VMC 211: General Veterinary Microbiology (1+1)

TYPES OF MEDIA

Classification  according to consistency of media

• Liquid media
  • A liquid medium is called a broth. Eg: Nutrient broth
• Semisolid media
  • The media contain agar at a concentration of 0.5 percent or less. These media have custard like consistency. It is useful for cultivation of microaerophilic bacteria and determination of motility.
• Solid media
  • They are useful for isolation of bacteria and determination of the characteristics of colonies. Agar is used at a concentration of 1.5 – 2 percent level. It forms a firm transparent gel. It is not degraded by most bacteria. Eg: Nutrient agar.
  • Classification according to the requirements
    • General purpose media/ basal media
    • Nutrient broth and nutrient agar

Composition of nutrient broth and nutrient agar

Component	Nutrient broth	Nutrient agar
Beef extract	5 g	5 g
Peptone	10 g	10g
Sodium chloride	5 g	5 g
Agar	-	15 g
Water	1000 ml	1000 ml

• Enriched media
  • These media contain extra nutrients in the form of blood, serum, egg yolk etc, to basal medium. Enriched media are used to grow nutritionally exacting (fastidious) bacteria. Eg: Blood agar, chocolate agar, serum agar 
• Selective media
  • The medium is designed to suppress the growth of some microorganisms while allowing the growth of others (i.e., they select for certain microbes). Solid medium is employed with selective medium so that individual colonies may be isolated. Any agar media can be made selective by addition of certain inhibitory agents that don’t affect the pathogen to be isolatead. Various approaches to make a medium selective include addition of antibiotics, dyes, chemicals, alteration of pH or a combination of these. Eg.: Mannitol salts agar (selects against non-skin flora), MacConkey agar (selects against gram-positives)
• Enrichment media
  • They differ from selective media in not out and out suppressing the growth of non-enriched microorganisms. Enrichment culture is used to increase the relative concentration of certain microorganisms in the culture prior to plating on solid selective medium. Unlike selective media, enrichment media are liquid media. Eg.: Selenite F broth, tetrathionate broth
• Differential media
  • Certain reagents or supplements when incorporated in to the media allow differentiation of different kinds of bacteria. Eg: blood agar. Some bacteria may hemolyse the red blood cells and others do not. So the hemolytic and non hemolytic bacteria can be differentiated.
• Special media
  • Media used for the cultivation of specific organisms. Eg: Dorset egg medium for Mycobacterium tuberculosis, PPLO agar for Mycoplasma, EMJH medium for Leptospira
• Transport media
  • Clinical specimens must be transported to the laboratory immediately after collection to prevent overgrowth of contaminating organisms or commensals. This can be achieved by using transport media. Such media prevent drying (desiccation) of specimen, maintain the pathogen to commensal ratio and inhibit overgrowth of unwanted bacteria. Some of these media (Stuart’s & Amie’s) are semi-solid in consistency. Addition of charcoal serves to neutralize inhibitory factors.