Staphylococcus aureus (staph) is a common bacterium that can live harmlessly on the skin or in the nose of approximately 30% of people. This organism can cause a range of infections if it enters the body through a cut or medical device. S. aureus exists in two major variants: one sensitive to standard antibiotics and one resistant. This key difference dictates how the infection must be treated.
Defining the Organisms
The distinction between Methicillin-Sensitive Staphylococcus aureus (MSSA) and Methicillin-Resistant Staphylococcus aureus (MRSA) lies in their genetic makeup regarding drug sensitivity. Both are the same species of bacteria, but MSSA remains susceptible to a class of antibiotics that includes methicillin and its derivatives.
Methicillin-Resistant S. aureus (MRSA) is the resistant variant, having acquired a mobile genetic element called the staphylococcal cassette chromosome mec (SCCmec). This element carries the mecA gene, which is the fundamental biological difference between the two strains. The mecA gene produces an altered penicillin-binding protein (PBP2a) in the bacterial cell wall.
This modified protein prevents beta-lactam antibiotics, such as methicillin, oxacillin, and penicillin, from effectively binding to and destroying the bacterial cell wall. The presence of the mecA gene provides MRSA with a defense mechanism, making the standard arsenal of antibiotics ineffective. Though the resistance is named after methicillin, the resistance extends to nearly all penicillins and cephalosporins, which are highly effective drugs against MSSA.
Clinical Manifestations and Transmission
Both MSSA and MRSA cause a wide spectrum of infections that are often clinically indistinguishable without laboratory testing. The most common manifestations are skin and soft tissue infections, which may appear as boils, abscesses, or cellulitis. These infections typically present with redness, swelling, warmth, and pain at the site, sometimes with a pocket of pus.
The illness can progress beyond the skin, leading to serious invasive infections like pneumonia, bone infections (osteomyelitis), or bloodstream infections (bacteremia and sepsis). When these advanced infections occur, symptoms may include fever, chills, and general malaise, regardless of whether the cause is MSSA or MRSA. A healthcare provider cannot visually determine which strain is responsible, making a bacterial culture and sensitivity test necessary to guide treatment.
S. aureus is primarily spread through direct contact, such as skin-to-skin touch or contact with contaminated objects like towels, razors, or gym equipment. Both MSSA and MRSA can be transmitted in community settings, known as community-associated strains (CA-MRSA), or in healthcare environments like hospitals and nursing homes, known as healthcare-associated strains (HA-MRSA). The skin and mucous membranes are usually effective barriers, but any break allows the bacteria to gain access and cause infection.
The Critical Difference in Treatment
The presence of the resistance gene in MRSA fundamentally alters the selection of effective antibiotics. For MSSA infections, the standard and most effective treatments are anti-staphylococcal penicillins, such as oxacillin or nafcillin, or a first-generation cephalosporin like cefazolin. These drugs are highly successful and generally preferred for MSSA because they clear the infection rapidly and are associated with better clinical outcomes.
When an infection is confirmed to be MRSA, the beta-lactam antibiotics used for MSSA are ineffective, requiring the use of different, often more potent drugs. Treatment for MRSA relies on antibiotics such as vancomycin, linezolid, daptomycin, or trimethoprim/sulfamethoxazole. Vancomycin is a common first-line choice for serious MRSA infections, but it must be carefully monitored because of potential side effects and is less effective than penicillins are against MSSA.
Linezolid is often preferred for MRSA pneumonia, as vancomycin can be inactivated by lung surfactant. Daptomycin is another powerful option but is not used for MRSA pneumonia for similar reasons. These MRSA-targeted antibiotics are often more expensive, require intravenous administration for severe infections, and carry a higher risk of toxicity or side effects compared to the standard MSSA drugs.
The clinical need to differentiate between the two strains is urgent because starting with the wrong treatment can lead to treatment failure and worsening of the patient’s condition. If a severe MRSA infection is mistakenly treated with an MSSA-effective drug, the bacteria will continue to multiply, potentially leading to widespread organ damage or death. Therefore, a correct and timely diagnosis of the resistance status guides the use of appropriate, life-saving antibiotics.