Staphylococcus

CASE 1

• A 36 year old diabetic man has a painful, soft nodule that developed on his right upper arm. The middle of the affected area is soft and flexible, suggesting a liquid center. There was some spontaneous drainage from the site (see below, left). He has no fever or other systemic symptoms. The patient has had several similar lesions on other parts of his body at various times during the past year, particularly under the arms, the back of the neck, and in the groin. After examining the patient, the physician opens the lesion with surgical blade and releases 10-15 cc of thick yellow pus (see photo of opened abscess, below right).

• A gram stain of the pus drained from the lesion has the appearance shown below. An antibiotic is prescribed, and the lesions quickly resolves.

Questions:

1. What species of Staphylococcus is responsible for these lesions?
2. Where did the organisms come from?
3. Why does this patient have this problem? Are there other situations that predispose to these infections?
4. What virulence factors permit this organism to cause abscesses like this one?
5. How would you isolate this pathogen in the laboratory? What tests would you use to establish its identity?
6. Which of the common antibiotics are unlikely to kill this pathogen? Why?

CASE 2

• A 25 year old man is admitted to the hospital with a high fever and shortness of breath. He is a farmer and recently sustained a cut to his right ankle while working in his field. A skin infection developed at the site of the cut, but the patient did not receive treatment for it. One week later, he began having sudden chills and fever. At the time of his hospitalization, he was breathing rapidly and was unable to walk up a flight of stairs without stopping to catch his breath.

• The past history is significant for an episode of rheumatic fever when the patient was 6 years old. He was told that he had a heart murmur ever since that episode.

• On examination, The patient has a few crackles at the bases of both lungs, but no signs consistent with pneumonia. His heart examination revealed two murmurs -- a loud systolic ejection murmur and a soft diastolic decrescendo murmur. The patient had a painless, black patches on the soles of both feet (see Janeway lesions, below). There are numerous splinter hemorrhages of his finger and toenails. His urine was positive for blood.

• Culture of both his blood and urine were positive for coagulase-positive Staphylococcus. An echocardiogram showed a large vegetation (collection of fibrin and platelets) on his aortic valve with significant incompetence of the valve and regurgitation of blood back into the left ventricle in diastole.

Questions:

1. Where is the source of Staphylococcus in this patient's blood and urine?
2. Why was the organism able to settle in this location?
3. Why does the patient have painful black spots on the feet? Why is there blood in the urine?
4. What will happen to this patient if he is not treated?

CASE 3

• A 38 year old Caucasian woman has high fever and dizziness for the past 12 hours. At the same time, she developed an intense rash over her body that looks like a sunburn and blanches momentarily when you press on it. There is also intense redness of her oral mucosa and bilateral conjunctivae (pictured below).

• Three days earlier, the patient underwent a tubal ligation (i.e., had her Fallopian tubes tied). Today, there is slight swelling and a small amount of drainage from the surgical wound.

• On examination her blood pressure is 80/60 mmHg (very low) and her pulse is 132/min (high). Her temperature is 39 degrees. Laboratory tests reveal decreased blood platelets, elevated liver injury tests, and evidence of renal dysfunction. Blood and urine cultures are negative.

Questions:

1. What is the most common cause of post-operative wound infections?
2. Why are the consequences of such a minor wound infection so sever in this case?
3. What is a superantigen, and what does that have to do with this patient's illness?