Staphylococcus Identification

There are many practical applications for the identification of prokaryotes.  Medical laboratories identify pathogenic organisms so that infections caused by the pathogen may be treated appropriately.  Environmental laboratories identify prokaryotes in natural samples so that environmental diversity and/or disturbance may be studied. 

Prokaryotes can be identified using a variety of different techniques.  Molecular diagnostic techniques involve isolating the DNA, RNA or protein of the prokaryotic cells and identifying them based on sequence motifs.  Serological diagnostic techniques involve observing reactions between prokaryotes and known antibodies.  The most traditional method of identifying prokaryotes, however, is by determining the morphological, biochemical and/or physiological properties of the organism and comparing patterns of those properties to those of known organisms. 

The three methods described above differ in accuracy, complexity, and cost. The molecular and serological methods are quite accurate and results can be obtained rather rapidly, however, they tend to be more expensive than the traditional methods.  The traditional methods of identification typically involve a great number of tests that require careful interpretation.  These tests are less expensive than the molecular and serological tests, but accurate results take up to 2 weeks to obtain.

Identification of an unknown prokaryote involves a few steps.  First, a sample must be taken and the prokaryote of interest must be enriched in culture.  Once the organism is enriched, then a pure culture must be obtained by streaking for isolation.  A diagnostic stain usually follows to make certain the culture is pure and to determine the cell wall type, morphology and/or presence of endospores.  Finally, appropriate biochemical tests are selected, performed and interpreted.  The results are compared to results of known organisms in the Bergeys Manual of Determinitive Bacteriology.  This book contains the biochemical properties of most of the common bacterial lab strains.

In this lab, you will identify the species of Staphylococcus that is part of your normal flora using a serological approach and a traditional biochemical approach.

Protocol

Day 1 (April 8): 

Today you will enrich for Staphylococci using m-Staphylococcus broth (Difco).  M-staph broth enriches for Staphylococci by providing nutrients appropriate for its growth.  Since staphylococci are salt tolerant, the addition of 7.5% salt makes this media selective.

1. Obtain 1 sterile swab and one M-staph broth.  Label your broth appropriately.
2. Moisten the swab by dipping it in your saline.
3. Swab in the interior of one of your nostrils.
4. Place the swab back in the broth and incubate at 37 degrees C for 24-48 hours.

Day 2 (April 13):

Today you will streak your Staphylococcus culture for isolation on Mannitol Salt Agar (MSA) media.  MSA also has nutrients appropriate for the growth of Staphylococcus and 7.5% salt, which will further select for Staph.  Additionally, this media contains mannitol and a phenol red indicator that will turn yellow in the presence of mannitol fermenting Staphylococcus.  Hence, this media is selective and differential.  It differentiates Staphylococcus on its ability to ferment mannitol.

1. Obtain your M-staph culture from the incubator and obtain an MSA plate.
2. Label your MSA plate.
3. Streak your plate for isolation and incubate at 37 °C.
4. Have a member of your row streak a control plate.  SA will be streaked to show a positive result for fermentation and SE will be streaked to show a negative result for fermentation.

Day 3 (April 15):

Today you will streak a Blood Agar Plate (BAP) and a slant.  BAP is a differential media.  It differentiates bacteria on their ability to lyse red blood cells.  The media contains sheep’s blood and nutrient agar.  If a bacterium lyses the RBCs then a zone of clearing (or sometimes a green zone) is formed around the colonies.  A novobiocin disk will also be placed on the BAP.  Novobiocin is an antibiotic that many Staphylococcus strains are sensitive to with the exception of one, Staphylococcus saprophyticus.  You will use the Kirby Baur method to determine the sensitivity of your Staph strain to novobiocin.  The slant will be used to create a culture that can be Gram stained and used for the serological test.

1. Obtain your MSA culture from the incubator.  Also obtain a slant and a BAP plate.
2. Observe and record the results of your MSA plate.  Compare your plate to the control plate.  SA on the control was positive for mannitol fermentation and SE was negative.
3. Label your plate and slant.
4. Streak a BAP plate for isolation and place a novobiocin disk in the first streak zone. 
5. Prepare a slant culture.
6. Put your cultures in the 37 °C incubator.
7. Have a member of your row prepare a control plate and a control slant (SA and SE). 

Day 4 (April 20)

Today you will do a Gram Stain and a coagulase test and determine the presumptive identification of your organism.  The Gram stain will confirm that you have a pure culture and it will also confirm that you have a Gram positive coccus (morphology and gram reaction for Staphylococcus).  The coagulase test is a differential test that is used to determine if bacteria produce the enzyme coagulase.  The test involves putting the bacterium in rabbit plasma and determining if it can produce a clot. 

1. Obtain your BAP culture and your slant from the incubator.  Also, obtain a coagulase tube.
2. Observe and record the results of your BAP plate with the novobiocin disk.  Compare your plate to the control plate.  SA on the control was positive for RBC lysis and SE was negative.
3. Do a Gram stain on your slant culture and observe under 100X.  Record the results.
4. Suspend one small loopful of your culture from the slant in the rabbit plasma.  Break up any clumps. 
5. Label your tube, put a piece of parafilm over the top to prevent evaporation, and incubate in the shaking incubator.
6. Have a member of your row prepare a control tube for coagulase (SA and SE). 
7. Save your slant by putting it in the fridge for next lab period.
8. Use Table 1 and Bergey’s manual to determine the identification of your Staphylococcus culture.

Day 4:  (April 20) Today you will also perform a serological test to confirm that you do or do not have Staphylococcus aureus.  You will be using a test called Staphyloslide.  Staphyloslide contains antibodies attached to microscopic latex beads.  The antibodies are specific to Staphylococcus aureus only.  If your test organism is Staphylococcus aureus then the antibodies will agglutinate the SA cells and clump the latex beads together to give a speckled appearance on the test card.

1. Obtain your slant from the fridge.
2. Put one drop of Staphyloslide reagent on one circle of the card.  Suspend a small amount of culture in the blue reagent and spread it around on the card.  Break up any culture clumps.
3. Wait about 5-10 minutes and observe clumping.  If blue speckles/clumps appear then it is positive for Stayphylococcus aureus.

April 22 (discussion of results from project)

1.  Obtain your coagulase test from the incubator.
2. Observe and record the results of your coagulase test.  Compare your tube to the control tubes.  SA is positive for coagulase and SE is negative.

Table 1.  Presumptive Identification Chart for Staphylococcus