Bacteria under Microscope

MRSA (methicillin-resistant Staphylococcus aureus)

methicillin-resistant Staphylococcus aureus   
                         microscopy, gram-positive cocci
Scanning electron micrograph (SEM) of Staphylococcus aureus. Numerous clumps of methicillin-resistant Staphylococcus aureus bacteria, commonly referred to by the acronym, MRSA; Magnified 2381×.
Text&Photo CDC, Public Health Image Library (PHIL)
This image is in the public domain and thus free of any copyright restrictions.
Public Health Image Library (PHIL)


Gram-positive cocci


Microscopic appearance:

Cocci in clusters, diplococci and single cocci


Clinical significance:

  • Staphylococcus aureus bacteria are found on the skin of nearly all individuals. However, MRSA is a strain of these bacteria that has become resistant to certain antibiotics. These antibiotics include methicillin and other more common antibiotics such as oxacillin, penicillin and amoxicillin. Staph infections, including MRSA, occur most frequently among persons in hospitals and healthcare facilities (such as nursing homes and dialysis centers) who have weakened immune systems, however, the manifestation of MRSA infections that are acquired by otherwise healthy individuals, who have not been recently hospitalized, or had a medical procedure such as dialysis, or surgery, first began to emerged in the mid- to late-1990's. These infections in the community are usually manifested as minor skin infections such as pimples and boils.
    Text: PHIL

Colony morphology:

Staphylococcus aureus identification
A Methicillin-resistant Staphylococcus aureus on sheep blood agar. Only weak beta-hemolysis. This strain produces only narrow zones of hemolysis that do not extend much beyond the edge of the colonies. Cultivation 24 hours, aerobic atmosphere, 37°C.  
B MRSA on oxacillin resistance screening agar. Medium contains 5.5% of NaCl (salt inhibits most bacteria other than staphylococci) and oxacillin at 2 mg/litre (inhibits methicillin sensitive Staphylococcus aureus). Typical colonies of methicillin-resistant Staphylococcus aureus(MRSA) are intense blue (mannitol fermentation in presence of aniline blue as a pH indicator). Cultivation 24 hours in an aerobic atmosphere, 37°C.  
C Disk diffusion susceptibility test on methicillin-resistant Staphylococcus aureus grown on Mueller-Hinton agar with co-trimoxazole (susceptible), erythromycin (resistant), clindamycin (resistant), gentamicin (susceptible), cefoxitin (resistant), and tetracycline (susceptible).