Antimicrobial heteroresistance: an emerging field in need of clarity

Abstract

"Heteroresistance" describes a phenomenon where subpopulations of seemingly isogenic bacteria exhibit a range of susceptibilities to a particular antibiotic. Unfortunately, a lack of standard methods to determine heteroresistance has led to inappropriate use of this term. Heteroresistance has been recognized since at least 1947 and occurs in Gram-positive and Gram-negative bacteria. Its clinical relevance may be considerable, since more resistant subpopulations may be selected during antimicrobial therapy. However, the use of nonstandard methods to define heteroresistance, which are costly and involve considerable labor and resources, precludes evaluating the clinical magnitude and severity of this phenomenon. We review the available literature on antibiotic heteroresistance and propose recommendations for definitions and determination criteria for heteroresistant bacteria. This will help in assessing the global clinical impact of heteroresistance and developing uniform guidelines for improved therapeutic outcomes.

FIG 1

Heteroresistant versus homogeneous responses to antibiotics. Dotted lines represent breakpoints for resistance. Homogeneous bacterial cultures can be either susceptible (A), of intermediate susceptibility (B), or resistant (C) to an antibiotic according to traditional in vitro susceptibility testing. Heteroresistant bacteria may be any of the following. (D) Bacteria are completely susceptible to an antibiotic, whereby the different subpopulations respond to antibiotic concentrations extending below the breakpoints. This form is less likely to be detected and is probably the least clinically important (unless the least responsive subpopulations develop resistance to the antibiotic). (E) Bacteria exhibit the more classical form of heteroresistance, in which the majority of the bacterial population is susceptible to an antibiotic, with a highly resistant minority. Antibiotic treatment guided by the traditional susceptibility testing breakpoints would select for the resistant subpopulation, leading to therapeutic failure. (F) The entire bacterial population, including the least resistant subpopulations, is resistant to the antibiotic. Chemical communication of antibiotic resistance from the more resistant members of the population protecting less resistant bacteria is the major concern of such bacterial populations.

FIG 2

Recommended scheme for determination of heteroresistance and interpretation criteria. Disc diffusion assays should be performed according to standardized procedures for antimicrobial susceptibility testing as recommended by agencies such as CLSI and BSAC. These procedures may be applied to Etest assays while taking into consideration the manufacturer's guidelines. PAP by CFU counts should be performed by plating aliquots of 10-fold serially diluted bacterial cultures on antibiotic-containing agar plates. Agar plate preparation should follow standardized guidelines for MIC determination by agar dilution assays. Turbidimetric PAP should follow standard guidelines for MIC determination by the broth dilution technique, with the exception of turbidimetric quantification of bacterial growth at each antibiotic concentration.