Laboratory Tests
The purpose of culturing wound infections is to identify the bacteria causing the infection and to determine the susceptibility of the microorganism to available antibiotics. If an infection is due to MRSA, it should be investigated to determine the source of the infection. This is especially important in CA-MRSA to prevent further cases from occurring.
The primary laboratory tests performed are:
- Cultures of the affected areas. Fluid or pus from a wound, sputum, blood, joint fluid, or even breast milk (in the case of an infected breast) is collected and spread onto a thin layer of nutrient gel and/or grown in a nutrient broth. Sometimes, multiple samples are collected to evaluate different body sites or to attempt to detect bacteria that may be present in small numbers.
- Nasal cultures (collected by inserting a swab inside the nose) used to screen healthy people may also be requested to determine whether someone has been colonized with MRSA and is a carrier. The cultured samples are incubated and examined for the growth of characteristic S. aureus colonies. If they are present, susceptibility testing is performed to determine whether the strain is MRSA.
- Nasal swabs may be collected to detect MRSA colonization based on rapid molecular tests, which do not grow the bacteria but detect their presence and antibiotic resistance by detecting the genes responsible for the meticillin resistance.
Identifying MRSA can sometimes be challenging. The population of S.aureus that a person has tends to be mixed. This means that even if a patient has CA-MRSA or hospital-acquired MRSA, not all of the S.aureus present will be equally resistant. Since resistant strains may grow more slowly than susceptible strains, laboratories may employ additional methods for detection.
A variety of methods may be used to track different strains of MRSA. These are used in the investigation of the spread of MRSA within a community or region but are not used in the treatment of an individual patient.