- Staphylococcus aureus are catalase-positive, gram-positive cocci which frequently colonize the skin and mucosa of humans and animals.
- However, it is a highly successful opportunistic pathogen and thus involved in various diseases including toxin-mediated diseases (food poisoning, toxic shock syndrome, scalded skin syndrome), pyogenic diseases (impetigo, folliculitis, furuncles, carbuncles, wound infections), and other systemic diseases.
- The species are characterized by the presence of coagulase, protein A, and species-specific ribitol teichoic acid with N-acetylglucosamine residues (“polysaccharide A”).
Image Source: Microbiology in pictures and Sanmukh Joshi
Laboratory Diagnosis of Staphylococcus aureus
- Microscopy is useful for pyogenic infections but not blood infections or toxin-mediated infections.
- A direct smear for Gram staining may be performed as soon as the specimen is collected.
- The Gram stain showing typical Gram-positive cocci that occur singly and in pairs, tetrads, short chains, and irregular grape-like clusters can be suspected to be S. aureus.
- The organism is isolated by streaking material from the clinical specimen (or from a blood culture) onto solid media such as blood agar, tryptic soy agar or heart infusion agar.
- Specimens likely to be contaminated with other microorganisms can be plated on mannitol salt agar containing 7.5% sodium chloride, which allows the halo-tolerant staphylococci to grow.
- The inoculated plates should be incubated at 35°C to 37°C for 24 to 48 hours.
- On Blood agar, growth occurs abundantly within 18 to 24 hours. Round, raised, opaque, yellow to golden yellow colonies of 1-2mm in diameter are seen with or without beta hemolysis.
- On Mannitol Salt Agar (MSA), a selective media, S. aureus being a mannitol fermenting bacteria, gives yellow or gold colonies.
- An 18 h to 24 h culture can be used as the inoculum for additional tests.
- Isolates should be subcultured at least once on nonselective medium after initial isolation before being used in a diagnostic test that requires pure culture or heavy inoculum.
The presumptive identification of S. aureus rests on the isolation of:
- Large mannitol fermenting colonies on MSA
- Gram-positive cocci in clusters
- Catalase-positive organisms
- Coagulase-positive organisms
Confirmatory tests include biochemical tests, molecular probes, or mass spectrometry.
Tests for clumping factor, coagulase, hemolysins, and thermostable deoxyribonuclease are routinely used to identify S. aureus.
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 the clumping of the latex particles by the toxins present in the samples.
- Commercial latex agglutination tests are available for this purpose.
Nucleic acid amplification tests
- Commercial nucleic acid amplification tests are available for the direct detection and identification of S. aureus in clinical specimens.
- Whereas the earlier versions of these tests required manual extraction of bacterial DNA and testing multiple specimens in large batches, integrated processing of specimens (extraction, gene amplification, and target detection) is now performed on highly automated platforms with disposable reagent strips or cartridges.
- They are useful for screening patients for carriage of methicillin-sensitive S. aureus (MSSA) and MRSA.
- Localized infections are managed by incision and drainage while antibiotic therapy is indicated for systemic infections.
- Oral therapy can include trimethoprim-sulfamethoxazole, doxycycline or minocycline, clindamycin, or linezolid; vancomycin is the drug of choice for intravenous therapy, with daptomycin, tigecycline, or linezolid acceptable alternatives.
- Hospital- and community-acquired infections with methicillin-resistant Staphylococcus aureus (MRSA) are a significant worldwide problem. Hence, empirical therapy should include antibiotics active against MRSA strains.
- Treatment is symptomatic for patients with food poisoning (although the source of infection should be identified so that appropriate preventive procedures can be enacted)
Prevention of Infection
- Proper cleansing of wounds and use of disinfectant help prevent infections.
- Thorough hand washing and covering of exposed skin help medical personnel prevent infection or spread to other patients.
- Patients and staff carrying epidemic strains, particularly MRSA, should be isolated.
- Patients may be given disinfectant baths or treated with a topical antibiotic to eradicate carriage of MRSA.
- Infection control programs should be used in hospitals.
- Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. (2013). Medical microbiology. Philadelphia: Elsevier/Saunders.
- BLAIR J. E. (1958). Laboratory diagnosis of staphylococcal infections. Bulletin of the World Health Organization, 18(3), 291–307.