with real wastewater as the feed. This component of the PhD project was only performed for carbapenems due to limited detection capacity of the UHPLC-MS/MS instrument for colistin, with the minimum quantifiable concentration being 0.5 - 0.7 µg/mL, which was identified to be too high for a laboratory bioreactor-based study. Batch tests were conducted in RM and GS sewer bioreactors fed with carbapenem-spiked wastewater to evaluate the stability of carbapenems and compared against those in a control reactor (CTL) without sewer biofilms, over a duration of 12 h. The findings contribute to the understanding of the fate of carbapenems in urban wastewater and the potential application towards epidemiological studies performed using wastewater analysis (Wastewater-based epidemiology, WBE).

FoR codes (2008)




Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.