Staphylococcus aureus

General:

Staphylococcus aureus commonly known as a “Staph” infection (1)

Gram positive, clusters, resembles grapes (2)
Forms large yellow colony on rich media (2)
Hemolytic on blood agar (2)
Facultative anaerobes grows by either aerobic respiration or fermentation yielding lactic acid (2)
Catalase positive (2)
Oxidase negative (2)
Coagulase positive (2)
Can grow at temps between 15-45°C (2)
Can grow in NaCl conc as high as 15% (2)

Risk:

Staphylococcus aureus is a very common microorganism It exists on skin, throat and nasal passages all the time. (1) Problems arise when bacteria enters body via open wound in skin due to injury or surgery. S. aureus can cause infections anywhere in the body. (1) Most people with healthy immune systems are not susceptible to staph infections; however, those at greatest risk are those with compromised immune systems, whose treatment requires an invasive device such as a cathter and those with chronic illnesses such as:

Surgrical patients—specifically orthopedic and cardiac (3)
Trauma and burn patients (3)
Those undergoing invasive outpatient procedures (3)
Patients receiving implanted medical device or prothetic (3)
Long-term care patients (3)
Kidney dialysis patients (3)
Type-(1) diabetics (3)
Immune-compromised patients such as those with HIV/AIDS or on immunosuppressive treatments (3)
Injection drug users (especially if reuse needles) (3)

Summary of Disease:

Infection occurs when skin barrier is broken due to injury or surgery and immune system cannot combat the bacteria. (3)

Methods of nosocomial contraction:

From another patient, heath care worker in hospital (3)
From any invasive device—cathter or syringe (3)
From home dare treatments like Total Parenteral Nutrition (TPN) (3)
From prosthetic devices (3)
Following surgical procedures (3)

Complications of Staph infection:

Minor complications:
- S. aureus causes a wide range of suppurate (pus forming) infections such as:
- Folliculitis-hair follicle—usually, shaving irritation, or chaffing form clothing (1)
- Boils--deeper infection of hair follicle—usually, face/neck (1)
- Sties--eyelash hair follicle infection (1)
- Impetigo--children’s mouths, noses—blisters and red scabby skin (1)
- Abscesses—pus, swelling in skin and in any organ (1)
More serious:
- Toxic Shock Syndrome (TSS) (2)
- Pneumonia (2)
- Bone infections (osteomyelitis) (2)
- Mastitis in nursing mothers (2)
- Endocarditis (infection of the inside of the heart) (2)
- Bacteremia (blood infection) (2)
- Hospital acquired (nosocomial) infection of surgical wounds (2)
- Infections associated with indwelling medical devices (2)

The bacteria is generally localized to site of infection, characterized by raised temperature at the site, swelling, pus accumulation, and necrosis of the tissue. (2) A fibrin clot may form, walling off the bacteria, forming an abscess or boil Septicemia may occur and is rapidly fatal (2) Bacteremia can fill in other internal abscesses, skin lesions, infections of the lungs, kidney, heart, skeletal muscles or meninges. (2)

Tests:

Swab taken from site; cultured to be identified by biochemical tests.

Treatment:

Antibiotics—some strains have become extremely resistant to antibiotics—especially in hospitals (1)
Resistant to antiseptics, disinfectants(2)

Antibiotic resistance:

70% of nosocomial strains are resistant to at least one antibiotic used to treat them (3)
Greater than 95% of patients worldwide with staph infections do not respond to first line antibiotics (penicillin or ampicillin) (3)
MRSA—Methicillin Resistant Staphylococcus aureus is wide-spread, if a strain is a MRSA, it is likely to be resistant to many other antibiotics. (2)
More than 30% of S. aureus infections are classified as MRSA (3)
Nosocomial cases can often only be treated with vancomycin (2) because hospital strains are usually resistant to many antibiotics, some are resistant to all but vancomycin. (2)
Some vancomycin-resistant strains have been reported in the US, Japan, and France. (3)
Cases contracted outside of a hospital can usually be treated with penicillinase-resistant ß-lactams (2)
However, MRSA strains, once localized to the hospital environment are making there way out into the community at large. (3)

Why is S. aureus so virulent?

S. aureus has developed drug resistance in the usual ways: mutations in genome followed by selection of resistant strains and acquisition of virulence genes in the form of plasmids, transducing particles, transposons, or other DNA inserts. (2)

Since the introduction of penicillin in the 1940s, S. aureus has been very quick to adapt to the introduction of new drugs. There are strains that have become resistant to most normal antibiotics, and there is fear that a new drugs are not on the horizon. Therefore, pharmaceutical companies are looking at drugs that would block certain molecular targets (i.e. active sites for enzyme binding) to combat the emerging resistant strains of S. aureus. (2)

A plasmid associated with vancomycin resistance has been found in enterococci that can be transferred to S. aureus in the laboratory, which may occur in the Gastrointestinal tract between S. aureus and normal flora. (2)

What about a Vaccine?

No vaccine is currently available for active immunity, however, when the exact molecular mechanism for the binding of S. aureus to its host is known, a synthetic inhibitor may be made to block the interaction to stop colonization. (2) There are ongoing clinical trials for a vaccine against S. aureus called StaphVAX. StaphVAX is S. aureus type 5 and 8 capsular polysaccharides conjugated to nontoxic recombinant Pseudomonas aeruginosa exotoxin A. This vaccine gives immunity for 40 weeks, and is ideal for a surgery patient setting because the patient doesn’t need immunity for the rest of his life, rather just for his stay in the hospital. (2)

Stats:

During the 1950s and 1960s Staph was the number one nosocomial infection. Gram-negative bacilli have since taken over the top spot for most common nosocomial infection, however, Staph continues to be a problem. (2)

Mortality rate for patients with Staph bacterimia ranges from 11%-43%. (3)
25-35% of Staph complications are from endocarditis, with a mortality rate of 20-44%. (3)
Nosocomial infections are the eighth leading cause of death in the US. (3)
The rate of MRSA infections doubled between 1987 and 1997 in intensive care units. (3)