Bacteriocins of gram-positive bacteria.

Dental plaque forms naturally on teeth and is of benefit to the host by helping to prevent colonization by exogenous species. The bacterial composition of plaque remains relatively stable despite regular exposure to minor environmental perturbations. This stability (microbial homeostasis) is due in part to a dynamic balance of both synergistic and antagonistic microbial interactions. However, homeostasis can break down, leading to shifts in the balance of the microflora, thereby predisposing sites to disease. For example, the frequent exposure of plaque to low pH leads to inhibition of acid-sensitive species and the selection of organisms with an aciduric physiology, such as mutans streptococci and lactobacilli. Similarly, plaque accumulation around the gingival margin leads to an inflammatory host response and an increased flow of gingival crevicular fluid. The subgingival microflora shifts from being mainly Gram-positive to being comprised of increased levels of obligately anaerobic, asaccharolytic Gram-negative organisms. It is proposed that disease can be prevented or treated not only by targeting the putative pathogens but also by interfering with the processes that drive the breakdown in homeostasis. Thus, the rate of acid production following sugar intake could be reduced by fluoride, alternative sweeteners, and low concentrations of antimicrobial agents, while oxygenating or redox agents could raise the Eh of periodontal pockets and prevent the growth and metabolism of obligately anaerobic species. These views have been incorporated into a modified hypothesis (the "ecological plaque hypothesis") to explain the relationship between the plaque microflora and the host in health and disease, and to identify new strategies for disease prevention.

Microbial ecology of dental plaque and its significance in health and disease.

This article describes the subgingival microflora of the healthy periodontium, gingivitis, advanced adult periodontitis, and juvenile periodontitis. A total of seven to nine subjects were examined in each of the four periodontal clinical entities listed. The individual bacteriological samples included material from the base of a single periodontal pocket. The sampling, the treatment of the samples, and the bacteriological cultivations were carried out using continuous anaerobic techniques.

Briefly, the healthy gingival sulcus harbored a scant microflora dominated by Gram-positive organisms (85%), usually Streptococcus and facultative Actinomyces species. The development of gingivitis was accompanied by a marked increase in the total number of Gram-negative organisms. Fusobacterium nucleatum, Bacteroides melaninogenicus ss. intermedius, Haemophilus species, and other Gram-negative organisms comprised about 45% of the total gingivitis isolates. Streptococcus and facultative and anaerobic Actinomyces species constituted the majority of the Gram-positive gingivitis isolates. The micro-flora of advanced adult periodontitis was comprised mainly of Gram-negative anaerobic rods (about 75%), B. melaninogenicus ss. asaccharolyticus and F. nucleatum being the most predominant isolates. The deep pocket microflora in juvenile periodontitis was also made up mainly of Gram-negative organisms (about 65%), but was of a nature different from that of adult periodontitis, being predominated by isolates of Bacteroides species and other organisms of unknown species.

The present article also concerns factors of importance for the colonization of Gram-negative anaerobic rods in the oral cavity and periodontal pockets. In vitro experiments showed that cells of B. melaninogenicus ss. asaccharolyticus and other Gram-negative organisms attached in high numbers to epithelial cells, hydrosyapatite (HA) surface, and Gram-positive bacteria when suspended in phosphate-buffered saline; however, the bacterial attachment to epithelial cells and HA was strongly inhibited in the presence of human saliva and serum. In contrast, saliva and serum had little effect upon the attachment of Gram-negative bacteria to Gram-positive bacterial cells. These findings agreed well with data from an in vivo study, in which streptomycin-labeled cells of B. melaninogenicus ss. asaccharolyticus were introduced into the mouth of two volunteers. A significantly higher number of B. melaninogenicus cells was recovered from dental plaque than from the other oral surfaces studied.

The present series of studies has pointed to certain Gram-negative organisms as potential pathogens in rapidly progressing periodontal lesions. The available data on oral microbial ecology suggest that the presence of dental plaque containing Gram-positive organisms may be essential for the attachment and colonization of several Gram-negative species after their initial introduction into the mouth and the periodontal pocket area. The clinical relevance of these findings is discussed.

K. Holmberg

Papers

Production of bactericidal concentrations of hydrogen peroxide by Streptococcus sanguis.