Answers - Case One

The patient was a 22-year-old female with a history of mitral valve prolapse (a defect of the valve between the left atrium and ventricle caused by a weakening of the tough, connective tissue of the valve leaflets, which allows the valve to project back into the left atrium). She was admitted with complaints of intermittent fevers for 1 month and headaches for 3 weeks. Two weeks before symptoms developed she had undergone a dental procedure.

Four blood cultures were performed on admission. All four blood cultures demonstrated Gram-positive cocci in chains. The diagnosis is bacterial endocarditis (a bacterial infection of the tissue lining the inside of the heart; usually involves the heart valves).

1. What is significant in the patient’s history? (what situations do you think predisposed this person to infection?) 2 things predisposed her to this infection: heart valve anomaly and recent dental procedure.

2. What organism is most likely to be causing her infection? Gram-positive cocci in chains that are catalase negative are streptococci. Two groups of streptococci are common causes of bacterial endocarditis (inflammation of the heart). They are viridans streptococci (the alpha-hemolytic or so-called "green" streptococci) and nonhemolytic streptococci (which include Group D streptococci and the enterococci). Because the individual had recently undergone a dental procedure, the organism causing the infection probably originated from the oral microbiota rather than the gut microbiota. The Group D streptococci and the enterococci are commonly found as part of the normal microbiota in the gut, whereas the viridans streptococci are common members of the oral microbiota. Streptococcus mutans, a viridans streptococcus, was the organism recovered from this patient. It commonly resides on the tongue and teeth. (What other common infectious disease does this organism cause? – dental caries (i.e., cavities)

During dental procedures, transient bacteremia occurs in up to 80% of individuals. (Transient bacteremia is defined as the presence of bacteria in the bloodstream for short periods. The organisms that cause this are generally of low virulence and are usually easily removed by the reticuloendothelial [filtering and phagocytic] system). The organisms that commonly cause this bacteremia are oral streptococci, which readily adhere to a variety of surfaces via a very sticky glycocalyx, so thick and sticky it is called a slime layer. In particular, abnormal heart valves may be affected because blood flow is slightly slowed and the streptococci have a better opportunity to adhere. Colonies may form and then endocarditis develops.

Bacterial endocarditis

Streptococci

Answers - Case Two

This 47-year-old man had a history of sickle cell disease that resulted in many previous hospitalizations for the management of painful crisis.

The patient had been admitted 9 days prior to the current admission for management of such a crisis, and a right port-a-cath (a central venous catheter that is designed to remain in place for a prolonged period) was placed in his right subclavian vein. He was discharged (with port-a-cath remaining) after a 4-day hospitalization. On the day of readmission, the patient had right arm discomfort and swelling, a temperature of 38.1C (normal is 37C), and chills. He presented to the hospital emergency room, where he was afebrile (not showing signs of fever). Physical examination was remarkable for right extremity swelling.

Two blood cultures were obtained (one set through the port-a-cath and one set via a peripheral vein). The two sets of blood cultures grew identical Gram-positive cocci that were catalase positive. The diagnosis is in-line sepsis.

1. Which organisms are most likely to cause this infection? On the basis of this patient’s blood culture results and the presence of a foreign body, the staphylococci are the most likely agents of his infection. The patient had in-line sepsis. The most common causes of in-line infection are the staphylococci, with the coagulase-negative staphylococci being more frequently recovered than Staphylococcus aureus. With the introduction and widespread use of intravascular prosthetic devices made of synthetic materials, the coagulase-negative staphylococci have become the most important causes of nosocomial (hospital-acquired) bacteremia, with these devices acting as the source of the bacteremia. There are in excess of 20 species of coagulase-negative staphylococci. Most laboratories do not identify these organisms to the species level, preferring to report them simply as coagulase-negative staphylococcus.

2. What risk factor does this patient have that predisposes him to this infection? Coagulase-negative staphylococci are normal inhabitants of the skin, mucous membranes, and nares. Any indwelling device introduced through the skin places an individual at risk for infection with this organism. The coagulase-negative staphylococci produce a slime layer that can enhance their adherence to a wide variety of plastic surfaces. Slime-producing strains of coagulase-negative staphylococci may also be more difficult to eradicate by antimicrobial therapy than non-slime-producing ones. The presence of a port-a-cath in this patient is an important risk factor for his development of this infection.

Staphylococcus epidermidis biofilm colonization of a catheter.

3. What is the significance of both blood cultures being positive? The diagnosis of line-related sepsis is made first by obtaining two blood cultures from a patient who develops fever and, in some instances, by observing localized signs of infection at the site of the intravascular line. One of the blood cultures is obtained through the line, and the other is obtained from a peripheral site. The reason for requiring cultures drawn from two sites is fairly straightforward. Since coagulase-negative staphylococci are commonly found on the skin and are the most frequent cause of blood culture contamination, a single positive blood culture with this organism may represent skin contamination rather than true infection. However, if blood cultures obtained from two separate sites grow the same organism, it is more likely that the recovery of the organism represents infection rather than contamination. The diagnosis of in-line sepsis can be confirmed by performing a semiquantitative culture on the catheter tip. The culture is done by aseptically cutting off the tip with sterile scissors, and transporting it to the laboratory for culture. In the laboratory the tip is rolled on the surface of an agar plate. After appropriate incubation, the number of colonies on the plate is counted. If 15 CFU (colony forming units) of a species is present per plate and the blood cultures are positive, the patient is said to have catheter- or line-related sepsis. The infection is treated by removing the catheter and initiating antimicrobial therapy.