Staphylococcus aureus, often called “Staph,” is a common type of bacteria that frequently lives harmlessly on the skin and in the nose of about one-third of the population. While typically causing only minor skin issues, this organism can become dangerous if it enters the bloodstream, a condition known as bacteremia. Methicillin-Sensitive Staphylococcus aureus (MSSA) is the form of this bacterium that remains susceptible to treatment with certain common antibiotics, distinguishing it from the more widely known Methicillin-Resistant Staphylococcus aureus (MRSA) primarily by its antibiotic response.
The Basics: What is MSSA Bacteremia
MSSA bacteremia is a serious bloodstream infection where bacteria circulate in the blood. Bacteremia describes the presence of bacteria in the bloodstream, sometimes referred to as blood poisoning or septicemia. MSSA is defined as a strain of S. aureus that is susceptible to methicillin and other related beta-lactam antibiotics.
This susceptibility is the defining factor setting MSSA apart from MRSA. While S. aureus frequently causes skin and soft tissue infections, it becomes life-threatening when it breaches the body’s defenses and enters the blood. Once in the circulation, the bacteria can spread rapidly throughout the body, causing systemic illness.
Entry Points and High-Risk Populations
The bacterium gains entry into the bloodstream through a break in the skin or mucous membranes, which serves as a protective barrier. Common entry points include surgical wounds, burns, and infected skin lesions like boils or abscesses. A significant pathway for MSSA to enter the blood is through indwelling medical devices that penetrate the skin.
Intravenous lines (central venous catheters, hemodialysis ports, and PICCs) represent common routes for infection, particularly in healthcare settings. The highest-risk populations are those with chronic contact with the healthcare system or with compromised immune function. Patients who are currently hospitalized, especially those in intensive care units, face an elevated risk.
Individuals with chronic medical conditions (diabetes, kidney failure requiring dialysis, or cancer) are highly susceptible to MSSA bacteremia. Those with a weakened immune system due to HIV/AIDS, chemotherapy, or immunosuppressive medications have a reduced capacity to fight off the infection. Any person with a prosthetic device, such as an artificial joint or a pacemaker, has a potential site where the bacteria can adhere and establish a persistent infection.
Recognizing Symptoms and Laboratory Confirmation
The symptoms of MSSA bacteremia are non-specific and reflective of a systemic infection, making early clinical diagnosis challenging. Patients commonly develop a high fever and experience chills as the body responds to the circulating bacteria. Signs of severe infection include a rapid heart rate, abnormally low blood pressure, and difficulty breathing.
Changes in mental status, such as confusion, sleepiness, or difficulty waking up, are frequently observed as the infection progresses. Because these signs can point to many different conditions, confirming MSSA bacteremia requires laboratory testing. The definitive diagnosis is made through a blood culture, where a sample of the patient’s blood is placed to encourage bacterial growth.
Once S. aureus is isolated from the blood culture, the lab performs susceptibility testing to determine which antibiotics are effective against the specific strain. This test confirms whether the bacteria are sensitive to methicillin-class antibiotics, establishing the diagnosis as MSSA rather than MRSA. Rapid molecular tests are increasingly used to identify S. aureus and its methicillin susceptibility faster, allowing for quicker initiation of targeted treatment.
Standardized Treatment and Duration
The standard approach to treating MSSA bacteremia involves the prompt administration of specific intravenous antibiotics to which the bacterium is sensitive. Since MSSA is susceptible to beta-lactam drugs, preferred first-line agents are antistaphylococcal penicillins like nafcillin or oxacillin. Cefazolin, a first-generation cephalosporin, is often used as an equally effective alternative, especially in cases of non-severe penicillin allergy.
Source control is a primary aspect of management alongside antibiotic therapy. If the infection originated from a medical device, such as an intravenous catheter or port, that device must be removed immediately to eliminate the source. If an abscess or collection of pus is present, surgical drainage is necessary to remove the infected material and allow the antibiotics to work effectively.
The duration of antibiotic treatment is substantial to ensure complete eradication of the infection and prevent relapse. For uncomplicated MSSA bacteremia, defined by criteria like negative follow-up blood cultures and no evidence of metastatic infection, a minimum of 14 days of intravenous therapy is required. Complicated cases, such as those involving persistent bacteremia or seeding of other body sites, necessitate a much longer course, ranging from four to six weeks.
Potential Complications and Prognosis
If MSSA bacteremia is not treated promptly, it can lead to severe complications as the bacteria spread and “seed” other organs. One feared complication is infective endocarditis, where the bacteria attach to and damage the heart valves or the inner lining of the heart. This condition is serious and requires an extended course of antibiotics.
The infection can spread to the bones, causing osteomyelitis, or to the joints, resulting in septic arthritis, both requiring prolonged treatment. The infection can also trigger sepsis, a life-threatening condition where the body’s response damages its tissues and organs.
Despite these serious risks, the prognosis for MSSA bacteremia is better than for MRSA bacteremia. Mortality rates for S. aureus bacteremia are substantial, with a 30-day all-cause mortality rate often cited around 20%. Studies have shown that MSSA bacteremia has a lower mortality rate compared to MRSA bacteremia, especially when appropriate beta-lactam antibiotics are administered early. The most favorable outcomes are achieved when the source of infection is controlled and the full, prescribed duration of targeted antibiotic therapy is completed.