By Dr Ananya Mandal, MD
Staphylococcus aureus can infect in a variety of ways leading to diverse manifestations. In addition, many humans carry strains of this bacteria on their skin, nose and pharynx as harmless commensal bacteria. This makes diagnosis of S. aureus from an infection difficult.
Strains of S. aureus are known to enter through the breaks in the skin to cause localized infections or spread via blood to cause more generalized infections like that of blood (sepsis), bone (osteomyelitis), brain (meningitis), lungs (pneumonia) etc. Individuals with a compromised immune system are particularly vulnerable.
For diagnosis, an important first step is isolation of the bacteria from appropriate specimens. This is followed up by identification of S. aureus toxins or measurement of antibodies in special cases, such as deep-seated infections or food poisoning.
Steps in diagnosis
Steps in diagnosis of S. aureus infections include:
Collection of specimens
This depends on the area of the body affected. For example, those with a skin infections or throat, nostrils and wound infections need to swabbed for pus and other discharge with the bacteria. Swabs consist of a sterile absorbent cotton tipped sticks. Those with a urinary tract infection need to provide a urine samples in sterile containers and those with a generalized blood infection need to provide blood samples. Blood samples are then transferred to blood culture bottle.
Identification of the bacteria
A small portion of the sample is swabbed onto a glass slide. This is then stained with Gram stain or dyes like crystal violet and basic fuschin and viewed under the microscope. S. aureus is Gram positive and stains blue or purple and appears as small round cocci or short chains and most commonly as grape-like clusters. Since S. aureus may be normally present on skin and mucous membranes, this test is not always confirmatory.
Confirmation of diagnosis
To confirm a diagnosis, the sample from the patient is placed onto a culture media. This could be a liquid or gel that provides sources of nutrition, carbon, energy and nitrogen for the bacteria to grow. For S. aureus, the medium used is suffused with blood and lactose. Also commonly used is the mannitol salt agar, which is a selective medium with 7–9% salt or sodium chloride that allows S. aureus to grow selectively. These media are placed on petri dishes and swabbed with the sample. The dishes are then incubated overnight at 37 degrees Celsius. After a set period of time the typical golden colonies of S. aureus are seen. These are then stained with Gram stain for confirmation and also undergo specific characteristic tests like the catalase test or the coagulase test for diagnosis.
Rapid diagnostic tests
These help in detection of the bacteria in real-time. These techniques include Real-time PCR and Quantitative PCR and are increasingly being employed in clinical laboratories.
Identification of toxins
This is important for more severe cases like toxic shock syndrome and food poisoning. Toxins produced by S. aureus, such as enterotoxins A to D and TSST-1 may be identified using agglutination tests. The tests are determined by clumping of the latex particles by the toxins present in the samples.
Antimicrobial assay studies
Many of the S. aureus strains are resistant to antibiotics. These assay studies help determine the specific susceptibility to antibiotics of the infected strain. Antibiotics like penicillin, amoxicillin, methicillin, first-generation cephalosporins, bactrim and vancomycin are commonly tested.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)