Question

1) What is the purpose of bacterial isolation?

 

2) Why is it necessary for medical diagnosis?

 

3) Please describe two selective media plates. How do they work and what information do they give you about the bacteria?

 

4) Please describe two differential plates. How do they work and what information do they give you about the bacteria?

 

5) Please describe two enrichment media plates. How do they work and what information do they give you about the bacteria?

Answer 1

1. Micro organisms are ubiquotous in nature and they generally found as mixed populations. So in order to study their growth,culture requirements, diseases caused by them,to study their roles in ecosystem etc.. they should be isolated from their environment.The isolation of bacteria requires maintainence of Pure cultures( culture of single strain).

The common methods employed to isolate the bacteria are SPREAD PLATE METHOD, POUR PLATE METHOD, which includes SERIAL DILUTION AND USE OF BACTERIAL LOOPS , STREAK PLATE METHOD,MICRO MANIPULATOR METHOD, ROLL TUBE METHOD.

2. In medical diagnosis the identification of the correct organism and their isolation plays a major role on which the entire treatment depends.Even the small excised part contains millions of microorganisms out of which the isolation of causitive organism is important.

Diagnostic medical microbiology deals with the study of etiological agents

3. A culture medium is a nutrient materiual used to grow the bacteria under laboratory conditions.Some bacteria can grow on any meida but some require special components to grow.The media can be solid, liquid or semi solid.

Selective media is a type of growth medium which allows the growth of the required bacteria by supressing the growth of the unwanted bacteria.For instance if a certain bacteria is resistant to any antibiotic like puromycin,ampicilin or carbanicilin etc then the particular organism can be grown by the addition of the paricular antibiotic on the other hand the bacteria which doesnt have that antibiotic resistance will fail to grow on it.

Examples:1 Bismuth sulfite agar is one of the medium used to identify the Gram negative bacterium Salmonella typhi from feces. Bismuth sulfite inhibits gram positive bacteria and other type of intestinal bacteria

2.Mannitol Salt Agar (MSA) is a selective media for pathogenic Staphylococci .It contains mannitol and Phenol red indicator.On this type of agar S.aureus grows by fermenting the mannitol which produce the acid which changes the colour of the medium from red to yellow

4.Differential media: This type of media is used to differentiate the closely related species of a bacteria or group of organisms.This media have certain compounds that allow bacteria to be visually distinguished from the surrounding organisms growing in the media

The 2 commomly used differential media under laboratory conditions are Blood agar media and EMB agar media

Blood agar media is used to identify the bacterial species which destroys the red blood cells such as Streptococcus pyogenes

EMB agar (Eosin MethyleneBlue agar) is used for the detection and isolation of coliforms.This media contains lactose as a sugar source. The 2 dyes utilised are toxic and dosent allow the gram positve bacteria to grow.The microbes that ferment the lactose will lower the pH of the media and appears to be coloured and the bacteria that doesn not ferment the lactose will increase the pH and appear to be colourless

5. Enrichment media:

When a basal media( simple media) is added with additional nutrients then it is called as Enriched media.The 2 such enriched plates are Chocolate agar, Thayer martin agar

Chocolate agar is used to grow the fastidious organisms like Nisseria or Haemophilus species.It is the blood agar in which the blood cells are lysed while preparing which gives the charecteristic chocolate colour appearance

Thayer-Martin Agar also called as VPN agar is Mueller hinton agar which has antibiotics and 5% sheep blood.Its is used for isolating pathogenic Nisseria species