George Baehr was an American physician and provided one of the most comprehensive descriptions of renal injuries in subacute bacterial endocarditis. In the introduction of his article, Baehr noted:
"In the kidneys of nearly all individuals dying during the course of subacute bacterial endocarditis, there exists a pathological lesion which affects one or more loops of a variable proportion of the glomeruli." He also mentioned that "these lesions, as a rule, have been called glomerulo-nephritis" and were commonly being revealed by hematuria.
Baehr also gave credit to the pioneering work of Lohlein who had already reported several cases of subacute bacterial endocarditis due to Streptococcus viridians which were complicated with glomerulo-nephritis.
Baehr performed histological studies of the kidneys of 34 patients with subacute bacterial endocarditis. The isolated microorganism was Streptococcus viridians in 25 cases. He defined these patients as "in the bacterial stage of the disease."
In his microscopic analysis, he observed the following renal injuries:
"The very earliest stage of the process appears to consist of a swelling of the glomerular epithelium in the involved loops...The capillaries are compressed, contain no blood, and their lumina are obliterated...Subsequently the outlines of the swollen cells become lost and the entire structure is fused into a homogenous finely granular material...the presence of a small amount of fibrin can often be demonstrated... Evolution of these lesions is characterized by epithelial proliferation and desquamation from Bowman's capsule...Eventually this epithelium undergoes necrosis."
He then noted that cellular debris was progressively replaced by a connective tissue network with occasional fibroblasts proliferation. Simultaneously, "There occurs an active growth of the epithelial cells of the adjacent intact portions of the parietal layer of Bowman's capsule. They advance over the inner aspect of the necrotic mass which by this time has usually separated somewhat from the rest of the glomerulus. The end stage is the formation of a healed lesion defined as a solid mass of dense fibrous material." Baehr wrote in his comments that he had observed these types of lesions in all his microscopic sections and that a common pathological feature was the association of lesions of various stages. He also commented that "usually a minority of the glomeruli were affected."
He also performed histological analysis of specimen in 7 patients with healed (or bacteria-free stage) subacute bacterial endocarditis. In six of these cases, he observed only healed lesions of glomerulonephritis as described above.