Reassessing the role of Staphylococcus aureus clumping factor and fibronectin-binding protein by expression in Lactococcus lactis

Since Staphylococcus aureus expresses multiple pathogenic factors, studying their individual roles in single-gene-knockout mutants is difficult. To ci rcumvent this problem, S. aureus clumping factor A (clfA) and fibronectin-b inding protein A (fnbA) genes were constitutively expressed in poorly patho genic Lactococcus lactis using the recently described pOri23 vector. The re combinant organisms were tested in vitro for their adherence to immobilized fibrinogen and fibronectin and in vivo for their ability to infect rats wi th catheter-induced aortic vegetations. In vitro, both clfA and fnbA increa sed the adherence of lactococci to their specific ligands to a similar exte nt as the S. aureus gene donor. In vivo, the minimum inoculum size producin g endocarditis in greater than or equal to 80% of the rats (80% infective d ose [ID80]) with the parent lactococcus was greater than or equal to 10(7) CFU. In contrast, clfA-expressing and fnbA-expressing lactococci required o nly 10(5) CFU to infect the majority of the animals (P < 0.00005). This was comparable to the infectivities of classical endocarditis pathogens such a s S. aureus and streptococci (ID80 = 10(4) to 10(5) CFU) in this model. The results confirmed the role of clfA in endovascular infection, but with a m uch higher degree of confidence than with single-gene-inactivated staphyloc occi. Moreover, they identified fnbA as a critical virulence factor of equi valent importance. This was in contrast to previous studies that produced c ontroversial results regarding this very determinant. Taken together, the p resent observations suggest that if antiadhesin therapy were to be develope d, at least both of the clfA and fnbA products should be blocked for the th erapy to be effective.