Lab Tests For Staph

1. Obtaining a Sample of Infected Tissue

For staph diagnosis, a sample of infected lesion (nasal secretion, pus, blood, cerebro-spinal fluid, urine, cell aspirate from lungs or bone) has to be obtained. Symptoms of staph infections are often caused by toxins, released from only few staphylococci, so a culture of obviously infected lesion may be negative! From this reason, samples from more than one lesion has to be provided, if possible.

What is Staphylococcus aureus?
Staph Epidemiology
Pictures of Staphylococcal Folliculitis

2. Gram Staining

A clinical sample is placed on a slide and rinsed with:

Crystal Violet
Iodine Solution (the Mordant)
Decolorizer (ethanol)
Safranin (the Counterstain)
Water

After this procedure, S. aureus and other staphylococci, should be blue, purple or violet under the light microscope, what is denoted as Gram-positive (Gram +), (picture 1). Bacteria, which are colored light pink are denoted as Gram negative (Gram -), (1). Gram staining is performed only, when mixed infection with Gram + and Gram - bacteria is expected.

Gram positive Staphylococcus aureus

Picture 1. Gram positive S. aureus,
light microscope, Magn. ~ 100x
Src: phil.cdc.gov

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What is Staphylococcus Aureus?

Staphylococcus aureus

Staphylococcus aureus (Greek staphyle = bunch of grapes, Latin coccus = spherical bacterium, aureus = golden), or golden staph (pronounced ’staff’), is the most common staphylococcus species causing infections in human.

Lab Tests for Staph
Staph Epidemiology

Staph Infections

S. aureus lives as the part of the normal skin flora in the nose or on the skin in 20-30% of healthy people (staph carriers), (1). However, in even slightly injured skin or mucosa, staph may cause styes, pimples, folliculitis, furuncles, boils (picture 1), swimmer’s ear, sinusitis, epiglotitis, whitlow, breast infection, impetigo, cellulitis, genital infection, or scalded skin syndrome. Infections of internal organs include urinary tract infections, arthritis, pneumonia, infection of veins (thrombophlebitis), lymph nodes (lymphadenitis) or lymph vessels (lymphangitis), bone infection (osteomyelitis) (picture 2), or life threatening sepsis (staph blood invasion), infection of heart valves (endocarditis), meningitis, “flesh eating bacteria” infection (necrotizing fasciitis), and toxic shock syndrome. S. aureus enterotoxins may cause food poisoning.

S. aureus and S. epidermidis are among the most common causes of hospital-acquired infections like bloodstream infections, infections of surgical wounds, or pneumonia (2). Staph may also appear in vascular and urinary catheters, prosthetic joints, and heart valves.

Staph skin infection usually appears as a red, warm, painful swelling with a blister, ulcer, or crust, and a drainage (picture 1). Infection may spread into deeper tissues, like bones (picture 2).