Cumulative survival profiling: a new PAP-based method for detecting heteroresistance in staphylococcal clinical isolates

The area under the population analysis profile (PAP) is used in the gold standard method for detecting heteroresistance in staphylococci. We tested the hypothesis that the initial inoculum strongly influences the area under the population analysis profile. We sought to interpret this dependence and develop a new metric that lacks this dependence to retrospectively detect heteroresistance to vancomycin in coagulase-negative staphylococcal (CoNS) isolates. We tested our hypothesis on 20 PAPs from the heteroresistant positive control isolate (Mu3) and 7 PAPs from one CoNS isolate which is associated with poor clinical response. The area under the PAP depended linearly (p<0.001) on the initial inoculum. We interpreted the slope to be the cumulative survival under vancomycine concentration gradient. The statistical distribution of the cumulative survival for Mu3 and the CoNS isolate constituted the cumulative survival profiles for each. The profiles reflect ed spectrum of response under vancomycine gradient with the left-tail of CoNS isolate profile located near the median of Mu3 profile indicating the heteroresistance of the CoNS isolate and that the most resistant in the spectrum are likely to be associated with poor clinical response. We estimated that about two-third of the CoNS from unique participants are heteroresistant with 80% of heteroresistant isolates may be associated with a poor clinical response.