Question

A patient in the hospital has an intravenous catheter inserted to allow for the delivery of medications, fluids, and electrolytes. Four days after the catheter is inserted, the patient develops a fever and an infection in the skin around the catheter. Blood cultures reveal that the patient has a blood-borne infection. Tests in the clinical laboratory identify the blood-borne pathogen as Staphylococcus epidermidis, and antibiotic susceptibility tests are performed to provide doctors with essential information for selecting the best drug for treatment of the infection. Antibacterial chemotherapy is initiated and delivered through the intravenous catheter that was originally inserted into the patient. Within 7 days, the skin infection is gone, blood cultures are negative for S. epidermidis, and the antibacterial chemotherapy is discontinued. However, 2 days after discontinuing the antibacterial chemotherapy, the patient develops another fever and skin infection and the blood cultures are positive for the same strain of S. epidermidis that had been isolated the previous week. This time, doctors remove the intravenous catheter and administer oral antibiotics, which successfully treat both the skin and blood-borne infection caused by S. epidermidis. Furthermore, the infection does not return after discontinuing the oral antibacterial chemotherapy. What are some possible reasons why intravenous chemotherapy failed to completely cure the patient despite laboratory tests showing the bacterial strain was susceptible to the prescribed antibiotic? Why might the second round of antibiotic therapy have been more successful? Justify your answers.

Answer 1

S. epidermidis is an opportunistic pathogen, being normal colonizers of the skin. The intravenous catheter could be the source of the Staphylococcus epidermidis. The antibiotics could clear the first infection as the S. epidermidis was sensitive to the antibiotic. This leads to the skin infection being treated and blood cultures being negative for the bacteria. However, the intravenous catheter was not removed and intravenous fluids may have been injected in the patient via this catheter. S. epidermidis can form biofilms in the catheter and thus, may have been able to resist the antibiotic within the catheter. This may have caused the bacteria adhering to the catheter, possibly as biofilms to re-enter the skin and cause reinfection. In the second treatment, the catheter was removed and oral antibiotics were administered. Thus, all bacteria could die and infection could be effectively treated.

Another possible scenario is that not all bacteria present on the skin were completely eliminated. Few of these bacteria may have been only bacteriostatically inhibited. As there was no growth of the bacteria, the skin infection healed. However, as soon as the antibiotics were withdrawn, the S. epidermidis could cause reinfection of the opening caused by the intravenous catheter. These bacteria were killed with second dose of the oral antibiotics. As intravenous catheter were withdrawn, there were no openings in skin to cause reinfection again.