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The oral biofilm · Microbiology · INBDE Patient Cases

Oral Microbiology INBDE Patient Cases

By Dr. Isaac Sun, DDS

10 ADA INBDE-format patient cases on oral microbiology. Each case is a shared patient box (chief complaint, history, medications, allergies, exam) followed by linked multiple-choice questions with full distractor explanations. Practice the way the real exam is structured.

10 patient cases50 linked questionsADA INBDE formatFull distractor explanations

Ten ADA INBDE-format patient cases connecting oral microbiology to dentistry: high caries risk and the cariogenic ecological shift toward acidogenic, aciduric Streptococcus mutans (with glucan adhesion and lactobacilli in lesion progression), dental plaque as a biofilm and why its matrix means mechanical disruption beats a mouthrinse, calculus as mineralized plaque and the microbial succession from early streptococci to Gram-negative anaerobes, chronic periodontitis as a dysbiosis driven by the red complex (Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola) in the low-oxygen pocket, aggressive molar-incisor periodontitis and Aggregatibacter actinomycetemcomitans with its neutrophil-killing leukotoxin, acute necrotizing ulcerative gingivitis (ANUG) as a fusospirochetal overgrowth, Candida-associated denture stomatitis and pseudomembranous candidiasis (thrush) after antibiotics or inhaled steroids, intraoral halitosis from anaerobic volatile sulfur compounds on the tongue, and root surface caries with xerostomia in an older adult. Topics include normal oral flora and colonization resistance, the acquired pellicle and plaque biofilm formation, the microbiology of caries and periodontitis, ANUG, and oral candidiasis.

Case Coverage Map
What each case is testing
High caries risk and the cariogenic shift:
S. mutans as initiator, acidogenic and aciduric, glucan adhesion, lactobacilli in progression, and the ecological shift from sugar frequency.
"Why won't mouthwash fix my gums?":
Plaque as a biofilm, why the matrix limits antimicrobial penetration, mechanical disruption, and rinses as adjuncts only.
Heavy calculus and how plaque matures:
Calculus as mineralized plaque and a retentive surface, early streptococcal colonizers, and the succession to Gram-negative anaerobes.
Adult bone loss and the anaerobic shift:
Chronic periodontitis dysbiosis, the red complex, the low-oxygen pocket, debridement as the cornerstone, and smoking as a risk factor.
Rapid bone loss in a young patient:
Aggressive (molar-incisor) periodontitis, A. actinomycetemcomitans and its leukotoxin, loss out of proportion to plaque, and adjunctive antibiotics.
Sudden painful, bleeding gums in a stressed student:
ANUG as a fusospirochetal overgrowth, punched-out papillae and fetor, stress and smoking, debridement, and when to add metronidazole.
Red palate under a full denture:
Candida-associated denture stomatitis, the denture as a reservoir, predisposing factors, denture hygiene, and diabetes.
White patches after a course of antibiotics:
Pseudomembranous candidiasis, wipeable plaques, antibiotics and inhaled steroids as causes, treatment, and recurrence as a sentinel sign.
Persistent bad breath despite brushing:
Intraoral halitosis from Gram-negative anaerobes, volatile sulfur compounds, the tongue-coating reservoir, and tongue cleaning.
Decay on exposed roots with a dry mouth:
Root surface caries, the higher critical pH of cementum and dentin, Actinomyces, xerostomia, sugar frequency, and fluoride prevention.
Patient case: High caries risk and the cariogenic shift
0 of 5 answered, 0 correct
Patient
Male, 22 years old
Chief Complaint
"I keep getting new cavities and I don't understand why."
Background and/or Patient History
  • Multiple new lesions over the past year
  • Sips sweetened coffee and energy drinks through the day
  • Brushes once daily, rarely flosses
  • Healthy, no medications
Allergies
NKDA
Medications
  • None
Current Findings
  • Several active enamel lesions and heavy plaque
  • Normal salivary flow
  1. Question 1
    Easy
    The bacterial species most responsible for initiating his enamel caries is:
  2. Question 2
    Moderate
    His frequent sugary drinks promote caries by causing an ecological shift that favors species that are:
  3. Question 3
    Hard
    S. mutans makes plaque stick firmly to enamel by converting sucrose into:
  4. Question 4
    Moderate
    The lesions of established, deeper caries are particularly associated with:
  5. Question 5
    Moderate
    The most effective long-term advice to lower his caries activity is to:

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Patient case: "Why won't mouthwash fix my gums?"
0 of 5 answered, 0 correct
Patient
Female, 37 years old
Chief Complaint
"My gums bleed, but I use mouthwash every day. Why isn't it working?"
Background and/or Patient History
  • Relies on an antiseptic rinse instead of flossing
  • Brushes quickly once a day
  • Gums bleed when brushing
  • No relevant medical history
Allergies
NKDA
Medications
  • None
Current Findings
  • Generalized marginal gingival inflammation and bleeding on probing
  • Visible plaque along the gumline and interproximally
  1. Question 1
    Moderate
    Dental plaque is a biofilm, which means it is a community of bacteria:
  2. Question 2
    Moderate
    Her daily mouthwash alone does not resolve the inflammation mainly because the biofilm matrix:
  3. Question 3
    Easy
    The most effective way to control her gingival biofilm is:
  4. Question 4
    Moderate
    Her bleeding on probing most directly reflects:
  5. Question 5
    Moderate
    An antiseptic mouthrinse such as chlorhexidine is best understood as:

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Patient case: Heavy calculus and how plaque matures
0 of 5 answered, 0 correct
Patient
Male, 55 years old
Chief Complaint
"There's hard buildup on my lower front teeth that I can't brush off."
Background and/or Patient History
  • Has not had a cleaning in several years
  • Notices hard deposits behind the lower incisors
  • Brushes but does not floss
  • Otherwise healthy
Allergies
NKDA
Medications
  • None
Current Findings
  • Heavy supragingival and subgingival calculus, lingual lower anteriors
  • Generalized gingival inflammation
  1. Question 1
    Easy
    The hard deposit he cannot brush away (calculus) is essentially:
  2. Question 2
    Moderate
    Calculus contributes to gum disease mainly because it:
  3. Question 3
    Moderate
    In plaque maturation, the first organisms to colonize the pellicle are:
  4. Question 4
    Moderate
    As the subgingival plaque around his teeth matures, the community shifts toward:
  5. Question 5
    Moderate
    The appropriate treatment for his calculus and inflammation is:

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Patient case: Adult bone loss and the anaerobic shift
0 of 5 answered, 0 correct
Patient
Female, 49 years old
Chief Complaint
"My gums have pulled back and a couple of teeth feel loose."
Background and/or Patient History
  • Gradual gum recession and occasional bad taste
  • Long-standing irregular dental care
  • Smokes half a pack a day
  • Otherwise healthy
Allergies
NKDA
Medications
  • None
Current Findings
  • Generalized 5 to 7 mm pockets with bleeding on probing
  • Radiographic horizontal bone loss; mild mobility of two molars
  1. Question 1
    Moderate
    Compared with periodontal health, her deep pockets are colonized by a flora that has shifted toward:
  2. Question 2
    Moderate
    The organisms most associated with chronic periodontitis (the red complex) include:
  3. Question 3
    Moderate
    The deep pocket favors anaerobes because it is:
  4. Question 4
    Moderate
    The cornerstone of treating her periodontitis is:
  5. Question 5
    Moderate
    Her smoking is relevant to the periodontitis because it:

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Patient case: Rapid bone loss in a young patient
0 of 5 answered, 0 correct
Patient
Female, 19 years old
Chief Complaint
"My front teeth and back teeth are getting loose and I'm only 19."
Background and/or Patient History
  • Rapid loosening over the past year
  • Good general hygiene, little visible plaque
  • Family history of early tooth loss
  • Healthy otherwise
Allergies
NKDA
Medications
  • None
Current Findings
  • Deep pockets and severe bone loss localized to first molars and incisors
  • Bone loss out of proportion to the modest plaque present
  1. Question 1
    Moderate
    The pattern of severe, rapid bone loss at the molars and incisors in a young patient suggests:
  2. Question 2
    Moderate
    The organism classically associated with this condition is:
  3. Question 3
    Hard
    A virulence feature of this organism that helps explain the rapid destruction is its:
  4. Question 4
    Moderate
    A notable feature in aggressive periodontitis is that the bone loss is often:
  5. Question 5
    Moderate
    Beyond mechanical debridement, management of aggressive periodontitis more often also includes:

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Patient case: Sudden painful, bleeding gums in a stressed student
0 of 5 answered, 0 correct
Patient
Male, 21 years old
Chief Complaint
"My gums suddenly hurt a lot, bleed, and my breath is terrible."
Background and/or Patient History
  • Two days of rapidly worsening painful gums during exam week
  • Smokes, sleeping poorly, eating little
  • No prior gum problems
  • Otherwise healthy
Allergies
NKDA
Medications
  • None
Current Findings
  • Punched-out, cratered interdental papillae with gray pseudomembrane
  • Spontaneous bleeding and strong fetid odor; low-grade fever
  1. Question 1
    Moderate
    This presentation is most consistent with:
  2. Question 2
    Moderate
    ANUG is a fusospirochetal infection, meaning it is driven mainly by:
  3. Question 3
    Moderate
    Which combination of his risk factors most predisposed him to ANUG?
  4. Question 4
    Moderate
    The primary local treatment of ANUG is:
  5. Question 5
    Moderate
    A systemic antibiotic (commonly metronidazole) is added to ANUG care mainly when there is:

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Patient case: Red palate under a full denture
0 of 5 answered, 0 correct
Patient
Female, 68 years old
Chief Complaint
"The roof of my mouth under my denture is red and a bit sore."
Background and/or Patient History
  • Wears a maxillary complete denture day and night
  • Rarely removes or cleans it thoroughly
  • Mild dry mouth
  • Type 2 diabetes
Allergies
NKDA
Medications
  • Metformin
Current Findings
  • Diffuse erythema of the palatal mucosa matching the denture outline
  • No wipeable plaques; denture surface coated with debris
  1. Question 1
    Moderate
    The palatal erythema confined to the denture-bearing area is most consistent with:
  2. Question 2
    Easy
    The organism most responsible is:
  3. Question 3
    Moderate
    Which factors in her history most favor Candida overgrowth?
  4. Question 4
    Moderate
    A key part of management, alongside any antifungal, is to:
  5. Question 5
    Moderate
    Her diabetes is relevant to recurrent candidiasis because:

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Patient case: White patches after a course of antibiotics
0 of 5 answered, 0 correct
Patient
Male, 60 years old
Chief Complaint
"I have white patches in my mouth and it feels sore."
Background and/or Patient History
  • Just finished a 10-day broad-spectrum antibiotic course for a chest infection
  • Uses an inhaled corticosteroid and does not rinse after
  • Reports a cottony, sore mouth
  • No known immune problems
Allergies
NKDA
Medications
  • Inhaled corticosteroid
Current Findings
  • Creamy white plaques on the buccal mucosa and tongue
  • Plaques wipe off leaving a red, slightly bleeding base
  1. Question 1
    Moderate
    White plaques that wipe off to leave a red base are characteristic of:
  2. Question 2
    Moderate
    His recent broad-spectrum antibiotics predisposed him to thrush by:
  3. Question 3
    Moderate
    His inhaled corticosteroid contributes to oral thrush; a simple preventive step is to:
  4. Question 4
    Moderate
    Appropriate treatment of his thrush is:
  5. Question 5
    Moderate
    If thrush recurred in a healthy-looking adult with no obvious cause, the dentist should consider:

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Patient case: Persistent bad breath despite brushing
0 of 5 answered, 0 correct
Patient
Male, 44 years old
Chief Complaint
"People tell me my breath is bad even though I brush my teeth."
Background and/or Patient History
  • Long-standing bad breath, socially embarrassing
  • Brushes teeth but never cleans the tongue
  • No reflux symptoms, no sinus complaints
  • Otherwise healthy
Allergies
NKDA
Medications
  • None
Current Findings
  • Thick coating on the posterior tongue dorsum
  • Mild gingivitis; no deep pockets or abscess
  1. Question 1
    Moderate
    Most intraoral halitosis is produced by bacteria that are:
  2. Question 2
    Moderate
    The malodorous compounds these bacteria generate are mainly:
  3. Question 3
    Moderate
    The single biggest intraoral reservoir for these odor-producing bacteria is usually the:
  4. Question 4
    Moderate
    The most effective addition to his routine is:
  5. Question 5
    Moderate
    Before attributing the halitosis solely to the tongue, it is reasonable to also check for:

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Patient case: Decay on exposed roots with a dry mouth
0 of 5 answered, 0 correct
Patient
Female, 72 years old
Chief Complaint
"I'm getting cavities on the roots of my teeth near the gumline."
Background and/or Patient History
  • Gum recession exposing root surfaces
  • Dry mouth she attributes to several medications
  • Snacks on crackers and sweet tea through the day
  • Brushes but finds the gumline tender
Allergies
NKDA
Medications
  • Several, including ones that cause dry mouth
Current Findings
  • Soft, cavitated lesions on exposed root surfaces near the gingival margin
  • Reduced salivary flow
  1. Question 1
    Hard
    Exposed root surfaces (cementum and dentin) are more caries-prone than enamel partly because they:
  2. Question 2
    Moderate
    Root surface caries is commonly associated with:
  3. Question 3
    Moderate
    Her dry mouth (xerostomia) increases caries risk because reduced saliva means less:
  4. Question 4
    Moderate
    Her frequent crackers and sweet tea matter because caries risk depends heavily on:
  5. Question 5
    Moderate
    A sensible prevention plan for her root caries includes:

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Oral Microbiology core recall

Refresh the anatomy facts these cases depend on: nerve numbers, foramina, functions, and lesion findings.

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