High-Yield Microbiology for the INBDE
INBDE microbiology is clinical microbiology. You need to know which organisms cause caries, periodontitis and root canal infections, how candida and the oral viruses look in the mouth, which organisms cause infective endocarditis, and your real infection risk after a needlestick. Deep immunology and lab culture details carry little weight.
This page organises the oral organisms by the disease you see in the chair, with the one fact each question usually hinges on. It is the clinical focus we use in The Study Boards INBDE course.
How The Study Boards helps you with INBDE microbiology
- Taught through the patient: our faculty-created Modules and Master Files teach organisms from the clinical picture, such as a failed root canal, a teenager with molar-incisor bone loss or a needlestick, and keep the topic to what is clinically relevant.
- Recall built in: daily tasks, nightly live tests at 9 PM New York time and the Master Test Series bring these organisms back until the answers come without thinking.
- Extremely personalised: red lines and assignments are given to you on the go, the course learns from the way you are studying, and US-licensed dentists answer your doubts.
See what is included in the INBDE course blueprint.
The caries organisms
Caries starts when acid-producing bacteria in plaque ferment sugar. Know who starts the lesion and who drives it deeper.
| Organism | Role | The fact to remember |
|---|---|---|
| Streptococcus mutans (and S. sobrinus) | Starts enamel caries | Strong acid producer. Makes sticky extracellular polysaccharides from sucrose, which help plaque stick. Caries usually follows 6 to 24 months after S. mutans appears in the mouth.1 |
| Lactobacillus species | Drives progression | Found in active and advanced lesions. Some strains bind type I collagen, which is exposed as caries moves into dentin.2 |
| Actinomyces (A. viscosus, A. naeslundii) | Classic root caries suspect | Long implicated in root surface caries, but later studies found them in high numbers on both sound and carious roots, so the link is uncertain.3 |
| Streptococcus sanguinis, Actinomyces | Early colonisers | The first bacteria to attach to the acquired pellicle. S. mutans then attaches to them.1 |
Periodontal pathogens: the complexes and Aggregatibacter
Socransky’s 1998 study of over 13,000 plaque samples grouped subgingival bacteria into complexes. The red complex is the one exams ask about.4
| Organism | Type | The fact to remember |
|---|---|---|
| Porphyromonas gingivalis | Gram-negative, black-pigmented anaerobe (red complex) | Makes gingipains, protein-cutting enzymes, and a collagenase. Rises with pocket depth.5 |
| Tannerella forsythia | Gram-negative anaerobe (red complex) | Formerly Bacteroides forsythus. Also found in root canal infections.57 |
| Treponema denticola | Spirochete (red complex) | Motile, spiral-shaped. The red complex as a group tracks with pocket depth and bleeding on probing.5 |
| Fusobacterium nucleatum | Gram-negative anaerobe (orange complex) | Bridges early colonisers and the red complex. The orange complex is the first linked to disease, then the flora shifts to red.5 |
| Aggregatibacter actinomycetemcomitans | Gram-negative | Its leukotoxin is a key virulence factor. Linked to localised aggressive periodontitis: bone loss at first molars and incisors in young patients, often with little visible plaque. The JP2 clone is highly leukotoxic and carries a much higher risk.6 |
To stage and grade the disease these bacteria cause, see periodontal classification, staging and grading for the INBDE.
Endodontic flora: primary, extraradicular and persistent
The classic Kakehashi study in germ-free rats showed that microorganisms are what turn an exposed pulp into an endodontic infection.7
Untreated necrotic canalPolymicrobial and dominated by obligate anaerobes: Prevotella, Porphyromonas endodontalis, P. gingivalis, T. forsythia, Dialister, Fusobacterium, Treponema.7
ExtraradicularBacteria that escape past the apex can form an acute apical abscess. Actinomyces and Propionibacterium propionicum are typical.7
Test these organisms under time
Our nightly live tests and Master Test Series turn every table on this page into clinical questions.
Oral candidiasis
Candida albicans is the usual cause. It is an opportunist: it causes disease when the host gives it a chance.9
| Form | What you see | Typical setting |
|---|---|---|
| Pseudomembranous (thrush) | Thick, curdy white plaques that wipe off with gentle scraping.9 | Infants, inhaled steroid users, HIV, cancer and immunosuppressed patients.9 |
| Erythematous | Flat red areas, often on the palate or dorsum of the tongue.1018 | Often painful or burning; common in HIV.18 |
| Denture stomatitis | Redness of the palate under a denture; Newton class I pin-point, class II diffuse, class III papillary.9 | Dentures worn at night or poorly cleaned.9 |
| Angular cheilitis | Red, macerated corners of the mouth on both sides.9 | A Candida-associated lesion.10 |
| Median rhomboid glossitis; chronic hyperplastic | Also classed as Candida-associated (median rhomboid) or primary chronic (hyperplastic, nodular or plaque-like) candidiasis.10 | Know them as names in the classification. |
Treatment: nystatin, miconazole and fluconazole are all effective. Oral fluconazole is used when topical treatment fails.11 Correct the cause too, such as poor denture hygiene or steroid inhaler use.9
Herpes viruses, enteroviruses and HPV
The location of the ulcers and the age of the patient answer most virus questions.
| Virus | Oral picture | Key clue |
|---|---|---|
| HSV-1, primary (herpetic gingivostomatitis) | Many vesicles and ulcers on movable and attached mucosa; the whole gingiva is red, swollen and painful.13 | Peak age 2 to 3 years. Mild cases settle in 5 to 7 days, severe ones in 2 weeks.13 |
| HSV-1, recurrent | Cold sore on the lip, or ulcers on keratinised mucosa bound to bone: hard palate and attached gingiva.1213 | Latent in the trigeminal ganglion; triggered by sun, stress, fever, trauma. Any site in immunocompromised patients.12 |
| Varicella-zoster (shingles) | Painful vesicles along one nerve’s territory.12 | Involvement of the ophthalmic division (V1) threatens the eye.12 |
| Epstein-Barr virus | Oral hairy leukoplakia: white, hair-like patch on the lateral tongue that cannot be scraped off.1218 | Also infectious mononucleosis, Burkitt lymphoma, nasopharyngeal carcinoma.12 |
| HHV-8 | Kaposi sarcoma: red-purple macules, plaques or nodules on the posterior hard palate, gingiva or tongue.12 | Strongly linked to AIDS.12 |
| Coxsackievirus A16, enterovirus A71 | Hand, foot and mouth disease: painful mouth blisters with a rash on the hands and feet.1416 | Children under 5; mild, lasts 7 to 10 days.16 |
| Enteroviruses (herpangina) | Vesicles and ulcers on the soft palate with fever and sore throat.15 | Lesions sit at the back of the mouth. |
| Human papillomavirus (HPV) | Oropharyngeal cancer at the base of the tongue and tonsils.17 | HPV is thought to cause 60 to 70% of US oropharyngeal cancers. It is not known to cause cancers of the lip, larynx or salivary glands.17 |
Oral signs of HIV
Oral lesions occur in up to 50% of people with HIV and up to 80% at the AIDS stage (CD4 below 200 cells per microlitre). They can be the first sign that leads to a diagnosis.18
Oral candidiasisThe most common oral lesion in HIV, seen in 17% to 75%. Risk rises when CD4 falls below 200.18
Oral hairy leukoplakiaCaused by EBV. White, hair-like patch on the lateral borders of the tongue; unlike thrush, it cannot be scraped off.18
Kaposi sarcomaRed-purple lesion, often on the palate or gingiva.12
Linear gingival erythema and necrotizing diseaseA 2 to 3 mm red band along the gingival margin; necrotizing gingivitis and periodontitis.18
Antiretroviral therapy has made candidiasis and Kaposi sarcoma less common, but it can reduce saliva flow.18
The endocarditis organisms
The organisms dental guidelines target are the viridans group streptococci, normal mouth flora that can enter the blood during dental procedures. The AHA’s 2021 statement kept the 2007 rules: prophylaxis only for the highest-risk heart patients, and good daily oral health matters for everyone.19 Clindamycin is no longer recommended as an alternative because of severe reactions, including C. difficile infection.20
For the full list of who needs cover and the drug doses, see antibiotic prophylaxis and dental trauma guidelines for the INBDE and high-yield antibiotics for the INBDE.
Needlestick risk and tuberculosis
Risk of infection after one needlestick or cut from an infected source, from CDC guidance.
| Virus | Risk per exposure | What to do and when |
|---|---|---|
| Hepatitis B | 6 to 30% for an unvaccinated person, depending on the source’s HBeAg status. Vaccinated people with immunity: virtually no risk.21 | Start post-exposure treatment as soon as possible, preferably within 24 hours and no later than 7 days.21 |
| Hepatitis C | About 1.8% in older CDC data; CDC’s 2020 guidance reports about 0.2%.2122 | No vaccine. Baseline test within 48 hours, HCV RNA at 3 to 6 weeks, final antibody test at 4 to 6 months if needed.22 |
| HIV | 0.3% (about 1 in 300) after a needlestick; about 0.1% after a mucous membrane splash.2123 | Start post-exposure prophylaxis as soon as possible, within hours rather than days.21 |
Tuberculosis in the dental office
Ask every patientAsk about TB history and symptoms at intake and at updates. Staff who see patients get a baseline TST, preferably two-step.23
Suspected active TBKeep the patient only as long as needed to assess and refer. Separate them from others and give them a surgical mask.23
Elective careWait until a physician confirms the patient is not infectious.23
Urgent careGive it in a facility with airborne infection isolation, such as a hospital. Staff need fit-tested N95 respirators; surgical masks do not protect against TB.23
Match the organism
Pick a clinical clue. The organism appears below.
The organism most associated with persistent infection after root canal treatment.
Leukotoxin-producing; the JP2 clone is the most aggressive.
Whole gingiva inflamed, both movable and attached mucosa ulcerated.
Oral hairy leukoplakia; it does not wipe off, unlike thrush.
Kaposi sarcoma.
Soft palate lesions without the hand and foot rash.
Three quick questions
Written by our faculty for this page. Not real exam items.
Which organism is most often recovered from root-filled teeth with persistent apical periodontitis?
Show answer
Answer: B E. faecalis survives canal disinfection and dominates failed cases; primary infections are dominated by Gram-negative anaerobes.8
An unvaccinated assistant gets a needlestick from a patient who is HBsAg and HBeAg positive. Which statement is correct?
Show answer
Answer: B Hepatitis B carries the highest needlestick risk (up to 30%), and treatment should start as soon as possible, preferably within 24 hours.21
Which bacterium belongs to the red complex?
Show answer
Answer: B The red complex is P. gingivalis, T. forsythia and T. denticola. F. nucleatum is orange; S. sanguinis is an early coloniser.5
How to prepare: our full plan
Biology of microorganisms is one of the INBDE’s foundation knowledge areas, and it is tested through clinical cases. Learn it alongside pathology and pharmacology, then test it under time.
INBDE Premium BoardsFrom the course blueprint:
- 12 sequenced course sections and 35,000+ focused questions
- Master Files on clinically relevant topics
- Grand Test Series of 8-hour mock exams
- Live tests every night at 9 PM New York time
Keep studying from hereRelated guides from our faculty:
Frequently asked questions
Which oral bacteria are most important for the INBDE?
How much immunology do I need for INBDE microbiology?
What is the needlestick transmission risk for HBV, HCV and HIV?
How do I tell primary herpes from herpangina?
How does The Study Boards help you prepare for microbiology on the INBDE?
Want a plan for the INBDE?
Message us on WhatsApp. Our Admin Team with Dr. T and Dr. Kaur will contact you over WhatsApp.
References
- Forssten SD, Bjorklund M, Ouwehand AC. Streptococcus mutans, caries and simulation models. Nutrients. 2010;2(3):290-298. PMC3257652
- Wen ZT, et al. Lactobacilli and human dental caries: more than mechanical retention. Microbiology. 2022. PMC10233465
- Dame-Teixeira N, et al. Actinomyces spp. gene expression in root caries lesions. J Oral Microbiol. 2016. PMC5027334
- Socransky SS, et al. Microbial complexes in subgingival plaque. J Clin Periodontol. 1998;25(2):134-144. PubMed 9495612
- Mohanty R, et al. Red complex: Polymicrobial conglomerate in oral flora: A review. J Family Med Prim Care. 2019;8(11):3480-3486. PMC6881954
- Aberg CH, Kelk P, Johansson A. Aggregatibacter actinomycetemcomitans: virulence of its leukotoxin and association with aggressive periodontitis. Virulence. 2015;6(3):188-195. PMC4601274
- Narayanan LL, Vaishnavi C. Endodontic microbiology. J Conserv Dent. 2010;13(4):233-239. PMC3010028
- The Influence of Enterococcus faecalis as a Dental Root Canal Pathogen on Endodontic Treatment: A Systematic Review. Cureus. 2020. PMC7152576
- Vila T, et al. Oral Candidiasis: A Disease of Opportunity. J Fungi. 2020;6(1):15. PMC7151112
- Singh A, et al. Oral candidiasis: An overview. J Oral Maxillofac Pathol. 2014;18(Suppl 1):S81-S85. PMC4211245
- Quindos G, et al. Therapeutic tools for oral candidiasis: Current and new antifungal drugs. Med Oral Patol Oral Cir Bucal. 2019. PMC6441600
- Viral infections of oral cavity. J Family Med Prim Care. 2020. PMC7014888
- Differential diagnosis between herpetic gingivostomatitis and herpetiform aphthous ulcerations. J Dent Sci. 2020. PMC7486510
- Hand, Foot, and Mouth Disease (HFMD) in India: A Review on Clinical Manifestations, Molecular Epidemiology, Pathogenesis. Indian Dermatol Online J. 2023. PMC10373810
- Large outbreak of herpangina in children caused by enterovirus in summer of 2015 in Hangzhou, China. Sci Rep. 2016. PMC5067559
- Centers for Disease Control and Prevention. About Hand, Foot, and Mouth Disease. cdc.gov
- Centers for Disease Control and Prevention. HPV and Oropharyngeal Cancer (17 September 2024). cdc.gov
- Oral Manifestations Associated with HIV/AIDS Patients. Medicina (Kaunas). 2022;58(9):1214. PMC9504409
- Wilson WR, et al. Prevention of Viridans Group Streptococcal Infective Endocarditis: A Scientific Statement From the American Heart Association. Circulation. 2021;143(20):e963-e978. PubMed 33853363
- American Dental Association. Oral Health Topics: Antibiotic Prophylaxis Prior to Dental Procedures. ada.org
- Centers for Disease Control and Prevention. Exposure to Blood: What Healthcare Personnel Need to Know. stacks.cdc.gov
- Moorman AC, et al. Testing and Clinical Management of Health Care Personnel Potentially Exposed to Hepatitis C Virus, CDC Guidance, United States, 2020. MMWR Recomm Rep. 2020;69(6):1-8. cdc.gov
- Kohn WG, et al. Guidelines for Infection Control in Dental Health-Care Settings, 2003. MMWR Recomm Rep. 2003;52(RR-17). cdc.gov
Current as of October 2026. This page is for exam study. Follow current CDC and local guidance after any occupational exposure.