The Resistant-Population Cutoff (RCOFF): a New Concept for Improved Characterization of Antimicrobial Susceptibility Patterns of Non-Wild-Type Bacterial Populations

Abstract

cThis study aimed to determine resistant-population cutoffs (RCOFFs) to allow for improved characterization of antimicrobial susceptibility patterns in bacterial populations. RCOFFs can complement epidemiological cutoff (ECOFF)-based settings of clinical breakpoints (CBPs) by systematically describing the correlation between non-wild-type and wild-type populations. We illustrate this concept by describing three paradigmatic examples of wild-type and non-wild-type Escherichia coli populations from our clinical strain database of disk diffusion diameters. The statistical determination of RCOFFs and ECOFFs and their standardized applications in antimicrobial susceptibility testing (AST) facilitates the assignment of isolates to wild-type or non-wildtype populations. This should improve the correlation of in vitro AST data and distinct antibiotic resistance mechanisms with clinical outcome facilitating the setting and validation of CBPs. In antimicrobial susceptibility testing (AST), clinical breakpoints (CBPs) are set with the intent of predicting the clinical outcome (1-3). Epidemiological cutoffs (ECOFFs), as defined by the European Committee for Antimicrobial Susceptibility Testing (EUCAST), separate wild-type from non-wild-type populations, reviving the concept of microbiological breakpoints (4-9). EUCAST uses the ECOFF as one of several tools in the process of CBP setting. Different methods have been suggested to determine ECOFFs (5, 10).For a clinical isolate, prediction of treatment success is usually based on a single laboratory AST result, which is subsequently classified according to the corresponding CBPs. However, both MICs and disk diffusion diameters show significant variations when tested repeatedly (1, 6). For wild-type populations, these variations have been shown to result from measurement imprecision and methodological and biological factors (5, 11, 12). As it is based on a single measurement, the true position of an isolate within a disk diameter or MIC distribution can only be approximated with a certain probability. In addition, it is unclear to what extent the in vitro susceptibility of a clinical isolate reflects the in vivo situation, as in vivo variations within a population are likely to be more pronounced than those observed in vitro in the standardized setting of AST.CBPs should preferably avoid splitting the wild-type population into different interpretative categories as the wild-type is regarded to be a genotypic entity despite population-intrinsic (micro-)variations in drug susceptibility (6). This principle of not splitting wild-type populations by CBPs is widely acknowledged by EUCAST guidelines (13). By logical consequence, these considerations should similarly apply to non-wild-type populations, which represent distinct genotypic/phenotypic entities ("resistotypes"). Thus, CBP setting should not only avoid splitting the wild-type into different interpretative categories but also CBPs should avoid splitting a resistotype into different clinical categories as far as possible in or...

MDR M. tuberculosis outbreak clone in Eswatini missed by Xpert has elevated bedaquiline resistance dated to the pre-treatment era

“…2). The NCOFFs for the Rv0678 M146T distributions for CFZ (part a) and BDQ (part b) were set using the eyeball method, and a corresponding idealized representation of the MIC distributions of the gWT and Rv0678 M146T populations for both drugs (and their area of overlap in grey) was prepared for illustrative purposes (part c) [25]. The MIC increases for M146T relative to gWT isolates were statistically significant for both CFZ and BDQ.…”

Evolutionary Pathways and Trajectories in Antibiotic Resistance

“…to recombination between different replicons and, consequently, to their evolvability (136,325). Mobile genetic elements carrying resistance genes frequently have sitespecific recombination systems and IS, whose location either in plasmids or in chromosomes favors homologous recombination, thereby favoring different events of integration or excision and interplay among different elements (17,(22)(23)(24)(25)(26).…”

On the Consequences of Poorly Defined Breakpoints for Rifampin Susceptibility Testing of Mycobacterium tuberculosis Complex

“…In plot B, the MIC distribution of the borderline resistance mutation B overlaps to a greater degree with the susceptible population. This cannot be compensated for by extending the ATU to the resistant cutoff value (RCV) for this mutation (i.e., the lower end of this mutation, which would be 0.125 mg/ml), given that this would classify too many susceptible isolates as uncertain, particularly if the isolate is otherwise pan-susceptible (29,31,32). Nevertheless, resistance can be ruled in reliably by the detection of the corresponding borderline mutation B. resistance (28).…”