33 Identification of staphylococci
33.1 Identification of staphylococci
In this laboratory session, you will be identifying a pathogen of the skin and circulatory system, Staphylococcus aureus, using differential media (Task 4A), biochemical assays (Task 4B), and a PCR-based technique called MVLA typing (Task 4C).
Staphylococcus aureus and Staphylococcus epidermidis are both members of the normal human skin flora (commensals), and dangerous pathogens often associated with nosocomial infections. Although S. aureus and S. epidermidis appear very similar when examined under the microscope (as in the Gram-staining procedure you have carried out previously, Section 18.3), they can be distinguished from one another, and from micrococci (also commonly found in the human skin flora) on the basis of several different phenotypic characteristics (Table 33.1).
| Characteristic | S. aureus | S. epidermidis | Micrococci |
|---|---|---|---|
| Catalase activity | + | + | + |
| Coagulase production | + | - | - |
| Thermostable nuclease production | + | - | - |
| Lysostaphin sensitivity | + | + | - |
| Anaerobic utilisation of glucose | + | + | - |
| Anaerobic utilisation of mannitol | + | - | - |
We will use these characteristics of the staphylococci to help identify an unknown in Case Study 5. Which are the most useful diagnostic characteristics?
CASE STUDY 5: Epidemiology of an MRSA outbreak
A number of patients at the local hospital have been acquiring secondary infections after seeing Dr. Staff for routine medical procedures. However, many of the patients also move in the same social circles, and so it is not clear whether these infections are nosocomial or community-acquired.
As a consulting microbiologist from the NHS, you have been asked to help the hospital discover the source of the pathogens, so that they can clean up their protocols for infection control if necessary. The first step is to determine whether the bacteria isolated from the patients are all genetically identical (from the same source) or different (from different sources).
You have already streaked your unknown organism on a selective and differential medium (MSA); you will now observe these plates and interpret the results (Task 4A).
Your goal is to identify the strains involved in the outbreak. You suspect that it is MRSA, but you must first confirm this. We have already determined that the patient isolates are in fact Staphylococci using a lysostaphin test (Section 34.1.1); you will next determine whether they are Staphylococcus aureus or other Staphylococci using a diagnostic coagulase test (Task 4B). Finally, you will further identify your Staphylococcus aureus isolate using MVLA typing (Task 4C).