Cocos Gram-positivos

Staphylococcus aureus

Gram-positive cocci in clusters

Identification & Growth

Identification

Catalase-positive, coagulase-positive (tube or latex test)
DNase-positive, ferments mannitol on mannitol salt agar
Remains susceptible to novobiocin

Growth Conditions

Blood agar: golden-yellow colonies, 1–3 mm, beta-hemolysis
Mannitol salt agar (7.5% NaCl): yellow colonies with medium colour change
35–37 °C, aerobic incubation, 18–24 h

Clinical Significance

Skin and soft-tissue infection, abscesses, furunculosis (outpatient setting)
Catheter-related bacteremia, endocarditis, osteomyelitis, pneumonia

Intrinsic Resistance

Aztreonam, colistin and nalidixic acid have no activity against Gram-positive organisms (EUCAST Expected Resistant Phenotypes v1.2)
Temocillin and the polymyxins should not be reported

Bench Alerts

Cefoxitin is the surrogate marker for MRSA: if resistant, report resistance to all beta-lactams (except ceftaroline)
Erythromycin R + clindamycin S requires a D-test for inducible MLSb resistance
Disk diffusion for vancomycin is not valid — MIC testing must be used

Clinical Notes

Clinical presentations

Causes skin and soft tissue infections (furuncles, cellulitis, abscesses), often community-associated (CA-MRSA).
It is a leading cause of intravascular catheter-associated bacteremia and acute endocarditis in people who inject drugs.
Causes severe necrotizing pneumonia, sometimes post-influenza, and hematogenous osteomyelitis/septic arthritis.
Produces toxin-mediated syndromes such as staphylococcal toxic shock syndrome and staphylococcal scalded skin syndrome.

Specimens and collection

Paired blood cultures (aerobic/anaerobic) before starting antibiotics are essential to confirm bacteremia and guide treatment duration.
Skin lesion swabs should be collected from the base of the abscess or drained purulent material, avoiding surface contamination.
Anterior nares swab is used for MRSA colonization screening in hospital surveillance protocols.
Synovial fluid and bone fragments should be sent promptly without transport media additives that could inhibit growth.

Epidemiology and at-risk populations

About 30% of the general population is asymptomatically colonized in the anterior nares.
People who inject drugs, hemodialysis patients, and central venous catheter carriers have increased bacteremia risk.
CA-MRSA (often PVL-gene positive) affects healthy young adults and athletes in close-contact settings.
Hospital MRSA outbreaks are linked to hand hygiene lapses and undetected colonization of healthcare workers.

Resistance and therapeutic implications

Methicillin resistance (MRSA), mediated by the mecA/mecC gene (PBP2a), confers class resistance to all beta-lactams except ceftaroline/ceftobiprole.
Strains with inducible clindamycin resistance (MLSb phenotype) must be detected by the disk approximation D-test before reporting clindamycin as susceptible.
Vancomycin-resistant isolates (VISA/VRSA) are rare but require confirmation and reporting, as they radically change therapeutic choice.
EUCAST recommends testing oxacillin/cefoxitin susceptibility as a screening marker for methicillin resistance.

Bench and reporting notes

Golden, beta-hemolytic colonies on blood agar, catalase-positive and coagulase-positive (tube or latex) confirm the species.
The cefoxitin disk test is the preferred method to detect mecA-mediated resistance in routine testing.
Blood isolates should have identification and susceptibility reported with priority due to the mortality impact of bacteremia.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Staphylococcus aureus in the lab?

Staphylococcus aureus is identified through catalase-positive, coagulase-positive (tube or latex test), dnase-positive, ferments mannitol on mannitol salt agar, remains susceptible to novobiocin.

What are the intrinsic resistances of Staphylococcus aureus?

This organism is naturally resistant to aztreonam, colistin and nalidixic acid have no activity against gram-positive organisms (eucast expected resistant phenotypes v1.2), temocillin and the polymyxins should not be reported. These drugs should not be reported as susceptible.

Where is Staphylococcus aureus commonly found?

It is typically associated with skin and soft-tissue infection, abscesses, furunculosis (outpatient setting), catheter-related bacteremia, endocarditis, osteomyelitis, pneumonia.

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