Several proteins from culture supernatants of Streptococcus sobrinus were able to bind avidly to Sephadex G-75. The proteins could be partially eluted from the Sephadex by low-molecular-weight ␣-1,6 glucan or fully eluted by 4 M guanidine hydrochloride. Elution profiles were complex, yielding proteins of 16, 45, 58 to 60, 90, 135, and 145 kDa, showing that the wild-type strain possessed multiple glucan-binding proteins. Two mutants of Streptococcus sobrinus incapable of aggregation by high-molecular-weight ␣-1,6 glucan were isolated. One mutant was spontaneous, from a cell suspension to which glucan had been added, whereas the other was induced by ethyl methanesulfonate. Both mutants were devoid of a 60-kDa protein, as shown by gel electrophoresis of culture supernatants and whole cells. Amino acid analysis showed that the 58-to 60-kDa protein and the 90-kDa protein were distinct, although both were N-terminally blocked. Both mutants retained their ability to adhere to glass in the presence of sucrose and to ferment mannitol and sorbitol. Both mutants retained their glucosytransferase activities, as shown by activity gels. Western blots (immunoblots), employing antibody against a glucan-binding protein of Streptococcus mutans, failed to reveal cross-reactivity with S. sobrinus proteins. The results show that even though S. sobrinus produces several proteins capable of binding ␣-1,6 glucans, the 60-kDa protein is probably the lectin needed for glucan-dependent cellular aggregation.Bacterial lectins seem to have evolved for the purpose of permitting the bacteria to adhere to glycoconjugate-containing surfaces. Members of the genus Streptococcus, found in the oral cavities of humans and animals, are able to adhere to a variety of surfaces when incubated in the presence of sucrose. The disaccharide sucrose serves as a substratum for glucosyltransferases (GTFs) to yield glucans rich in ␣-1,6 and/or ␣-1,3 linkages. Many of the streptococci produce glucan-binding lectins (GBLs) or glucan-binding proteins (GBPs). Several years ago, McCabe and colleagues were able to purify streptococcal GBPs by using Sephadex as an affinity matrix (12,16,17). Others have now used similar chromatographic techniques to purify streptococcal GBPs (2,14,22,23,28,29). Banas et al. (3) were able to clone and sequence a GBP from Streptococcus mutans Ingbritt. In addition to GBLs, the streptococcal GTFs possess glucan-binding domains (19,27). Many of the bacteria which possess cell surface GTFs are not aggregated by glucans, suggesting that the GTFs do not act as lectins. The purpose of the present study was to define which surface protein of Streptococcus sobrinus is involved in glucan-dependent aggregation. To avoid confusion, a GBP is defined as any protein capable of binding reversibly to Sephadex, whereas a GBL is defined as any Sephadex-binding protein capable of conferring the ability of the bacteria to be aggregated by ␣-1,6 glucan. It is here shown that S. sobrinus possesses at least five GBPs, but only one of these, the 60-kDa band of ...

Lactoferrin (LF) is an iron-binding glycoprotein common to exocrine secretions and the specific granules of neutrophils. Each molecule is capable of high-affinity coordinate-binding of two ferric ions with two bicarbonate or carbonic anions. The initial aspect of the present study was directed at determining the nature of anion involvement in LF bactericidal activity. It was found that selective anions were capable of inhibiting the expression of bactericidal activity by LF on S. mutans 10449. The ability to block LF expression was directly related to the capacity of the anion to serve as a coordinate ion in iron-binding by the transferrin molecules. These data support the hypothesis that the LF target site on the bacterial surface is anionic. There has been controversy in the literature regarding LF involvement in hydroxy radical generation. The second phase of these studies indicated that treatment of S. mutans with LF under anaerobic conditions abrogated the bactericidal effect of this molecule. LF-killing could be enhanced by the presence of thiocyanate and inhibited by catalase and lactoperoxidase; however, bovine serum albumin was equally effective as an inhibitor. The apparent requirement for oxygen in LF bactericidal effect on S. mutans is not inconsistent with a hydroxy radical mechanism.

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