What is MSSA?

June 18, 2020

Nurse analyzing MSSA bacteria

Staphylococcus aureus (S. aureus) is categorized as a commensal bacterium because it is able to gain benefits from living with a host but does not typically cause any harm or provide any benefits for the host. The bacteria colonize inside of the nose (the nasal vestibule) and interact with nasal epithelial cells that line the vestibule. The presence of colonized S. aureus  does not necessarily lead to an infection. Most healthy people will be asymptomatic carriers of S. aureus at some point throughout their lifetime while around 30% of people are permanently colonized [1]

Most types of bacteria, including S. aureus, have different subtypes that are sensitive to certain antibiotics. S. aureus with methicillin susceptibility can be treated with the antibiotic methicillin, while S. aureus without methicillin susceptibility is resistant to the effects of methicillin treatment[2]

What is MSSA?

Methicillin-susceptible Staphylococcus aureus (MSSA) is a type of S. aureus that is sensitive to the antibiotic methicillin. MSSA is commonly found in the environment and generally does not pose a significant threat to healthy people. However, MSSA is an opportunistic pathogen, meaning that it has the potential to cause an infection if the protective skin surface is disrupted or penetrated, for example by a deep scrape, cut, incision or puncture and an adequate amount of bacteria is present. For this reason, even healthy people can become infected with MSSA.  However, those with heavily burdened or compromised immune systems are at increased risk of developing these potentially life-threatening MSSA infections [1].

  1. S. aureus bacteria, including MSSA, are spread very easily. Since the bacteria colonize inside the nostrils, when someone who is a carrier of the bacteria touches their nose and then touches something else, the bacteria will transfer. While MSSA does not lead to infection in a large percentage of healthy individuals, serious infection can still occur in individuals with no prior health conditions if the bacteria come in contact with an open wound. MSSA infections often look like pimples or small abscesses, and the surrounding skin will be red and sensitive [3]. Cuts or wounds that are infected with MSSA will feel warm to the touch and pus will develop. This is one reason why preventing MSSA transfer with regular handwashing and the use of alcohol-based sanitizers is so important. 

If MSSA overpowers the immune system, the resulting infection can be very serious, even life-threatening. MSSA can cause bacteremia (bloodstream infection), sepsis (blood poisoning), endocarditis (infection inside the heart) and pneumonia, all of which can be fatal if not treated quickly [4]

How serious is MSSA?

MSSA colonizes the skin and nose of many people without causing infection [1]. However, if one of these individuals gets a cut and MSSA is transferred to it, they may develop an infection. Transfer of a pathogen from one part of the body to another part is called self-infection, and about 80% of S. aureus infections are caused by self-infection involving the nose as a source [5]. When the MSSA bacteria get into a cut, immune cells act to quickly attack the bacteria, limiting its ability to spread throughout the body. An MSSA skin infection often looks likes a pimple or small abscess causing the surrounding area to be red and tender, with pus production. 

MSSA that gets into the bloodstream is far more dangerous [6]. A bloodstream infection (called bacteremia) is a systemic infection that poses a threat to the respiratory, cardiovascular and other systems of the body. Bacteremia induces a large-scale response from the immune system, which releases infection-fighting chemicals and cells into the bloodstream. If these chemicals are not able to get the infection under control, they will continue to accumulate until they reach a dangerously high level. This is called sepsis, and it can lead to severe organ damage and death.

In individuals with compromised immune systems, even a superficial MSSA skin infection can be serious [1,7]. A major responsibility of the immune system is keeping bacteria populations in check. When the immune system is not working properly, the bacteria can overwhelm its capabilities and cause serious often widespread infection. A MSSA infection in an immunocompromised person can quickly lead to bacteremia, sepsis and/or pneumonia, all of which can be life threatening. There is evidence that people with autoimmune disorders (e.g. lupus, rheumatoid arthritis) have higher rates of S. aureus colonization than the general public, which suggests that S. aureus, including MSSA, may influence the immune system and contribute to the pathology of autoimmunity [8]. Additionally, MSSA is one of the most common hospital-associated infections (HAIs). Patients who have had surgery or who have an invasive medical device (e.g. an IV line or a urinary catheter) are at a much higher risk for developing a serious MSSA infection [4, 6]

There is a misconception that, because MSSA is susceptible to methicillin, it is less dangerous than methicillin-resistant Staphylococcus aureus (MRSA). This is only partly true. While MRSA is associated with increased mortality compared to MSSA, MSSA is often more widely associated with bacteremia, sepsis and endocarditis that are acquired in a hospital setting [4]. Thus, while MSSA may not be as lethal as MRSA, it is still a very dangerous pathogen that results in pain and suffering.

Can MSSA be prevented?

MSSA is transferred through skin-to-skin contact, touching a contaminated object or inhalation. The spread of MSSA bacteria can be reduced with proper hand hygiene, using contact precautions (gloves, masks) and using alcohol-based cleaning agents on countertops, doorknobs and other surfaces. 

Recently, a groundbreaking infection control technique was introduced to prevent the spread of S. aureus, including MSSA and MRSA. Alcohol-based nasal decolonization targets bacterial colonies in the nasal vestibule, to prevent MSSA colonization before it leads to self-infection or is transferred to others or the environment. Nasal decolonization aims to reduce S. aureus levels in hospitals, in order to prevent or limit HAIs like surgical site infections, bacteremia and pneumonia [9,10,11]

How to Treat Patients with MSSA

Healthy people who get minor MSSA skin infections generally do not require antibiotics or intensive treatment [2]. Keeping minor MSSA infection sites clean, dry and covered will help the site heal and prevent cross-transfer of bacteria. Serious MSSA infections can be treated with antibiotics like penicillin, methicillin or cefazolin [2]

Immunocompromised individuals who acquire S. aureus infections are at a higher risk for serious complications and must be treated quickly. MSSA that is not contained by an effective immune response can quickly overwhelm the body and cause life-threatening sepsis, pneumonia and endocarditis [1,7]. However, it is important to reduce the sources of S. aureus on the skin of the body and nose in all individuals at risk of developing infections not just those who are immunocompromised. Nasal decolonization reduces the likelihood of re-inoculation of the infection site by reducing the prevalence of S. aureus in the nasal vestibule [9]. Alcohol-based nasal sanitizers minimize the risk of MSSA transfer, which prevents self-infection and protects patients from infection. These products can be used broadly and prophylactically to help in the reduction of S. aureus infections before the use of antibiotics is necessary.


Methicillin-susceptible Staphylococcus aureus (MSSA) is a commensal type of bacteria that is frequently found in the nasal vestibule. Minor Staphylococcus aureus infections will typically resolve without requiring antibiotics or serious medical treatment, but if antibiotics are required penicillin, methicillin and cefazolin are all effective in MSSA infection control. 

While people with compromised immune systems have a higher risk for complications related to MSSA colonization, serious opportunistic infections could occur in any individual. If the immune system is unable to limit the spread of an MSSA infection, life-threatening bacteremia, sepsis, endocarditis or pneumonia may develop. MSSA is among the most common causes of HAIs. It is important to limit the spread of HAIs including MSSA by promoting awareness of bacterial infections to all patients, workers and guests to limit the chances of opportunistic MSSA infections.

Nasal decolonization has proven to be a highly effective strategy to minimize MSSA in hospital settings. The principle underlying nasal decolonization is that by targeting a primary reservoir for MSSA, hand-to-nose MSSA spread can be stopped at the source reducing the risk of serious infection.


  1. Sakr, A., et al. (2018). Staphylococcus aureus Nasal Colonization: An Update on Mechanisms, Epidemiology, Risk Factors, and Subsequent Infections. Frontiers in Microbiology. doi:10.3389/fmicb.2018.02419
  2. Gudiol, C., et al. (2017). Pharmacotherapeutic options for treating Staphylococcus aureus bacteremia. Expert Opinion on Pharmacotherapy 18(18). doi:10.1080/14656566.2017.1403585
  3. Lowy, F. D. (1998). Staphylococcus aureus infections. New England Journal of Medicine, 339(8), 520–532. doi:10.1056/nejm199808203390806 
  4. David, M. Z., Boyle-Vavra, S., Zychowski, D. L., & Daum, R. S. (2011). Methicillin-susceptible Staphylococcus aureus as a predominantly healthcare-associated pathogen: a possible reversal of roles?. PloS One, 6(4), e18217. doi:10.1371/journal.pone.0018217
  5. Coates, T., Bax, R., & Coates, A. (2009, July). Nasal decolonization of Staphylococcus aureus with mupirocin: strengths, weaknesses and future prospects. Journal of Antimicrobial Chemotherapy. 64(1), 9-15. doi: 10.1093/jac/dkp159
  6. Centers for Disease Control and Prevention. (2019). Strategies to Prevent Hospital-onset Staphylococcus aureus Bloodstream Infections in Acute Care Facilities. Accessed February 6, 2020.
  7. Thammavongsa, V., Kim, H. K., Missiakas, D., & Schneewind, O. (2015). Staphylococcal manipulation of host immune responses. Nature Reviews. Microbiology, 13(9), 529–543. doi:10.1038/nrmicro3521
  8. Ceccarelli, F., Perricone, C., Olivieri, G., Cipriano, E., Spinelli, F. R., Valesini, G., & Conti, F. (2019). Staphylococcus aureus Nasal Carriage and Autoimmune Diseases: From Pathogenic Mechanisms to Disease Susceptibility and Phenotype. International Journal of Molecular Sciences, 20(22), 5624. doi:10.3390/ijms20225624
  9. Steed, L. L., et al. (2014). Reduction of nasal Staphylococcus aureus carriage in health care professionals by treatment with a nonantibiotic, alcohol-based nasal antiseptic. American Journal of Infection Control. https://doi.org/10.1016/j.ajic.2014.04.008
  10. Arden, S. (2019). 567-Does Universal Nasal Decolonization with an Alcohol-Based Nasal Antiseptic Reduce Infection Risk and Cost? Poster presented at: IDWeek; 2019 October 2-6; Washington D.C.
  11. Septimus, E. J., & Schweizer, M. L. (2016). Decolonization in Prevention of Health Care-Associated Infections. Clinical microbiology reviews, 29(2), 201–222. doi:10.1128/CMR.00049-15

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