Virulence Factors of Mutans Streptococci Role of Molecular Genetics

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Transcript of Virulence Factors of Mutans Streptococci Role of Molecular Genetics

Critical Reviews http://cro.sagepub.com/ in Oral Biology & Medicine

Virulence Factors of Mutans Streptococci: Role of Molecular GeneticsHoward K. Kuramitsu CROBM 1993 4: 159 DOI: 10.1177/10454411930040020201 The online version of this article can be found at: http://cro.sagepub.com/content/4/2/159

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Critical Reviews in Oral Biology and Medicine, 4(2):159-176 (1993)

Virulence Factors of Mutans Streptococci: Role of Molecular GeneticsHoward K. KuramitsuDepartments of Pediatric Dentistry and Microbiology, University of Texas Health Science Center, San Antonio, TX ABSTRACT: Biochemical approaches were utilized initially to identify the virulence factors of the mutans streptococci (primarily Streptococcus mutans and 5. sobrinu). Traditional mutant analysis of these organisms further suggested the important role of several of these factors in cariogenicity. However, because these mutations were not clearly defined, the utilization of cloned genes was necessary to verify their significance. The introduction of molecular genetic approaches for characterizing these factors has led not only to a clearer understanding of the role of these virulence factors in cariogenicity but has also suggested some novel approaches for reducing further the incidence of dental caries. KEY WORDS: Streptococcus mutans, S. sobrinus, molecular genetics, adhesins, sucrose-enhanced colonization of teeth, glucosyltransferases.

I. INTRODUCTION Despite the recent decline in dental caries frequency among children in technologically advanced societies, tooth decay still remains a major health problem (Loesche, 1986; Tanzer, 1992). In addition, it is not clear yet whether or not this decline will continue into the next century. The significant improvement in the oral health status of Western children has been attributed primarily to the widespread utilization of fluoride together with improvements in oral care (Glass, 1982). Nevertheless, there still remains a sizeable proportion of the population that is at risk of developing carious lesions (Krasse, 1985). Therefore, a more detailed understanding of the bacterial-host interactions that lead to dental caries may be of value in developing additional anticaries strategies that may further decrease the frequency of this disease. In this respect, the recent introduction of molecular genetic approaches for examining the virulence of mutans streptococci (Loesche, 1986) may yield innovative methods for both the identification of children at high risk of developing dental caries as well as the prevention of decay. Several recent reviews have addressed various aspects of this topic (Curtiss, 1985;

Macrina et ai, 1990; Russell, 1990), and this article focuses primarily on the relationship between the results of these newer approaches to the earlier investigations (Loesche, 1986; Tanzer, 1992). Furthermore, since the publication of these reviews, the results of animal studies utilizing defined mutants constructed from cloned Streptococcus mutans genes are now available. In addition, there is a discussion of several unresolved issues relating to the cariogenicity of the mutans streptococci.

II. VIRULENCE PROPERTIES OF MUTANS STREPTOCOCCI IDENTIFIED FOLLOWING COMPARISON WITH OTHER ORAL BACTERIA Despite the complexity of the human oral flora, pioneering animal model studies (Fitzgerald and Keyes, 1960), as well as human epidemiological surveys (Loesche, 1986), have strongly implicated mutans streptococci (primarily S. mutans) as the principal etiological agents in human dental caries. Therefore, for the past several decades there have been extensive efforts to identify the virulence factors of these organisms as

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159

potential targets for anticaries prophylaxis. Initially, potential virulence properties of the mutans streptococci were identified following comparison of these organisms with other oral streptococci. One of the first putative virulence factors of the mutans streptococci identified by this approach was the ability of these organisms to colonize smooth surfaces in vitro in the presence of sucrose (Gibbons etal, 1966). Subsequently, this property was shown to be dependent on the synthesis of water-insoluble glucans from the disaccharide. Although several other oral streptococci (5. sanguis, S. salivarius) also have the ability to synthesize these polysaccharides (Hamada and Slade, 1980), only the mutans streptococci generally display sucrose enhanced colonization (colonization is defined in this review as the aggregation of bacteria onto hard surfaces following initial attachment and subsequent accumulation) of smooth surfaces (Loesche, 1986). One recent exception to this rule has been reported for S. gordonii strains in vitro fVickerman et ai, 1991), but its significance in vivo has yet to be established. Therefore, it was quite evident almost 30 years ago that the mutans streptococci are capable of synthesizing colonization promoting glucans from sucrose. Because of the extensive epidimeological evidence linking the incidence of human dental caries with sucrose consumption (Newbrun, 1982), major emphasis was placed on examining the mechanism of glucan synthesis. However, despite detailed investigations regarding the synthesis of these exopolysaccharides and the role of glucosyltransferases (GTF) in this process (Loesche, 1986), the chemical nature of the glucans involved in colonization have not been precisely defined yet. Because of this unique colonization property exhibited by the mutans streptococci, it was not surprising that these organisms also exhibited clumping (aggregation) when grown in the presence of sucrose. This characteristic could be rationalized as another example of sucrose-dependent attachment of cells to solid surfaces due to insoluble glucan formation. However, in addition, many of these organisms also aggregated when cells were incubated in the presence of high molecular weight glucans (especially water soluble high molecular weight dextrans) (Gibbons and Fitzgerald, 1969). This property was also unique to the mutans streptococci and was postulated to160

play a role in colonization. More recently, it has been suggested that such interactions may also aid in the initial attachment of the mutans streptococci to glucans formed within the pellicle of teeth (Schilling and Bowen, 1992). Therefore, two of the earliest virulence properties associated with these organisms were their ability to synthesize colonization, promoting insoluble glucans, and their interaction with these polysaccharides. Despite the initial emphasis on the role of insoluble glucan formation in the colonization of tooth surfaces by the mutans streptococci, both in vitro (Staat et al., 1980) as well as in vivo (Van Houte et aL, 1976) results have suggested that these organisms do not require sucrose for colonization. Subsequent approaches from several laboratories (Douglas and Russell, 1984; Russell, 1986, Lee et aL, 1989) have suggested that these organisms are capable of attaching to the pellicle of teeth by means of putative adhesin-like cell surface molecules. As detailed below (Section IV), molecular genetic approaches have suggested several candidates for this phenomenon. Furthermore, Gibbons et al. (1986) have demonstrated differences in tooth colonization mechanisms that depend on the group of mutans streptococci involved: S. mutans strains apparently attach by both adhesin and glucan mediated mechanisms, whereas S. sobrinus strains utilize primarily the latter process. Because dental caries is ultimately related to the acidogenicity of plaque bacteria, the fermentation patterns of the mutans streptococci have been examined extensively (Hamada and Slade, 1980). These organisms were demonstrated to ferment a wide variety of sugars and it was of special interest that they appear to metabolize sucrose to lactic acid more rapidly than other oral bacteria (Minah and Loesche, 1977). This property of the mutans streptococci is undoubtedly related to the multitude of enzyme systems expressed by these organisms that are capable of both transporting and metabolizing sucrose (Loesche, 1986). It was also reasonable to assume that an important virulence property of cariogenic bacteria should be their ability to continue fermentation in the absence of exogenous food supplies (conditions that are likely to be most conducive for tooth demineralization due to the reduction in salivary flow during these periods) (Loesche, 1986). There-

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fore, the observation that some strains of S. mutans that were highly virulent in rats were capable of intracellular polysaccharide storage (IPS) (Tanzer et a/., 1976), whereas others that were not capable of IPS were avirulent, suggested another potential virulence property of these organisms. Because dental plaque pH becomes acidic in the presence of a fermentable carbon source (Loesche, 1986), the relative aciduricity of cariogenic bacteria may also serve as a virulence factor. A comparison of the relative aciduricity of oral bacteria (Harper and Lo