Detection of anti-teichoic acid immunoglobulin G antibodies in experimental Staphylococcus epidermidis endocarditis

Getting to the heart of the problem: serological and molecular techniques in the diagnosis of infective endocarditis

Infective endocarditis is diagnosed using the Duke criteria, which rely predominantly on cardiac imaging and recovery of a causative organism from the bloodstream. These criteria can be inconclusive, particularly when blood cultures remain sterile either due to the fastidious nature of the infecting organism or prior antibiotic therapy. Serology and, more recently, molecular techniques have been investigated as a solution to the problematic negative blood culture. The detection of elevated antibody levels has proved particularly useful in the diagnosis of those patients infected with organisms that cannot be cultured using standard laboratory methods, whilst molecular methods have been successfully used in the detection of both fastidious pathogens and those inhibited by prior antibiotic therapy. In view of recent and ongoing developments in the field of molecular diagnostics, these techniques will become increasingly important not only in the routine investigation of infectious disease, but specifically the diagnosis of endocarditis.

Rapid serodiagnosis of gram-positive bacterial endocarditis

OBJECTIVES

To characterize a serological test for diagnosing endocarditis caused by Gram-positive cocci.

METHODS

We have developed an indirect enzyme-linked immunosorbent assay (ELISA) for the serological detection of Gram-positive infections. The test measures serum IgG directed towards lipid S, a recently identified exocellular glycolipid antigen which is related to lipoteichoic acid. We have previously shown the test to be of value in serodiagnosis of central venous catheter-associated sepsis and infection of orthopaedic prostheses caused by coagulase-negative staphylococci. We now describe the application of this test in endocarditis.

RESULTS

Serum IgG levels to lipid S were significantly elevated in 34 patients with Gram-positive bacterial endocarditis confirmed as 'definite' by the Duke criteria as compared to 50 control patients. The test had a sensitivity of 88% and a specificity of 88%.

CONCLUSIONS

The assay is independent of culture results or endocardial imaging, making it complementary to currently used investigations. It may therefore be possible to refine the current Duke criteria for diagnosing endocarditis. We describe an algorithm which incorporates lipid S serology into a positive diagnostic strategy.

The pathogenic role of Staphylococcus epidermidis capsular polysaccharide/adhesin in a low-inoculum rabbit model of prosthetic valve endocarditis

BACKGROUND

The capsular polysaccharide/adhesin (PS/A) antigen of Staphylococcus epidermidis was required to produce endocarditis in a rabbit model in which infection resulted from hematogenous spread of bacteria from a contaminated catheter in the jugular vein. However, many prosthetic valve endocarditis (PVE) infections probably result from direct contamination of the valve with small numbers of bacteria during surgery. The role of PS/A in this situation was evaluated by modifying a rabbit model of endocarditis to partially mimic PVE.

METHODS AND RESULTS

A Teflon catheter was contaminated with graded inocula of either PS/A-positive S epidermidis strain M187sp11 or the PS/A-negative, isogenic strain M187sn3 and inserted into the left ventricle through the aortic valve. The PS/A-positive strain had a 50% infectious dose of 1.1 x 10(2) cfu (95% CI, 3.3 to 3.7 x 10(3)) compared with 8.5 x 10(4) cfu of the PS/A-negative strain (95% CI, 8.6 x 10(3) to 8.5 x 10(5)). The odds for developing endocarditis were estimated to be 42 times higher for any given inoculum level of the PS/A-positive strain (P = .1). When the PS/A-positive strain was adherent to a catheter surface it survived in rabbit blood, whereas under the same conditions the PS/A-negative strain was killed approximately 90% in 1 hour.

CONCLUSIONS

Direct contamination of an intraventricular foreign body by low levels of PS/A-positive S epidermidis results in endocarditis in rabbits, but at suitably high doses PS/A-negative strains have sufficient virulence to infect cardiac vegetations. PS/A enhances but is not absolutely required for bacterial virulence in a rabbit model of PVE.

Long-term antibiotic treatment of chronic bacterial prostatitis. Effect on bacterial flora

The bacterial flora in patients referred with chronic bacterial prostatitis were studied. Only 13% had Gram-negative bacteria in significant numbers but 43% had Gram-positives using the same criteria. Half of the patients were symptom-free by the end of a 12-week course of antibiotics and remained so after 6 months; in one-third the symptoms were unchanged 6 months after completing treatment. The relief of symptoms correlated with the disappearance of white blood cells in the expressed prostatic secretion (EPS) and with a lowered pH in the EPS. Thus only 1 of the 14 patients without symptoms at 6 months had a significant growth of bacteria at the prostatic level, whereas 7 of 10 patients with unchanged symptoms had a significant bacterial colonisation. Although the initially infecting organism was eliminated in about half of the patients, new Gram-positive bacteria were isolated after treatment in 13 of 29 patients; 12 of these resolved spontaneously within 6 months. Five patients with Gram-positive bacteria were colonised with new Gram-negatives at the end of treatment. Two healed spontaneously but 3 remained colonised with Gram-negatives at the end of the follow-up period. These findings make it likely that many patients infected with Gram-positive bacteria benefit from antibiotic treatment. However, disturbances in the bacterial flora by antibiotic treatment may facilitate invasion by new types of bacteria.

Enzyme-linked immunosorbent assay for detection of Staphylococcus epidermidis antibody in experimental S. epidermidis endocarditis

The immune response against Staphylococcus epidermidis, as determined by an enzyme-linked immunosorbent assay, was evaluated in experimental S. epidermidis infections in rabbits. Antigens from 8 of 10 clinical S. epidermidis strains detected significant antibody production in five rabbits immunized with different strains of S. epidermidis and in five of six rabbits with experimental endocarditis caused by four different strains. The antigens from two strains detected antibody production in all rabbits, and strain ATCC 14990 discriminated best between positive and negative samples. Consecutive blood samples from rabbits with endocarditis and control rabbits with bacteremia, which were successfully prevented from developing endocarditis by using prophylactic antibiotics, were examined by using an enzyme-linked immunosorbent assay and an ultrasonic extract of strain ATCC 14990 as the antigen. This assay discriminated between rabbits with endocarditis and rabbits with uncomplicated bacteremia. Antibody production was detected as early as 3 days after the onset of infection in rabbits with endocarditis.

Enzyme-linked immunosorbent assay for detection of immunoglobulin G and M antibodies to teichoic acid in intravascular staphylococcal disease

An enzyme-linked immunosorbent assay (ELISA) for detection of IgG and IgM antibodies to cell-wall teichoic acids of Staphylococcus aureus and three defined coagulase-negative staphylococci was tested using serum samples from 11 cases of intravascular coagulase-negative staphylococcal infections, 13 cases of Staphylococcus aureus endocarditis, and 24 patients with no evidence of infection. IgG antibody titers to all four teichoic acids in the 13 patients with Staphylococcus aureus endocarditis were significantly different from those in noninfected control patients (p less than 0.0001). In contrast, IgG antibody titers in serum from 11 cases of intravascular coagulase-negative staphylococcal infection were not significantly different from those in control sera. There were no differences in IgM antibody titers of the three groups. Although the ELISA was sensitive in detecting Staphylococcus aureus endocarditis, it was not reliable in the detection of intravascular coagulase-negative staphylococcal infections, even when tested with specific teichoic acid.