Interpretative reading: recognizing the unusual and inferring resistance mechanisms from resistance phenotypes

Abstract

If isolates are speciated and if a sufficient range of antibiotics is tested, underlying resistance mechanisms can often be inferred from the antibiogram data. This allows: (i) anomalous combinations of phenotype and organism to be reconsidered; (ii) prediction of further antibiotics that deserve testing; and (iii) the suppression of susceptibilities that are anomalous in the light of the inferred mechanism. This 'interpretative reading' is widely undertaken in France but is largely precluded in the UK by limited speciation and the testing of narrow ranges of antibiotics. Nevertheless, UK laboratories should be aware of: (i) grossly anomalous combinations of species and phenotype, demanding reference laboratory confirmation; (ii) useful indicator drugs, where resistance implies a mechanism conferring other resistances that may be less obvious in direct tests; and (iii) antibiotics that are prone to select resistant mutants of particular species during therapy. Details of these combinations of organism and resistance are presented. Relationships between antibiogram and mechanism are also presented to allow full interpretative reading for those testing wide panels of drugs versus speciated isolates.

Extended-spectrum beta-lactamases in Klebsiella spp and Escherichia coli obtained in a Brazilian teaching hospital: detection, prevalence and molecular typing

“…The prevalence of K. pneumoniae around the world varies greatly although it tends to be less than 25% in many countries (1,3,13,16,24). We, therefore, considered that the prevalence of ESBL was quite high among isolates of K. pneumoniae in this study.…”

The aac(6')Ib gene in Proteus mirabilis strains resistant to aminoglycosides.

“…The minimum inhibitory concentration value of gentamicin, tobramycin, amikacin, and netilmicin in three positive isolates (Pm1, Pm2, and Pm6) were ≥256 µg/ml. The enzymes produced by various bacteria can often be predicted from the antibiogram data [4], but hidden genes could be present. One Proteus mirabilis strain (Pm5) had non-acitve gene aac(6')-Ib which MIC values of Amgs were located in a criterion of the susceptibility.…”

“…The AAC(6`)-I enzyme confers resistance to tobramycin, amikacin, netylmicin, and kanamycin. Only one component (gentamicin) can remains active but in vivo use best avoided [1,4]. Of all Amgs amikacin is the most resistant to the action of AMEs.…”

“…and Gramnegative, including Proteus spp. strains [1][2][3][4]. These enzymes are often plasmid-encoded but may also be associated with transposable elements facilitating the rapid dissemination of the genes in various bacterial populations [1,5].…”

Resistencia Antimicrobiana en Unidades de Cuidado Intensivo de Bogotá, Colombia, 2001-2003

“…Para el análisis se consideró Pseudomonas multirresistente, a aquellos aislamientos de Pseudomonas spp. resistentes a cuatro o más antibióticos (piperacilina o piperacilina/tazobactam, ceftazidima, imipenem o meropenem, ciprofloxacina y amikacina) (8), posible fenotipo Amp C, a los aislamientos de enterobacterias corresistentes a cefamicinas (cefotetan) y oximinocefalosporinas (ceftriaxona ó Ceftazidima); posible fenotipo de betalactamasa de espectro extendido (BLEE) a los aislamientos de Escherichia coli, Klebsiella spp. y Enterobacter spp.…”