Glycosides of fluorinated p-nitrophenol offer improved sensitivity for detection of β-galactosidase and β-glucuronidase in Escherichia coli and other Enterobacterales

We describe the synthesis and evaluation of six halogenated nitrophenyl glycosides for detection of β-galactosidase and β-glucuronidase enzyme activity among Enterobacterales ("coliforms") and Escherichia coli, respectively. These were evaluated alongside the established substrates; o-nitrophenyl-β-D-galactopyranoside (ONPG), p-nitrophenyl-β-D-galactopyranoside (PNPG) and p-nitrophenyl-β-D-glucuronide (PNP-GUR). The evaluation was performed using 30 isolates of Enterobacterales including 19 isolates of E. coli. Hydrolysis of 2-fluoro-p-nitrophenyl-β-D-galactopyranoside (2-fluoro-PNPG) yielded a significantly stronger yellow coloration after a six-hour incubation period compared to hydrolysis of ONPG and PNPG, potentially allowing for a more sensitive detection of Enterobacterales. Similarly, hydrolysis of the novel substrate 2-fluoro-p-nitrophenyl-β-D-glucuronide sodium salt (2-fluoro-PNP-GUR Na) by producers of β-glucuronidase also yielded a significantly stronger yellow colouration, potentially allowing for a more sensitive detection of E. coli. The yellow chromophore 2-fluoro-PNP retained high colour intensity at reduced pH when compared to o-nitrophenol and p-nitrophenol. Both substrates potentially offer enhanced sensitivity for the detection of Enterobacterales and E. coli in environmental samples as markers of faecal pollution.