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Métodos sustitutos del antibiograma

Métodos sustitutos del antibiograma
Hello, everyone. Today, we’re going to talk about surrogate phenotypic methods.
With surrogate AST methods, we mean methods which are different from the reference broth microdilution method, but which are calibrated to the reference broth microdilution method. The most commonly used surrogate methods are disc diffusion, gradient tests, and semi-automated devices such as Phoenix, Vitek 2, and MicroScan.
This diffusion of the output is an inhibition zone diameter measured in millimetres. And that is, in turn, converted to an S, I, and R via a zone diameter breakpoint, which is part of the EUCAST breakpoint table. That breakpoint is calibrated to the MIC breakpoint in the same table. Gradient tests provide inhibitory concentrations, which lack the status of formal MIC values, but which are taken to the MIC breakpoint and interpreted into an S, an I, or an R. You need to remember that making the concentration scale of a gradient test work for a multitude of species and across the whole scale is very tricky.
So you need to be a little bit suspicious of gradient tests, and you should ask the manufacturer exactly which species and for which concentrations on the scale the manufacturer is willing to guarantee the result. Semi-automated devices often have a very limited array of inhibitory concentrations in the plastic cards or trays, which are the basis for the method and in which the organism is placed. In one way or another, the inhibition of growth is measured as part of the method. And then, using breakpoints in the machine, these are converted to– the concentrations are converted to S, I, and R. Dilutions are never full scale simply because the manufacturer and the customer wants to save on material, number of cards.
They want to put as many antibiotics on the card as is possible. And that means compromising. So a lot of the data you get out of these machines are MIC values of some sort listed as less than or equal to or more than a certain or a named value. These values lack the status of MIC values, and they are never accepted by organisations such as EMA, EUCAST, et cetera as true MIC values. Quality control of procedures is also very difficult. If you’re interested in setting up broth microdilution or disc diffusion and you’re a beginner and you don’t know exactly how to do it, you can go to the EUCAST web page.
And, as you can see, there are numerous documents on media preparation and MIC determination and distribution methodology and how to implement this in your lab, et cetera, et cetera.

Los métodos sustitutos del antibiograma son métodos que son diferentes del método de microdilución en caldo de referencia, los cuales se calibran tomando como referencia el método de microdilución en caldo.

Los métodos más utilizados son:

  • Difusión con discos

  • Pruebas de gradiente como Etest y MTS

  • Dispositivos semiautomatizados como Phoenix, Vitek2 y MicroScan

Difusión con discos

El resultado de este método son los halos (mm) que se convierten en S, I y R a través de puntos de corte de los halos, los cuales se calibran según los puntos de corte de CMI.

Pruebas de gradiente

Estas proporcionan concentraciones inhibidoras que carecen del estado de los valores de CMI formales, los cuales se llevan a los puntos de corte de CMI y se interpretan en S, I o R. Es importante recordar que es muy difícil conseguir que la escala de concentración de una prueba de gradiente funcione para una multitud de especies y a través de toda la escala.

Dispositivos semiautomatizados

Los dispositivos semiautomatizados a menudo tienen una gama limitada de concentraciones inhibidoras en «tarjetas/bandejas de plástico», las cuales son la base del método y donde se coloca el organismo. La inhibición del crecimiento se mide como parte del método y los puntos de corte dentro de la máquina los convirtieron en S, I y R. Sin embargo, las diluciones nunca son a gran escala, ya que a menudo el fabricante/cliente desea ahorrar material y aumentar la cantidad de antibióticos. Este método carece del estatus de los valores CMI y no es aceptado por organizaciones como EUCAST como verdaderos valores CMI.

Si estás interesado en establecer CMI o difusión en discos y eres principiante, las páginas web de EUCAST de aquí disponen de información y documentos sobre la preparación de medios, la determinación de CMI, la terminología de distribución y cómo puedes instalarlos en tu laboratorio.

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