Two modified disk diffusion methods may substantially shorten antibiotic susceptibility testing for suspected urinary tract infections while maintaining accuracy, according to a study published in Microbiology Spectrum.
“Urinary tract infections are among the most prevalent bacterial infections worldwide, typically diagnosed using clinical symptoms, dipstick tests, and laboratory methods requiring standardized bacterial suspensions for antibiotic susceptibility testing (AST),” first author Henning Sabersky-Müssigbrodt, of Technical University of Munich, Germany, and colleagues wrote.
The researchers noted that conventional antibiotic susceptibility testing (AST) requires bacterial isolation and adjustment to a 0.5 McFarland standard before testing, a process that can delay targeted therapy and promote empiric antibiotic use. Prior attempts at direct susceptibility testing from urine have shown inconsistent performance, particularly at high or low bacterial loads, they added.
Using 27 clinical urine isolates and eight reference strains, Sabersky-Müssigbrodt and colleagues demonstrated that inhibition zone diameters vary predictably with bacterial concentration across common urinary tract infection (UTI) antibiotics, including fosfomycin, nitrofurantoin, and mecillinam. Without adjusting for concentration, resistant isolates were correctly identified in only about two-thirds of cases.
The researchers evaluated two approaches to address this limitation. A threshold-adapted method used concentration-specific breakpoints and achieved more than 93% categorical agreement with standard AST for both susceptible and resistant isolates. A regression-based method mathematically converted inhibition zone diameters to values equivalent to a 0.5 McFarland standard, correctly identifying all susceptible isolates and approximately 88% of resistant isolates.
By accounting for bacterial concentration (the inoculum effect), both strategies enable direct susceptibility testing from urine, allowing culture, quantification, identification, and AST to proceed in parallel without overnight inoculum standardization. According to the authors, this may enable physicians to receive targeted susceptibility data up to 24 hours earlier, supporting earlier optimization of therapy and improved antimicrobial stewardship in UTIs.
The researchers also described several study limitations, including challenges in accurately defining inhibition zones at low bacterial concentrations. “At these lower concentrations, the edges of inhibition zones can become less distinct, complicating the interpretation of results,” they explained. “This issue is particularly pronounced with swarming pathogens such as Proteus mirabilis, where extensive spreading across the medium can make inhibition halos difficult to define. These challenges could lead to discrepancies between the rapid method and traditional AST.”
Looking ahead, the researchers noted that additional refinement of breakpoints and regression models is needed, along with validation across a broader range of pathogens and bacterial concentrations. They also indicated that integrating concentration-adjusted direct susceptibility testing into automated laboratory systems could improve adoption across health care settings and expand access to faster diagnostic testing.
This research was supported by funding from the Bavarian Ministry of Economic Affairs, Regional Development and Energy. The authors had no conflicts of interest to declare.
Source: Microbiology Spectrum