Patient X has a large wound with opaque, yellowish pus oozing form the wound. A sample is sent to your lab. Outline the steps you would take to identify the organism causing a suspected infection. Include in your outline stains that would be used and what information you would gather from the results of the stains. For example, would the reaction be positive or negative, and why? Consider cell shape and cellular arrangement. Explain why certain types of media and biochemical tests would be used. Describe cultural and physiological characteristics of each organism.

This question is fairly dense, but the steps taken will be fairly straightforward in a prototypical case. If Patient X is discharging a yellowish pus from a wound, we can be certain that there is an infection. To outline what we would need to identify said infection, there are tests we would need to order to identify the strain of bacterium causing pathogenesis. With that said, if a patient presents with a yellow discharge oozing from a wound, my initial thought as a health are provider is that my patient could be presenting with a staphylococcal infection of the skin. Staphylococcus aureus is incredibly contagious and spreads quite easily in a hospital setting. Methicillin-resistant S. aureus (MRSA) is especially dangerous in the hospital or community setting.
To be able to stop these specific strands of infection, as health care providers, we are required to find the specific antibiotic to combat them; this is where lab testing becomes paramount.
We would first take a culture of the infection to see if the strain of bacteria is gram-positive or gram-negative. Under a microscope, we would see that gram-positive Staph species have cellular arrangements that are grape-like in appearance. On a blood agar test, we would then see that the gram-positive strain has an opaque-white or cream color. We would then use a catalase test to distinguish Staph from Streptococcus, which is also a genus of gram-positive cocci and a common cause of skin infections. Streptococcus can cause “strep throat” and toxic-shock syndrome if left untreated, though S. aureus can cause toxic-shock as well. Staph species are catalase-positive, while Strep species are catalase-negative. Again, it is important that we distinguish between the two due to the required antibiotics to treat the infection. Other steps would then be taken to see if the strain of bacteria is coagulase-positive or coagulase-negative, which is unique to certain species of bacteria. Passive agglutination is the test that would be used to identify whether a strain is coagulase-positive or negative. If the sample is coagulase-positive, it is typically S. aureus.
Another important test in determining the strain of bacteria is the mannitol salt agar (MSA). The MSA test is another way of testing whether a strain is coagulase-positive or negative. Many staph species are able to grow on an MSA medium because they can exist in high concentrations of sodium chloride or specifically, (7.5% NaCl). The difference comes in, in that coagulase-positive strains of staphylococcus such as S. aureus ferment mannitol. Mannitol will turn yellow on the plate and distinguish itself as being S. aureus. A coagulase-negative staph infection will stay red and will be readily identifiable.
This is the basic outline of a test called for in identifying a bacterial infection, using staph as the main example, as it is so often encountered. With these markers that each strain of bacteria produces, common physical presentations on the patient often manifest themselves and help in identifying the underlying pathology.