Staph is short for the bacteria Staphylococcus, which can cause a wide variety of infections including skin infection, pneumonia and blood poisoning. Staphylococcal infections are diagnosed using patient's sample collected from the site of infection. Along with some preliminary tests, a small portion of the sample is placed on growth medium plates, which contain gels that promote the growth of bacteria. These plates are incubated at 98.6 degrees Fahrenheit for 18 hours and observed for the presence of staphylococcal colonies. These colonies are further confirmed using biochemical tests.
Catalase
Catalase is an enzyme that breaks down a chemical called hydrogen peroxide into water and oxygen, and is commonly used to differentiate staph from another bacteria known as streptococci. According to Georgia Highland College, a small of the bacterial colony is taken and mixed with a small amount of hydrogen peroxide on a slide and observed for the presence of bubbles, which indicates a positive test for staph.
Fermentation Tests
Fermentation tests check for the ability of staph strains to break down certain sugars, such as mannitol and trehalose, into alcohol. A small amount of staph colony is placed into another tube containing growth medium with 10 percent sugar. The tubes are incubated at 98.6 degrees Fahrenheit for 18 hours. If the tube turns yellow, the bacterium is confirmed to be staph.
Coagulase Test
A small amount of the bacterial colony is taken from the original growth medium plate and mixed with latex particles on a slide. The slide is then rotated for 20 seconds and observed for clumping. According to Mackie and McCartney's "Manual of Practical Medical Microbiology", although 15 percent of ordinary strains of staph and many more of Methicillin-resistant Staphylococcus aureus (MRSA) may give negative reactions, this is one of the main methods used in the laboratory to identify and confirm staph.
Novobiocin Sensitivity
Sensitivity to the antibiotic novobiocin is used to distinguish between different species of Staphylococcus. A small amount of the bacterial colony isolated from the patient is placed in growth medium plates with the antibiotic novobiocin, and presence of growth after incubation at 98.6 degrees Fahrenheit for 18 hours indicates the presence of the species Staphylococcus saprophyticus.
Urease Test
Urease broth is a type of growth medium that tests the ability of bacteria to produce an enzyme, called urease, which breaks down urea into ammonia and carbon dioxide. The broth contains a dye called phenol red that turns pink in presence of ammonia, thereby indicating a positive reaction.
According to an article published in August 2007 edition of the "Journal of Clinical Microbiology," the urease test is used to distinguish between different species and subspecies of staph such as S. cohnii ssp. Cohnii, S. cohnii subsp. ureolyticus, and S. epidermidis isolates that have atypical results for other tests.
References
- Catalase Test
- "Mackie and McCartney's Manual of Practical Medical Microbiology"; J. G. Collee, A. G. Fraser, B. P. Marmion, A. Simmon; 1996
- "Journal of Clinical Microbiology"; Simplified and Reliable Scheme for Species-Level Identification of Staphylococcus Clinical Isolates; Natalia Lopes Pontes Iorio et al; August 2007
