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Métodos de prueba de sensibilidad a los antimicrobianos (AST)

En este vídeo, el profesor Gunnar Kahlmeter analiza brevemente algunos de los tipos de los métodos AST.
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Hello, everyone. I’m Gunnar Kahlmeter. I’m a clinical bacteriologist, and I’ve been dealing with antimicrobial susceptibility testing for the better part of my life. I’m going to talk to you about methods. We can divide methods into phenotypic methods, genotypic methods, mechanistic methods, and expert rules methods. Let’s start with phenotypic AST. It really means measuring the activity of the agent against the isolate bug in question. Phenotypic AST is good because it predicts resistance and susceptibility, so you can actually exclude resistance with phenotypic methods. It’s also quantitative, which is a great advantage. You can say things like, this is more susceptible than that. And that way, you can quantitate your possibilities of success. The reference method is broth microdilution.
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It has been described both by the American organisation, by the European organisation, and they’ve come together united in an ISO description of how that method should be performed. All other methods are surrogate tests.
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Genotypic AST is the detection of the gene– the whole gene or an important part of the genes, which will convey specificity. It predicts resistance, but cannot really guarantee susceptibility. It’s not quantitative to any great extent. Breakpoints are not required, but when you set it up, you need to validate the detection of the gene against clinical susceptibility or resistance. There is so far no reference method available, but this is a volatile field so eventually there will probably be one. Mechanistic AST that is when you detect the end result of the resistance gene if you like.
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So if you have a gene that conveys the production of an enzyme, which, in turn destroys the agent, then by detecting the enzyme, you can predict that the isolate will be resistant to that specific agent.
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It’s not quantitative and it does not require breakpoints, but again, you need to initially calibrate it against clinical significance if you like. Then you can use something called expert rules, and what it really means is that if you know that something is resistant to one agent, you can deduce that it will also be resistant to another agent, normally in the same class. And you write rules that are formulated in the form of if, then. If resistant to erythromycin, then also resistant to clarithromycin. Not quantitative, and it’s not really trustworthy because there will be changes over time.

En este vídeo, el profesor Gunnar Kahlmeter analiza brevemente algunos de los tipos de los métodos AST.

Para ver el vídeo completo, haz clic en el enlace a continuación.

AST fenotípico – se basa en la medición de la actividad del agente frente al microorganismo, CMI y los puntos de corte.

AST genotípico – detección de un gen resistente (mecA, vanA/anB, etc.) en el genoma o en elementos genéticos transferibles.

AST mecanicista – detección de un mecanismo de resistencia mediante la detección del producto del gen resistente (enzima).

*Reglas expertas AST – sensibilidad basada en el conocimiento empírico.

El profesor Gunnar Kahlmeter continúa explicando la concentración mínima inhibidora (CMI) y su importancia.

CMI es la concentración más baja de un fármaco necesaria para inhibir la división celular en el entorno en el que se realizó la prueba. Es un caldo de microdilución y es la base de todas las pruebas de sensibilidad. Todos los demás métodos se clasifican como métodos sustitutos y se tratan en un punto posterior. El antibiograma clínico debe producir una recomendación clínica para determinar si se debe tratar o no y si la dosis estándar será suficiente o si se requiere una mayor exposición.

El CMI no es un valor absoluto y se ve afectado por factores como:

  • El inóculo
  • El tiempo de incubación
  • El caldo utilizado y su composición
  • La atmósfera
  • El pH

La prueba de muchas cepas bacterianas de la misma especie contra un agente definido puede determinar la distribución natural de la especie para ese agente en particular. Las cepas naturales son aquellas que carecen de mecanismos de resistencia. Una vez que se establece la distribución natural y se determinan los valores de corte epidemiológicos (ECOFF), la CMI más alta de la distribución natural, entonces se pueden clasificar las cepas sucesivas como naturales o no naturales.

Para ver el vídeo completo, haz clic aquí.

Cuéntanos en la sección de comentarios a continuación, ¿de qué tipo de método has oído hablar o has usado con anterioridad?

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