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Motivation and Goal:
Why do we care about superbugs?
In 2014, the World Health Organization reported that ‘When bacteria become resistant, it will cause a great threat to public health if there are no antibiotics to use.’ In a report to the United Nations in 2019, they warned that, without urgent action, by 2030, antimicrobial resistance could force up to 24 million people into extreme poverty, and by 2050 drug-resistant bacteria could cause 10 million deaths each year and damage the economy catastrophically [1].
▲ Figure 1:
A grouping of methicillin-resistant Staphylococcus aureus (MRSA) bacteria.
Methicillin-resistant Staphylococcus aureus (MRSA), one of the best-known superbugs, results from decades of often unnecessary antibiotic use and has been associated with high mortality rates every year across the globe. According to the Center for Disease Control and Prevention, around 5% of the world population chronically carries this type of staph bacteria [2]. Without proper treatment, MRSA can rapidly spread from infected regions to organs and other body areas, causing severe organ damage and even death. MRSA infection has caused up to 19,000 deaths per year [3].
▲ Figure 2:
A cutaneous abscess on the foot post packing.
Superbugs: MRSA
Staphylococcus aureus (S. aureus) is a major human pathogens, causing a wide range of clinical disease symptoms. Generally, the immune system in the human body can efficiently defend against S. aureus infection and forms a balance with S. aureus on the skin without causing further damage. However, weakened immunity or an open wound can break the balance, and promote S. aureus infection. The infection of S. aureus can cause mild clinical symptoms, such as swelling or suppuration, to more serious health problems, such as bacteremia, endocarditis, pleuropulmonary, and device-related infections [4].
Antibiotic treatment is the standard therapeutic strategy for S. aureus infection. Therefore, the overall infection rate of S. aureus has remained consistent over the past few decades. However, the improper use of antibiotics promotes the appearance of methicillin-resistant S. aureus, namely MRSA. The ratio of MRSA has increased year after year, and MRSA has become an emerging problem in clinical trials.
Notably, MRSA also causes latent infection. In the human immune system, macrophages are responsible for removing infective pathogens (e.g., MRSA) through phagocytosis and lysosomal degradation. Recent studies indicate that S. aureus develops protective mechanisms to escape from phagosomes or is resistant to lysosomal degradation, and remains latent in macrophages. The latent MRSA, which escapes from immune defense and antibiotics treatment, spreads through the blood, causing more serious health problems [5].
MRSA caused symptoms- Bacteremia
One well-known disease associated with MRSA infection is bacteremia, which is the uncontrolled spread of infectious S. aureus in the human body. S. aureus bacteremia (SAB) is difficult to cure and has a high mortality rate. Thus SAB places a huge cost and resource burden on healthcare systems. SAB is able to induce life-threatening complications in the human body, including infective endocarditis and other bacterial infections [6]. Antibiotic treatment of SAB caused by S. aureus is not very efficient, and the situation is even worse in the case of MRSA infection. Generally, the incidence of SAB in western countries is from 20 to 80 per 100,000 people (Table 1).
| Region
|
Time period (yr)
|
Incidence per 100,000 person-year |
| Europe |
|
|
| Sweden |
2003–2005 |
33.9 |
| EU 27-country |
2007 |
26.1 |
| Finland |
2004–2008 |
21.8 |
| Denmark |
2007–2008 |
21.8 |
| Iceland(adults) |
2008 |
28.9 |
| England |
2008 |
22 |
| Netherlands |
2009 |
19.3 |
| Scandinavian |
- |
26 |
| North America |
|
|
| USA |
2004-2005 |
31.8 |
| Quebec, Canada |
2005 |
32.4 |
| Canada |
2008 |
19.7 |
| USA |
- |
50 |
| Oceania |
|
|
| New Zealand |
1998-2005 |
21.5 |
| Australia |
2009 |
154 |
| Australia |
2019 |
86.4 |
▲ Table 1: The worldwide incidence of SAB per 100,000 person-years.
An examination of SAB risk groups suggests that age, gender, ethnicity, and chronic conditions such as diabetes and Acquired Immunodeficiency Syndrome (AIDS) are important factors [4]. Age is the most powerful determinant of SAB incidence. The 30-day mortality of SAB is less than 10% for patients under 35. Importantly, SAB mortality increases with age, from around 20% at age 35 to 35% at age 75, and over 50% for patients older than 85 (Figure 3) [7].
▲ Figure 3: The 30-day mortality of bacteremia in different ages of patients.
Regarding the chronic conditions, diabetes is associated with significantly higher mortality in patients with SAB [8]. Patients with diabetes may have increased susceptibility to SAB for a number of reasons including tissue hyperglycemia and decreased oxygenation, and generally reduced immunity. High age, coexisting morbidities, and diabetes complications may further increase the risk of SAB [9].
Similarly, S. aureus infections account for significant morbidity in patients with AIDS. The incidence of SAB in AIDS patients is significantly higher than other patients, 1,960 per 100,000 person-years, which is 24 times that of the non-HIV-infected population [10]. Although the increase of SAB incidence may result from the injection drugs used in AIDS patients, non-injection drug-using AIDS patients also exhibit higher rates of SAB than those in the non-HIV-infected population [4].
The incidence of SAB is also associated with gender, males consistently showing a higher risk of SAB incidence. The ratio of SAB of males to females is ~1.5, although the reason is unclear. Ethnicity is also associated with SAB incidence. In the United States, SAB incidence in the black population (66.5 per 100,000 person-years) is over twice that in the white population (27.7 per 100,000 person-years). In Australia, the incidence of SAB in the indigenous population is 5.8 to 20 times that of nonindigenous Australians [4].
MRSA caused symptoms- Infective endocarditis
Infective endocarditis (IE) is a rare but severe disease. IE is caused by bacterial infection of the heart lining. When the immunity of patients is weak, the infectious bacteria may overcome the immune defense and infect other regions of the patient’s body through the bloodstream. The incidence per 100,000 person-year of IE is from 3.2 to 14.4, with a mortality rate in the hospital of about 20% and a 5-year mortality rate of 40% [7]. Importantly, S. aureus is the pathogen most frequently associated with IE [4], with the proportion of IE caused by S. aureus ranging from 26% to almost 50% (Table 2) .
| Region
|
Time period (yr)
|
Incidence per 100,000 person-year |
The proportion of IE cases due to S. aureus |
Mortality
(in-hospital) |
| Europe |
|
|
|
|
| Veluto, Italy |
2006-2008 |
4.9 |
40% |
19.1% |
| France |
2008 |
3.2 |
26% |
21.2% |
| England |
2008 |
13.1 |
- |
16.7% |
| Germany |
2014 |
14.4 |
- |
17% |
| North America |
|
|
|
|
| USA |
2009 |
12.7 |
49.3% |
13% |
| Oceania |
|
|
|
|
| Australia |
2000-2006 |
4.7 |
32% |
18% |
▲ Table 2: The incidence and mortality rate of IE in the hospital.
Design and rationale:
How do we solve the superbug problem?
AgenT dressing- the antimicrobial dressing to stop the MRSA infection from wounds on patients with weak immunity or chronic disease.
Inspired by the 2019 iGEM team Linkoping_Sweden, we decided to develop AgenT dressing, to solve both extracellular and intracellular MRSA infections. The design can be briefly divided into antimicrobial components and supporting components.
For antimicrobial components, we apply antimicrobial peptides (AMPs) to kill extracellular MRSA and other bacteria by disrupting the bacterial membrane or interfering with bacterial metabolism. For intracellular bacteria, we linked AMP with cell-penetrating peptide (CPP), and a lysosomal cathepsin S (CTSS) site to stimulate AMP release. Once CPP brings AMP into the macrophage by macropinocytosis, the phagolysosomal CTSS will cleave the CTSS site and release AMP to exert its function.
The supporting components include an outer layer of PU film, a middle layer for absorption of wound exudate, and an inner collagen layer containing AMPs and CPP-linked AMPs. The PU film is waterproof and breathable, which can block external infection. The middle layer is composed of alginate and chitosan, which are both biocompatible. The collagen layer bridges the middle absorption layer and AMPs or CPP-linked AMP. The collagen also stimulates the deposition and organization of newly formed collagen fiber and granulation tissue in the wound bed.
Finally, we use the Collagen Binding Domain (CBD) from human fibronectin to anchor our AMPs or CPP-AMPs onto the collagen layer. To release our AMPs or CPP-AMPs to the wounds, we designed a linker peptide with a thrombin cleavage site. When our dressing contacts the thrombin present in wounds, thrombins will cut the linker and release AMPs or CPP-AMPs to kill both extracellular and intracellular bacteria.
Thus, CCU_Taiwan iGEM team provides a new treatment option for patients who are in the high risk group of MRSA infection.
