Background and Introduction:

A. Staphylococcus spp.and Streptococcus spp.

In the previous lab, coagulase tubes were inoculated with Staphylococcus spp. Coagulase is an enzyme produced by Staphylococcus aureus that effectively clots blood plasma. Coagulase positive Staphylococus spp. form a clot around themselves to protect against the host's immune defenses. A positive coagulase test is indicated by solidification of the rabbit plasma following inoculation and incubation.

Two separate tests for antibiotic susceptibility were also performed. These tests are used to differentiate between alpha and beta hemolytic organisms. Alpha hemolytic organisms (Streptococcus pneumoniae and Streptococcus mitis) are tested for their susceptibility to Taxos P (optochin). The beta hemolytic organisms (Streptococcus pyogenes and Streptococcus agalactiae) are tested with Taxos A (bacitracin). Examine the growth of bacteria around the Taxos A and Taxos P discs. A zone of inhibition around the Taxos disc indicates that the organism is susceptible/sensitive to that antibiotic.

Expected Results

Staphylococcus aureus: coagulase positive (plasma will form a clot in the tube)

Staphylococcus epidermidis: coagulase negative (plasma will remain fluid)

Streptococcus pneumoniae: Taxos P (optochin) sensitive (zone of inhibition)

Streptococcus mitis: Taxos P resistant (no zone of inhibition)

Streptococcus pyogenes: Taxos A (bacitracin) sensitive (zone of inhibition)

Streptococcus agalactiae: Taxos A resistant (no zone of inhibition)

B. Enterobacteriaceae and Pseudomonas

In the previous lab, Kligler's Iron Agar (KIA) was inoculated with various enterics and Pseudomonas aeruginosa. KIA is commonly used to distinguish members of Enterobacteriaceae from other Gram-negative bacilli and to identify lactose negative enterics, which are often more serious pathogens of the Gastrointestinal tract. KIA medium measures an organism’s ability to ferment glucose and lactose, to utilize protein, and to produce H 2S. Although interpretation of the medium is somewhat complicated, the ability to determine these three properties all in one test makes KIA media particularly useful. Please refer to the Atlas, the web page or the class notes to better learn how to read the results.

Expected Results

Escherichia coli: A yellow slant and a yellow butt indicate that both glucose and lactose are fermented. Bubbles, mostly in the butt of the tube, specify gas production.

Proteus mirabilis: A red slant and a yellow butt show that only glucose is fermented. Bubbles and a black precipitate in the butt indicate that gas and H 2S, respectively, are produced.

Shigella dysenteriae: A red slant and a yellow butt indicate that only glucose is fermented.

Pseudomonas aeruginosa: A red slant and a red butt show that no fermentation occurs.

Rachel Watson, M.S.
AG 5010
766-3524
Cell: 307-760-2942
rwatson@uwyo.edu