Lab 6

Lab 6 – Selective and Differential Media

  Ninoska Garcia-Ortiz 063 053 2

Objective: • •

To learn the difference between selective and differential media. To learn how to use both b oth types of media

Theory:

Selective media – one or several chemical inhibiting growth of most types of microorganisms, exception select few Differential media – recognition (differentiation) between 2 or more types of microorganisms. Some media can be both b oth selective and differential. Four types of agar: Mannitol Salts Agar (MSA) – used for Staphylococcus. for Staphylococcus. Differentiates between S. aureus and S. Epidermis. High concentration (7.5%) of sodium chloride selects for micrococci staphylococci staphylococci.. Mannitol Mannitol acidifiers acidifiers (eg. S. aureus) aureus) have have yellow yellow zones zones around around their  their  colonies due to change of the phenol red indicator by the acid. Macconkey Agar (MA) – used for Gram Gram negative enteric bacteria. Differentiates between lactose lactose fermenters fermenters and and nonfermenter nonfermenters. s. Gram – enteric enteric bacilli bacilli grow. grow. Gram + bacteri bacteriaa growth growth inhibited by crystal violet. violet. Differenti Differentiates ates between brick red lactose fermenting fermenting (lac+) and transparent transparent lactose lactose nonfermenting nonfermenting (lac-) bacilli. bacilli. Pathogenic Pathogenic enteric bacteria bacteria tend to be lac- bacilli (eg. Shigella and Salmonella) Tellurite Glycine Agar (TGA) – used for coagulase-positive staphylococci. Will grow grow as  black colonies. Coagulase-negative and other microorganisms microorganisms are inhibited. Blood Agar Agar (BA) – used to grow nutritional nutritionally ly demanding demanding organisms. organisms. Different Differentiates iates   between between those that lyse red blood and those that do not. Alpha-hemoly Alpha-hemolytic tic organisms organisms  produce green, olive zone around colonies due to oxidative effect of peroxide waste on heme. Beta-hemolytic organisms produce clear zone around their colonies, due to lysis of  red blood cells by bacteri bacterial al exoenzymes exoenzymes known as hemoly hemolysin sins. s. GammaGamma-hem hemoly olytic tic organisms will exhibit no detectable change around colonies.

Questi Que stions ons :

1 –  Mannitol Salts Agar – selective media. Specifically selects for micrococci and staphylococci by way of the nutrients that it • contains. Macconkey Agar - both selective and differential. Selective Selective due to the fact that it allows Gram negative negative enteric bacilli to grow and inhibits • the growth of Gram positive bacteria. Differential because it allows for differentiation of lactose fermenting and non lactose • fermenting Gram negative enteric bacilli such as E.coli as E.coli and P.vulgaris and P.vulgaris Tellurite Glycine Agar - selective media. specifically specifically tests for coagulase-po coagulase-positiv sitivee staphylococci staphylococci (which will grow after 24hrs.) • Coagulase-negative staphylococci and other organisms are inhibited. Blood Agar – differential media. Enriched medium which will allow for the growth of most micro-organisms. • Serves to differentiate between alpha-hemolytic organisms that produce a green olive • zone, beta-hemolytic organisms that will produce a clear zone and gamma haemolytic organisms that will exhibit no detectable change. 2 –  Alpha hemolysis - caused by oxidative effect of peroxide waste on heme Beta-hemolysis – caused by lysis of the red blood cells b y the bacterial exoenzymes known as hemolysis. 3 –  Macc Macconk onkey ey Agar, Agar, whic which h is both both a sele select ctiv ivee and a diff differ erent entia iall medi media, a, could could be used used to   E. coli. This medium allows for the differentiation of lactose determine the presence of  E. fermenting and non lactose fermenting Gram negative enteric bacilli such as E.coli as E.coli and  P.vulgaris. 4 –  The results for all the plates were as expected, except the growth on the Tellurite Glycerine Agar    plates. plates. The plate plate with with the the S. eareus gave the expected results, however; the plate with the S. epidermis had similar results. results. There was not supposed to be any growth in this plate. The growth can perhaps be explained by the fact that the plate was not inspected in the recommended 24 hour  time frame, frame, but instead 1 week later. later. This theory is further further enforced enforced by the evident “slower “slower”” growth on the plate that should not have had any growth.

Conclusion:

The objective of the lab, • •

To learn the difference between selective and differential media, and To learn how to use both b oth types of media,

was achieved. The use of four different types of Agar were used to attain the objective. In the case of the Tellurit Telluritee Glycerine Glycerine Agar, Agar, we saw the importance of analyzing analyzing results of time sensitive experiments. The assumption that the growth was due to prolonged exposure is is based on the fact that we were already told that coagulase-positive staphylococci would grow black  colonies within 24 hours. The fact that there was growth in both plates, plates, is an indication that the experiment experiment was not done correctly. correctly. In this case, we are able to identify the infraction, infraction, analysis analysis one week later, instead of 24 hours later.