The difference between selective and differential media is mainly due to the following factors:
Intended use of the media: The purpose of both selective and differential media differs. A selective media only permits the development of particular organisms, while a differential media discerns one species of the organism from the others that are growing on a similar substrate.
Growth pattern: Different microorganisms show different growth patterns in the selective as well as differential media, which can be characterized by the size, shape, texture, colour and other notable features.
In this post, we will study the key differences between selective and differential media along with the comparison chart. Also, the definition and the examples of the two have been explained.
Content: Selective Vs Differential Media
|Properties||Selective media||Differential media|
|Meaning||It selectively isolates the particular strains of the organism||It separates closely related species|
|Alternative names||Specialized media||Indicator media|
|Growth pattern||It is characterized by the growth of single organism showing different growth patterns||It is characterized by the colour change in the media as well as the developing bacterial colonies|
|Supplements||Selective media is supplemented with certain growth promoting or inhibiting substances that can be organism or species specific||Differential media is supplemented with the indicator dyes that differentiates the bacterial isolate based on colour changes|
|Examples||Mannitol salt agar, tellurite media, Lowenstein-Jensen media, thiosulphate-citrate bile sucrose agar etc.||Eosin methylene blue agar, blood agar, deoxycholate citrate agar etc.|
|Specificity||It is more specific as it permits the growth of single microorganism||It is less specific, as closely related species may also develop|
Definition of Selective Media
It refers to the agar-based medium, which favours the growth of specific organisms by restricting the development of other undesired bacteria. Antibiotics in selective media are sensitive to the target organisms and resistant to the growth of other organisms.
It suppresses undesired commensals and contaminants. It also provides an enrichment broth. Generally, any media can be made selective by rendering some inhibitory substances, which in turn restricts the undesired organism’s growth. The selectivity of the medium depends on the addition of antibiotic agent, chemicals, indicator dyes and media with altered pH.
Definition of Differential Media
It refers to the agar-based media that discriminate one type of microorganism from the others growing in the same medium. The differentiation of the microorganisms depends upon their biochemical reaction with the indicators like neutral red, phenol red, methylene blue etc.
The differential media also detects the closely related organisms and the recombinant bacterial strains. The bacterial isolates exploit the dyes, metabolic substrates of the growth media.
The tendency of the bacteria to carry out specific biochemical processes is a property that helps in differentiating organisms. Hemolytic reactions, fermentation of specific sugar etc., are the factors deciding the growth pattern of the cultured bacterial isolates.
Examples of Selective Media
- Added with antibiotic: Charcoal blood agar media contain cephalexin that selectively isolates Bordetella pertussis. Kinsley’s medium contains polymyxin-B that enables the isolation of Bacillus anthracis.
- Added with chemicals: Mannitol salt agar medium contains 10% sodium chloride that helps in the isolation of Staphylococcus aureus. MacConkey agar media contains bile salts (less selective for isolating different members belonging to the Enterobacteriaceae family). Potassium tellurite media comprises 0.04% of potassium tellurite that is selective for the growth of Clostridium diphtheria.
- Supplemented with dyes: Crystal violet agar medium comprises 0.0002% CV that helps in the isolation of Streptococcus pyogenes. Lowenstein Jensen agar media comprises malachite green, selective for the isolation of Mycobacterium tuberculosis.
Examples of Differential Media
- With altered pH: Thiosulfate citrate bile salt sucrose agar media has an altered pH of 8.6 that supports the growth of vibrio cholera. Monsur’s tellurite taurocholate gelatin agar has an altered pH of 8.5 that promotes the growth of V. cholera. Lead acetate agar differentiates paratyphoid strains by forming lead sulphide. Eosine methylene blue agar helps in discriminating the colon typhoid organisms from pathogenic materials. Endo’s agar medium facilitates differentiation of colon colonies, particularly from water suspected of pollution.
Key Differences Between Selective and Differential Media
- Selective media selectively isolate the particular strains of the organism, while differential medium discriminates between the familiar species.
- The growth of a single organism showing different growth patterns indicates selective growth. In contrast, the growth of different bacterial colonies shows distinct growth patterns, and colour changes indicate differential growth.
- Selective media is incorporated with certain growth-promoting or inhibiting substances that can be organism or species-specific. Oppositely, the differential media is mainly incorporated with indicator dyes that differentiate the bacterial isolates based on the colour changes.
Despite many differences, both selective and differential media are commonly used in the medical field to check water pollution, food and dairy labs etc.