100+ Free DAT Reading Comprehension Practice Questions

PASSAGE 2 (Questions 26–50): The human oral cavity harbors one of the most complex microbial ecosystems in the body, hosting more than 700 distinct bacterial species collectively known as the oral microbiome. This community is not merely a passive resident; it plays an active role in both maintaining health and initiating disease. Under normal homeostatic conditions, commensal bacteria outcompete potentially pathogenic species, maintaining a stable ecological balance. However, disruptions to this balance—a process called dysbiosis—are central to the development of the two most prevalent oral diseases: dental caries and periodontitis. Dental caries is associated with a shift in the oral microbiome toward acid-tolerant, acid-producing organisms. Streptococcus mutans and Lactobacillus species are among the most studied cariogenic organisms. These bacteria possess the enzymatic machinery to ferment dietary sugars into organic acids, most notably lactic acid. Crucially, they also tolerate low-pH environments that inhibit competing, less acid-tolerant species. As the ecological balance tips toward these aciduric organisms, acid production becomes more sustained, creating a self-reinforcing acidic niche that progressively demineralizes enamel. Periodontitis, by contrast, involves a dysbiotic shift in the subgingival microbiome—the community living beneath the gumline. Key pathogens include Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola, collectively termed the 'red complex.' These anaerobic, gram-negative bacteria release lipopolysaccharides (LPS) and proteolytic enzymes that trigger a chronic inflammatory response in the periodontal tissues. It is important to note that tissue destruction in periodontitis is not primarily caused directly by bacteria but by the host's own immune response to the dysbiotic community—a distinction with major therapeutic implications. Emerging research highlights bidirectional systemic connections between oral dysbiosis and whole-body health. Periodontal pathogens and their metabolic products can enter systemic circulation through inflamed periodontal tissues, potentially contributing to cardiovascular disease, adverse pregnancy outcomes, and glycemic dysregulation in diabetic individuals. These connections are not fully proven causally, but epidemiological evidence consistently links poor periodontal health with elevated risk for these systemic conditions. Interventions targeting oral dysbiosis focus on mechanical biofilm disruption (brushing, scaling), antimicrobial agents (chlorhexidine), and emerging probiotic strategies. Probiotics introduce beneficial bacteria that compete with pathogens for adhesion sites and nutrients, nudging the microbiome toward a healthier composition. However, the long-term efficacy of probiotic approaches remains under investigation, and no probiotic product has yet received regulatory approval specifically for treating periodontitis. According to the passage, what is the term used to describe a disruption in the normal ecological balance of the oral microbiome?