Methicillin Resistant Staphylococcus aureus (MRSA):

“The Superbug”

Reejan Shrestha

Truman State University

Author Note

Reejan Shrestha, Truman State University.

This paper was written for Dr. Carolina Sempertegui – Sosa’s General Microbiology course,

BIOL 304, Section 02.

Correspondence concerning this paper should be addressed to Reejan Shrestha,

Truman State University, Kirksville, MO 63501.

Contact: rs2363@truman.edu

718-663-1998

Table of Contents

Introduction 2

Structure and Taxonomy 3

How General Antibiotic Resistance occurs 4

Evolution 6

Epidemiology 8

Symptoms 10

Diagnosis 11

Treatment 12

Conclusion 13

References 14

1

Introduction

One of the biggest global issues is the antibiotic resistant microorganisms. These new forms of

antibiotic resistant bacteria can cross international borders and spread world-wide with ease. World

health leaders have described these antibiotic resistant bacteria as “Nightmare Bacteria” as they possess

catastrophic threat to human all around the world. Methicillin resistant Staphylococcus aureus (MRSA)

is considered as the most problematic antibiotic resistant bacteria (Frieden, 2013). In addition, MRSA

has become established as a veterinary pathogen in pets and livestock, presenting a concern for public

health as it can serve as reservoir that can infect humans and as a source of transferrable resistance

genes. MRSA causes wide range of diseases, including endocarditis, osteomyelitis, toxic-shock

syndrome, pneumonia, food poisoning and carbuncles (Juhasz et al., 2007).

Antimicrobial resistance is one of the most common health problems and some pathogens have

become resistant to multiple types of antibiotics. Due to antimicrobial resistant microorganisms, first-

line and second- line antibiotic treatment are limited. Thus, more toxic antibiotics are used which are

expensive and less effective. However, even with this alternative treatment, research has shown that

patients with resistant infections have low chances of survival, considerably longer hospital stays and

long-term disability (Baddour, 2010).

In the United States, CDC estimates more than two million people are ill every year due to

various antibiotic- resistant infections, with at least 23,000 dying as a result. Likewise, CDC estimates

that around 80,000 invasive MRSA infections and 11,000 related deaths occurred in 2011(Frieden,

CDC, 2013).

2

Structure and Taxonomy

Staphylococcus aureus (S. aureus) are microscopic gram-positive coccus bacteria that look like a

golden cluster seed. It is yellow in color and is a commonly found on our body. They form bunches,

like grapes, as they divide in two planes. The S. aureus genome is typically around 2.8 Mb in size and

carries 2800 genes (Ferreira JP et al., 2011). In addition, they are facultative anaerobes and mainly grow

by aerobic respiration, or fermentation that produces lactic acid. Furthermore, they have a thick

peptidoglycan layer and teichoic acid in their cell wall. The peptidoglycan is made of a polysaccharide

that gives the bacterium structure and rigidity. In addition, it induces the release of cytokines. Similarly,

the purpose of teichoic acid is to aid in adhesion of the cocci to the host cell surface as it is an antigenic

component. These bacteria lack flagella, thus they are non-motile (Baddour, 2010).

Their surface proteins interact with specific host molecules, thereby either concealing the

bacterial surface from the defense system or promoting adhesion of the pathogen to host tissues.

Staphylococcus aureus produces leukocidin, which destroys the leukocytes of the host cells allowing it

to escape from phagocytosis. In addition, leukocidin is produced in skin lesion (boils) that results in

destruction of white blood cells and forms pus (Anonymous, 2008). Furthermore, the bacteria have

numerous secretions that help in surface associated adhesions, endotoxins, exoenzymes, capsular

polysaccharide. The outer most capsule is responsible for enhanced virulence of a mucoid strain (Heidi,

2008). The following Table provides that complete taxonomic classification of S. aureus (Table-1).

3

Domain Bacteria

Kingdom Eubacteria

Phylum Firmicutes

Class Baccilli

Order Bacillales

Family Staphylococcaceae

Genus Staphylococcus

Species Staphylococcus aureus

Table 1 Taxonomic classification of Staphylococcus aureus.

How General Antibiotic Resistance Occur

There are numerous kinds of germs present all around us, some of them are useful where as some

are harmful to our health. Similarly, there are numerous microbes that are resistant to drug as they have

developed resistance against particular drugs. Most antibiotics that are presently kill bacteria causing

illness, as well as good bacteria protecting our body. When antibiotics lost its ability to effectively

control bacterial growth and it continues to multiply in the presence of therapeutic levels of a prescribed

antibiotic, there is a development of a drug-resistant bacterial. These bacteria grow and takes over the

system since it can resist the drug. Then these drug resistant characters are passed on to the daughter

cells leading to more antibiotic resistance bacteria, eventually creating “Superbugs.”

4

Figure 1. Examples of How Antibiotic Resistance Spreads. (Frieden, CDC, 2013).

Figure 1, illustrates how most antibiotic resistance spreads in the community. This is common

way for the MRSA to spread and it is further discussed later in detail. The figure illustrates that the

animal, environment and the infected person plays vital role in the spread of the bacteria. Thus, to

prevent the wide spread, we should try to contain the bacteria to limited number of host.

5

Evolution

Staphylococcus aureus was discovered in the 1880s, as a potential pathogenic causing minor skin

infections and post-operative wound infections. Prior to the introduction of penicillin in 1940 to cure S.

aureus infections, the mortality rate was about 80 % with the infected person. With the introduction of

penicillin, the infection was under control for a while. However, within the few years of introducing

penicillin, the first penicillin resistant S. aureus was isolated in a hospital. Soon after, penicillin-resistant

S. aureus strains were observed in the community. Since 1960, almost 80% of S. aureus strains are

resistant to penicillin. Thus, methicillin, a stronger form of penicillin, was introduced to fight the

penicillin-resistant S. aureus. And in 1961, after two year of introducing methicillin, penicillinase-

resistant penicillin, S. aureus developed methicillin-resistance, giving birth to Methicillin-Resistant

Staphylococcus aureus (MRSA) (Heidi, 2008).

Staphylococcus aureus is a significant pathogen not only in humans but also in numerous

animals. It has continuously evolved on all hosts through genetic variation and subsequent selection of

the fittest. Various selective forces like environmental conditions, host factors, competition with other

microbes or antimicrobial usage may differ in animal and human hosts (McCarthy et al., 2012).

Over the next three decades, the prevalence of Methicillin Resistant S. aureus isolation

increased, especially being hospital-acquired. Likewise, there was an outbreak of MRSA among

intravenous drug users in 1982 making it a community-associated. During last half century, various

Hospital Acquired-MRSA clones spread worldwide. Slowly, the MRSA had emerged epidemiologically

started occurring first in health care-associated setting then in the community. It has been suggested by

epidemiologist that the primary MRSA reservoir were hospitals where patients, health care workers and

inanimate environment plays crucial role. Mostly, the patients and health care workers may have

6

become colonized with MRSA and therefore served as sources of transmission (Hawkes et al., 2007). In

1998, Herold and his team reported an increase in MRSA infection cases, primarily of the skin and soft

tissue, among children from community who had no predisposing risk factors. The prevalence of MRSA

increased in patients: with weak immune systems, treated with multiple antibiotics, or who had been

under procedures that punctured the skin, and lived in the hospitals, nursing homes or dialysis centers

(Herold et al., 1998).

We know that bacterial genomes can evolve by point mutation or by horizontal gene transfer of

DNA. Point mutation usually occurs by environmental exposures that causes DNA mutation or errors in

genetic replication. Likewise, horizontal gene transfer mechanisms in S. aureus are conjugation and

transduction. Conjugation is a process of encoded by plasmid-carried genes, it is the transfer of DNA

from donor to recipient bacteria through a specialized pores or pilus. And, Transduction requires the

pack-aging of DNA into a bacteriophage particle, lysis of the donor bacteria cell, and delivery of DNA

into a recipient host by bacteriophages (McCarthy et al., 2012).

Likewise, the extensive use of antimicrobials and heavy metals in hospitals and farm animals has

undoubtedly generated a stronger selective pressure on bacterial population for the acquisition and

maintenance of Mobile Genetic Elements (MGE) encoded resistance genes. Good evidence for this

association comes from an intervention study in a hospital, where discontinuation of fluroquinolones for

clinical use over 1 year significantly reduced MRSA rates (Ferreira et al., 2011).

The first reported MRSA in United States was in 1968, which was resistant to an entire class of

penicillin-like antibiotics called beta-lactams. It is a clonal bacterial with mutation and is going through

a classical Darwinian survival process by not responding to the intensive antibiotic selective pressure.

Thus, the scientists working in this field have de-demonizing MRSA as a “SUPERBUG” (Davis, 2014).

7

Epidemiology

Staphylococcus aureus is called as a “Super-bug” due to its three distinct characteristics among

most clinically important bacteria. First, it can express a variety of virulence factors. S. aureus causes

wide range of infections from minor skin condition to life-threatening sepsis. MRSA has numerous

virulence factors; the one protein that is widely studied is Panton Valentine Leukocidin (PVL). Panton

Valentine Leukocidin is a kind of a toxin that forms pores in leukocyte membranes of host cells causing

it to burst. In addition, another protein virulence factor is α-toxin, which targets leukocytes and platelets.

Furthermore, biofilm formation, on medical implants has helped bacterial cells to show much greater

resistance to antibiotics than free living cells. Second, the ability of Staphylococcus aureus to develop

and expand resistance to a broad spectrum of antimicrobial drug classes makes it unique. The increased

resistances for antibiotics are encoded by a transposon, which is inserted into a conjugated plasmid that

also encoded resistance to other factors. Third, MRSA expresses a modified penicillin-binding protein

encoded by mecA gene that was brought by many evolutions through horizontal gene transfer to a wide

variety of methicillin susceptible stains (Murugans et al., 2008).

Hospital- acquired MRSA (HA-MRSA) is acquired in the hospital setting and is one of many

hospital-acquired infections exhibiting increased antimicrobial resistance. It has been in rise due to

various factors like an increased number of immunocompromised and elderly patients; an increase in

invasive procedures; and failures in infection control measures in past decade. MRSA accounts for up to

43% of the S. aureus isolates from diabetic foot infection (Abbanat et al., 2003).

Furthermore, the community-associated MRSA (CA-MRSA) is caused by newly emerging

strains that infect a healthy person with no contact with hospital settings (Capriotti, 2003). It usually

causes skin or soft tissue infections and develops into life-threatening infections. A number of risk-

8

factors are associated with CA-MRSA colonization, which includes gastrointestinal disease, intravenous

drug use, direct contact with CA-MRSA, indirect contact with contaminated objects that are shared

(Hawkes et al., 2007). In a study, CA-MRSA was known to have caused more than 50% of all

suppurative skin infections presented to emergency room in 11 US metropolitan centers (Capriotti,

2003). This CA-MRSA seems to occur with people who come in close physical contact, such as athletes,

soldiers, childcare workers and residents of long-term care facilities.

Likewise, in clinical samples of animals that were infected, infection with MRSA is evident and

is in increasing numbers. The genotypic characterizations of MRSA from sites of infection in these pets

reveal the predominant occurrence of well-known Hospital-acquired (HA-MRSA). In the research done

by Vincze, S. and his group in 2012, on MRSA Variant in Companion Animals in Germany, found

important and impact of MRSA, as a cause of wound infections in dogs, cats and horses. Thus,

transmission from humans will continue to play an important role in the success of MRSA as an

opportunistic pathogen (Ferreira JP, et al. 2011).

The research done by Ferreira, P concluded that companion animals of MRSA- infected patients

could be culture-positive for MRSA, representing a potential source of infection or re-infection for

humans from the animals. Similarly, research done by Eva Juhasz, and Szilard Jonosi on “MRSA

transmission between cows and humans,” concluded that several cases of subclinical mastitis in cows on

a farm in Hungary were caused by MRSA and that these strains were indistinguishable from MRSA

isolated from a carrier working in close contact with the cows. However, the direction of transfer, i.e.

cow to human or human to cow, could not be proven by that experiment (Juhász-Kaszanyitzky et al.,

2007).

9

Symptoms

MRSA is commensal bacterium that can be found anywhere in the world. It colonizes the nares,

axillae, vagina, pharynx, nose, and armpit and skin surface and thus has various symptoms. MRSA often

appears like a skin infection, which could appear like a boil or abscess (Anonymous, 2008). There are

numerous symptoms that one needs to keep on mind. The one of the most common symptoms of MRSA

are the swelling of the wounds that turns red and fills with pus. In addition, one must be careful if they

appear as abscesses, filled with larger pus-filled and closes the hair follicles, particularly in the groin

area, under the arms, or on the scalp. Likewise, as infection progress it will lead to painful eye styes that

can irritate the eyeball, and make it difficult to wear contact lenses. Most MRSA might also cause

cellulitis under the skin, which is a widespread swollen rash and it is very warm to the touch. As the

infection progress, there will be more symptoms like coughing, shortness of breath, and wheezing. If not

diagnosed on time, the infection might spread to the whole body causing high fever and body chill,

possibly accompanied by urinary tract infection. In rare and problematic cases, the symptom of

necrotizing fasciitis could occurs, which means the flesh-eating disease (Abbanat et al., 2003)

To summarize the symptoms, if an individual experiences shortness of breath, cough chest pain,

fever, body aches and rashes all over the body, there is a higher change for that person to have MRSA

present in their system. If individuals experience any of these symptoms, they should immediately go for

check-up.

10

Diagnosis

Most physicians start with a whole history and physical exam of the patient to identify any skin

changes that may have cause by MRSA, especially if the patients or caretaker mentions a close link with

a person who has been diagnosed with MRSA. They send a skin sample, sample of pus from a wound,

or blood, urine or biopsy tissue sample to the lab for S.aureus culture. If they are able to grow the

bacteria in the petri disk, the bacteria will be exposed to different antibiotics. If the bacteria grow well

when methicillin is in the culture, then they term it as MRSA, and conclude that the patient is suffering

from MRSA infection. Medical practitioner usually recommends tests, after initial examination. Doctors

might firstly recommend skin cultures from the infected site to confirm the disease. Furthermore, they

might ask for culture of the fluid from the infection. In addition, blood culture, Urine Culture and

Sputum culture helps the physician confirm the presence of S.aureus in the individual. These culture

tests usually take about 48 hours to confirm. However, there are numerous researches and development

is underway to determine rapid and less expensive molecular tests (Baddour, 2010).

The Clinical and Laboratory Standards Institute (CLSI) recommends the cefoxitin disk screen

test, the latex agglutination test for PBP2a, or a plate containing 6µg/ml of oxacillin in Mueller-Hinton

agar supplemented with 4% NaCl as alternative methods of testing for MRSA (CDC, 2013).

11

Treatment

The essence of classifying a Staphylococcus aureus isolate as methicillin resistant

Staphylococcus aureus (MRSA) lies in its resistance to methicillin, a semisynthetic penicillin. In

addition, MRSA is resistant to other antibiotic, like penicillins, cephalosporins and cabapenems (Davis,

2014). It’s resistance to all these forms of penicillin has earned it the title of “Superbug.” Thus, it is

twice as fatal as other staph infections. Thus, the treatment of MRSA infections has changed in the last

decade.

Many MRSA can still be treated by certain specific antibiotics like Vancomycin, Linezolid and

others often in combination with vancomycin depending on the strand of the bacterium. Most of the

moderate to severe infections are treated with intravenous antibiotics, usually in the hospital setting. But

some CA-MRSA is susceptible to trimethoprimsulfamethoxazole (Bactrium), doxycycline

(Vibramycin), and clindamycin (Cleocin). However, it has been recently discovered that some strains of

MRSA are resistant to vancomycin (Sharpe et al., 2005).

Due to CA-MRSA, isolation of the pathogen is imperative and knowledge of local epidemiology

and susceptibility is essential to make the choice of antimicrobial, route of administration and duration

of the therapy. Thus, individualized therapy is conducted with relation with host setting, clinical

presentation, and situation of the patient. The open-label study comparing oral linezolid and intravenous

vancomycin for management of complicated skin and soft-tissue infections caused by methicillin-

resistant Staphylococcus aureus was conducted. They found out that linezolid was associated with

greater rates of clinical cure and improvement, a 3-day shorter median length of stay, and reduced

outpatient charges. Vancomycin therapy was associated with more treatment failure and subsequent

lower-extremity amputations (Capriotti et al. , 2003).

12

Conclusion

Staphylococcus aureus is a ubiquitous bacterium that has been continually evolving and will

continue to adapt to its human and animal hosts leading to more virulent and new resistant strains. These

strains would present different clinical and socio-economic problems in human and veterinary medicine.

It has been studied that the main mode of transmission of MRSA is via hands of health care workers

with may have become contaminated by contact with colonized or infected patients (Davis et al., 2014).

Thus waterless, alcohol-based antiseptic hand rubs should be easily available for these workers to

minimize the spread of the bacteria. Compliance with strict infection control procedures is imperative in

the hospital settings to decrease the HA-MRSA. The best alternative is to prevent it from spreading.

Thus, proper care should be provided in healthcare settings to stop it from spreading. We should perform

proper hand hygiene after touching blood, body fluids, secretions, excretions and contaminated items. In

addition, we should wear protective gears for eyes, nose, mouth and proper gowning to protect the skin

from contamination.

In addition, special care must be provided to individual that are suffering CA-MRSA by

facilitating those persons with appropriate medication and containing them so that the bacteria does not

spread in the community. We should try not to share products that come in contact with skin, like soap,

lotions, creams, cosmetics and unwashed towels with others. And, keeping short fingernails could

prevent the growth of bacteria under the nails. Thus, minor details in cleanness can play a major role in

prevention for the spread of CA-MRSA.

Thus, overall goal of safe and effective therapy of MRSA infections in patients requires the

clinician consideration. This would facilitate to all the pertinent treatment for a rapid eradication of the

pathogen. However, the best option for all of us would be to control and minimize the spread of the

bacteria to prevent it from developing into a stronger microbe.

13

Reference

Abbanat D., Macielag M., Bush K. 2003. Novel antibacterial agents for the treatment of serious Gram-positive infections. Expert Opin Invest Drugs 12:379-399

Anonymous. 2008. Combating MRSA: The drug-resistant "superbug.” Harvard Women's Health Watch 15(7):4-5. Available from: Academic Search Complete, Ipswich, MA. Accessed March 1, 2014.

Baddour, M. 2010. Public Health in the 21st Century: MRSA (Methicillin Resistant Staphylococcus aureus) Infections and Treatment. Nova Science Publishers, New York, NY.

Capriotti T. 2003. Preventing nosocomial spread of MRSA is in your hands. Dermatology Nursing [serial online]. 15(6):535-538. Available from: Academic Search Complete. Ipswich, MA. Accessed March 18, 2014.

Centers for Disease Control and Prevention (CDC). 2013. Methicillin-resistant Staphylococcus aureus (MRSA) infection. Retrieved from http://www.cdc.gov/MRSA/lab/index.html#a6 on March 3, 2014.

Charbonneau, P., Parienti J., Thibon P., Daubin, C., Cheyron, D. et al. 2006. Fluoroquinolone Use and Methicillin-Resistant Staphlococcus aureus Isolation Rates in Hospitalized patients: A Quasi-Experimental Study. Clinical Infections Diseases 42(6):778-784. Available from: Academic Search Complete, Ipswich, MA. Accessed March 12, 2014.

Davis. C.P., Stoppler, M. C. (ED). 2014. MRSA infection. Retrieved from http://www.medicinenet.com/mrsa_infection/page6.htm on March 18, 2014.

Ferreira, J., Anderson, K. L., Correa, M. T., Lyman, R., Ruffin, F., Reller, L., & Jr., V. 2011. Transmission of MRSA between Companion Animals and Infected Human Patients Presenting to Outpatient Medical Care Facilities. Plus ONE, 6(11), 1-6. doi:10.1371/journal.pone.0026978

Frieden, T. (Director of CDC) 2013. Antibiotic Resistance Threat in the United States, 2013. Centers for Disease Control and Prevention (CDC). Antibiotic resistance threats in the United States, Retrieved from http://www.cdc.gov/drugresistance/threat-report-2013/pdf/ar-threats-2013-508.pdf on February 4, 2014.

Hawkes M., Barton M., Conly J., Nicolle L., Barry C., & Ford-Jones E. 2007. Community-associated MRSA: Superbug at our doorstep. CMAJ: Canadian Medical Association Journal Supplement [serial online]. 54, 56. Available from: Academic Search Complete, Ipswich, MA. Accessed March 1, 2014.

14

Heidi S. 2008. MRSA. Retrieved from: http://site.ebrary.com/lib/truman/docDetail.action?docID=10676730 on Feburary 2, 2014.

Herold, B., Immerguluck L., Marannan M., Lauderdale, D., Gaskin, RE., Leitch, CD., & Daum, R. 1998. Community-acquired methicillin-resistant Staphylococcus aureus in children with no identified predisposition risk. Jama 279:593-8.

Juhasz-Kaszanyitzky E, Janosi S, somogyi P., Engeline, V. D., Wagenaar, J., & Dan, A. 2007. MRSA Transmission between Cows and Humans. Emerging infectious disease. 13(4).

Murugans S, Mani K.R., UmaDevi. P. 2008. Prevelence of methiciling resistanct Staphylococcus aureus among diabetes patients with foot ulcers and their antimicrobial suspceptibility pattern. Journal of Clinical and Diagnostic Research. 2:978-982.

McCarthy, A.J., Lindsay, J.A., & Loeffler, A. 2012. Are all methicillin-resistant Staphylococcus aureus (MRSA) equal in all hosts? Epidemiological and genetic comparison between animal and human MRSA. Veterinary Dermatology, 23(4), 267.

Sharpe J., Shively E., & Polk H. 2005. Clinical and economic outcomes of oral linezolid versus intravenous vancomycin in the treatment of MRSA-complicated, lower-extremity skin and soft-tissue infections caused by methicillin-resistant Staphylococcus aureus. The American Journal of Surgery [serial online]. 189 (4): 425-428. Available from: Academic Search Complete, Ipswich, MA. Accessed March 16, 2014.

Vincze S., Walther B., Wieler L.H.,Brandenburg A., Kopp P.A.,Kohn B., Semmler T., & Becker A. 2012. MRSA Variant in Companion Animals. Emerg Infect Dis: 18. Accessed March 1, 2014.

15