Un medio de cultivo es un sustrato o solución de nutrientes en los que crece y se multiplican los microorganismos en el laboratorio, con el objeto de aislar diferentes especies bacterianas, con el din de identificarlas y realizar estudios complementarios.
Clasificación
Pueden clasificarse según su:
- Consistencia
- Origen
- Composición
- Utilización
Consistencia
- Líquidos: contienen nutrientes a los cuales se les adicionan sustancias con la capacidad de mantener
estables un pH, entre ellos podemos encontrar el caldo nutritivo y caldo peptona. - Solidos: se obtienen agregando agar (sustancia de tipo polisacárido que se obtiene de algas marinas) a un medio liquido determinado. Al tener un medio sólido, nos facilita obtener colonias bacterianas aisladas.
- Semisólidos: similares a los medios sólidos, la única diferencia es que a estos se les disminuye la cantidad de agar, lo que permite conservar cepas bacterianas y la observación y registro de bacterias móviles.
Origen
Según su origen los medios de cultivo se dividen en medios sintéticos y medios naturales.
- Los medios sintéticos o químicamente definidos son medios compuestos por productos químicos conocidos y se utilizan para realizar estudios metabólicos, por ejemplo: agar blando, caldo nutritivo, agar eosina azul de metileno, etc.
- Los medios naturales o químicamente no definidos son aquellos medios que se preparan a partir de sustancias naturales animales o vegetales, por ejemplo: suero, leche, papa, etc.
Composición y utilización
Según su utilización y su composición los medios de cultivo se pueden clasificar en:
- Simples
- Enriquecidos
- Selectivos
- Diferenciales
- Enriquecimiento
- Medios Simples
- Poseen los requisitos nutricionales para permitir el desarrollo bacteriano general, ejemplos: agar nutritivo, caldo nutritivo, entre otros.
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- Medios enriquecidos
- Son medios simples o comunes, a los que se le añaden ciertos elementos como sangre, suero, líquido ascítico, huevo, glucosa, vitaminas, etc. lo que permite el aporte de factores de crecimiento o sustancias que neutralizan agentes inhibidores del crecimiento en bacterias exigentes nutricionalmente, entre algunos ejemplos: agar sangre, medio de Löwenstein-Jensen, enriquecido con huevo para facilitar el crecimiento de Mycobacterium; agar desoxicolato-lactosa (DLA), enriquecido con lactosa y desoxicolato.
- Medios selectivos
- Se consiguen añadiendo al agar nutritivo compuestos químicos nocivos para algunas bacterias cuyo crecimiento no es de interés o también mediante una alteración de las condiciones físicas del medio, entre algunos ejemplos tenemos
- El agar Mac Conkey que contiene cristal violeta.
- Thayer-Martin medio selectivo para Neissar, ya que este posee antibióticos como la vancomicina colistina y nistatina.
- Caldo lactosado bilis verde brillante en donde la bilis inhibe el crecimiento de bacterias gran positivas.
- Löwenstein-Jensen con verde de malaquita es selectivo para Mycobacterium.
- Medios diferenciales
- A estos medios se le adiciona sustancias para que solo crezcan ciertas bacterias y estas, al actuar sobre alguna de las sustancias adicionadas, permiten observar macroscópicamente ciertas propiedades de crecimiento que ayudan a diferenciar sus colonas de otras especies diferentes, entre algunos ejemplos encontramos:
- Agar eosina azul de metileno (EMB), diferencia E. coli (color verde metálico brillante) de colonias de Enterobacter aerogenes (color rosado, consistencia mucosa y crecimiento abundante).
- Agar sangre, diferencia variedades de Streptococcus pyogenes en Streptococcus betahemolíticos de Streptococcus alfa hemolítico y gama hemolítica.
- Agar Salmonella-Shigella tiene lactosa y un indicador de pH que permite diferenciar las colonias de bacterias fermentadoras de este disacárido.
- Medios de enriquecimiento
- Son medios líquidos que favorecen o permiten la multiplicación de las bacterias cuando la muestra obtenida es muy pobre, por ejemplo:
- Caldo tioglicolate, favorece el crecimiento de las bacterias anaeróbicas.
- Caldo tetrationato favorece crecimiento de bacterias microaerófilas.
- Los caldos bilis y de Muller-Kauffman, favorecen el crecimiento del género Salmonella.
- Caldo o agua peptonada se utiliza para el crecimenot de Vibrio Cholerade.
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Muy interesante el tema de los medios de cultivos ya que no todos los bacteriologas están familiarizados con el tema personalmente muy enriquecedor y de un gran fortalecimiento en el conocimiento
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