What Is Antibiotic Resistance?
Sir Alexander Fleming's discovery of Penicillin ushered in the modern antibiotic era. Antibiotics have made many infectious diseases treatable, including those caused by S. aureus. On the other hand, the rapid acquisition of antibiotic resistance by S. aureus poses a significant problem for treating human infections caused by this organism. Here is a timeline illustrating the emergence of antibiotic-resistant S. aureus following the introduction of key antibiotics:
Mobile genetic elements (MGEs) play an important role in S. aureus' ability to adapt to environmental stresses such as antibiotic exposure. MGEs are the most common way bacteria exchange genetic information via horizontal gene transfer. In general, S. aureus strains contain many MGEs, including transposons, plasmids, bacteriophages, staphylococcal cassette chromosomes, and pathogenicity islands. Plasmids and staphylococcal cassette chromosomes, in particular, have been implicated in the transmission of resistance to β-lactam antibiotics and Vancomycin.
What Are Vancomycin-Resistant Staphylococcal Infections?
Staphylococcus bacteria cause staphylococcal infections. These bacteria are commensals to the skin and noses of healthy individuals. Most of the time, Staphylococcus bacteria are harmless or result in minor skin infections. On the other hand, Staph infections can be fatal if the bacteria spread deeper into the body, bloodstream, joints, bones, lungs, or heart. As a result, many otherwise healthy people are becoming infected with potentially fatal staph infections. Staph infections can range from a simple boil to antibiotic-resistant and flesh-eating infections. The difference is the severity of the infection, how deep it goes, how quickly it spreads, and how treatable it is with antibiotics. Different strain emergence events have been identified in the evolution of Staphylococcus aureus during the modern antibiotic era, including antibiotic resistance acquisition.
Although Vancomycin was approved for human use in 1958, it became the antibiotic of choice for treating MRSA infections in hospitals in the late 1980s. However, resistance to Vancomycin was discovered in enterococci in the 1980s, which raised serious concerns about the future use of Vancomycin as an effective treatment for MRSA. In addition, S. aureus isolates with reduced susceptibility to teicoplanin, a structural relative of Vancomycin, appeared in Europe shortly after. In 2002, the first VRSA isolates in the United States were reported. There have been 14 isolates reported in the United States since then.
How Are Vancomycin-Resistant Staphylococcal Infections Diagnosed?
Based on laboratory tests, Staph bacteria are classified as VISA (Vancomycin-intermediate Staphylococcus aureus) or VRSA (Vancomycin-resistant Staphylococcus aureus). Laboratories run tests to see if staph bacteria are resistant to antimicrobial agents that could be used to treat infections. Laboratories determine how much Vancomycin and other antimicrobial agents are needed to inhibit the growth of an organism in a test tube. The test result is typically expressed as a minimum inhibitory concentration (MIC) or the amount of antimicrobial agent required to inhibit bacterial growth in the test tube. As a result, staph bacteria are classified as VISA if the vancomycin MIC is 4 to 8 g/ml and VRSA if the vancomycin MIC is 16 g/ml.
What Are the Symptoms of Staphylococcal Infections?
Staph infections can cause everything from minor skin irritation to endocarditis, potentially fatal infection of the heart's inner lining (endocardium). As a result, depending on the location and severity of the infection, the signs and symptoms of staph infections vary greatly.
1. Infections of the Skin:
Staph bacteria cause skin infections, including,
- Boils - The boil, a pocket of pus that develops in a hair follicle or oil gland, is the most common type of staph infection. Typically, the skin over the infected area becomes red and swollen. If a boil bursts, it will most likely drain pus. Boils are most commonly found under the arms, around the genital area, or on the buttocks.
- Impetigo - Staph bacteria can cause this contagious, often painful rash. Impetigo is characterized by large blisters that may ooze fluid and form a honey-colored crust.
- Cellulitis - Cellulitis is the infection of the deep layers of skin that causes redness and swelling on the skin's surface. Sores or areas of oozing discharge may also appear.
- Skin Scalding Caused by Staphylococcus Aureus - Toxins produced by a staph infection may result in staphylococcal scalded skin syndrome. This condition, which primarily affects babies and children, is characterized by a fever, a rash, and, in some cases, blisters. When the blisters rupture, the top layer of skin peels away, revealing a red, raw surface that resembles a burn.
2. Poisoning From Food:
One of the most common causes of food poisoning is staph bacteria. Symptoms appear quickly, usually within hours of consuming tainted food. Symptoms usually fade quickly, lasting only a half-day at most. A staph infection in food does not usually result in a fever.
However, one can expect the following signs and symptoms if they have this type of staph infection:
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Diarrhea.
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Vomiting and nausea.
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Low blood pressure.
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Dehydration.
3. Bacteremia:
Bacteremia, also known as bloodstream infection, occurs when staph bacteria enter a person's bloodstream. Bacteremia is characterized by fever and low blood pressure.
The bacteria can spread deep within the body and cause infections that affect:
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Internal organs like the brain, heart, and lungs.
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Devices that are surgically implanted, such as artificial joints or cardiac pacemakers.
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Muscles and bones.
This potentially fatal condition is caused by toxins produced by certain strains of staph bacteria and has been linked to tampons, skin wounds, and surgery.
It usually manifests itself abruptly with:
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A high temperature.
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A rash that resembles sunburn is seen on the palms and soles of the feet.
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Vomiting and nausea.
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Muscle pain.
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Confusion.
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Stomach ache.
4. Septic Arthritis:
A staph infection is frequently the cause of septic arthritis. Bacteria frequently attack the knees, shoulders, hips, toes, or fingers.
Among the signs and symptoms are:
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Swelling of joints.
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Severe pain in the affected joint.
What Are the Treatment Options for Vancomycin-Resistant Staphylococcal Infections?
The mainstay of VRSA treatment is the treatment of underlying comorbidities, antimicrobial therapy, and if clinically indicated, surgical intervention. Because VRSA disease epidemiology is still evolving, with high mortality rates, real-time information on the efficacy of available drugs is limited. Several agents, including modifications of Glycopeptides, Carbapenems, Oxazolidinones, Quinolones, and Tetracyclines, are being researched and developed for highly resistant gram-positives, including VRSA.
Conclusion:
S. aureus is well-known for its ability to acquire and develop antibiotic resistance. This characteristic, combined with the high prevalence of S. aureus infections, poses a treatment challenge. Given the importance of Vancomycin in treating severe MRSA infections, the ability of S. aureus to become completely resistant to Vancomycin is concerning. Fortunately, despite the widespread use of Vancomycin to treat severe MRSA infections, strains with complete resistance to Vancomycin (VRSA) are uncommon.
