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Key Terminology in Medical Bacteriology

• Gonorrhea: A sexually transmitted infection caused by Neisseria gonorrhoeae.

• Adrenal hemorrhage: A condition associated with Neisseria meningitidis (Waterhouse-Friderichsen Syndrome).

• Epiglottitis: Inflammation of the epiglottis, often caused by Haemophilus influenzae.

• Pertussis: Commonly known as whooping cough, caused by Bordetella pertussis.

• Localized aggressive periodontitis: A specific form of periodontal disease associated with Aggregatibacter actinomycetemcomitans.

• Enteric fever: Also known as typhoid fever, caused by certain Salmonella species.

• Bacillary dysentery: Bloody diarrhea caused by Shigella species invading the ileum and colon.

• Nosocomial infection: Hospital-acquired infections; Pseudomonas aeruginosa is a primary opportunistic agent.

• Cholera: An acute diarrheal disease caused by Vibrio cholerae.

• Actinobacillus: Former name for the genus now classified as Aggregatibacter.

• Porphyromonas and Prevotella: Genera of anaerobic bacilli frequently involved in oral and periodontal diseases.

Family Neisseriaceae

• General Characteristics:

  • Gram-negative, aerobic diplococci.

  • Some members are capnophilic (require increased $CO_2$ levels).

  • Primary genera include Neisseria and Moraxella.

• Neisseria gonorrhoeae:

  • Normal Habitats: Found in the urogenital tract, mouth, rectum, and nasopharynx.

  • Transmission: Transmissible primarily via sexual intercourse.

  • Morphology: Non-motile and non-capsulated.

  • Clinical Significance: The causative agent of gonorrhea. It can lead to septicemia, arthritis, pelvic inflammatory disease (PID), and sterility in women.

  • Virulence Factors:

    1. Pili (for attachment).

    2. IgA protease (to evade mucosal immunity).

    3. $\beta$-lactamase (resistance to penicillins).

    4. Cytotoxin (damages fallopian tubes, leading to sterility).

  • Treatment: Cephalosporins are considered the best drugs for treatment.

• Commensal Neisseria:

  • N. sicca, N. mucosa, and N. subflava are members of the microflora of the oral mucosa and the female genital tract.

• Neisseria meningitidis:

  • Normal Habitat: Nasopharynx.

  • Transmission: Transmissible via respiratory droplets.

  • Morphology: Non-motile but capsulated.

  • Virulence and Pathogenesis: The primary virulence factor is endotoxin. The bacteria spread from the nasopharynx into the bloodstream and subsequently to the meninges.

  • Complications: Death results from meningitis or adrenal hemorrhage (also known as Waterhouse-Friderichsen Syndrome).

  • Treatment: Penicillin and cefotaxime are the drugs of choice.

• Moraxella:

  • General Characteristics: Gram-negative cocci.

  • Normal Habitat: Respiratory tract.

  • Clinical Significance: Acts as an opportunist causing endocarditis, otitis media, sinusitis, and meningitis.

Veillonella species

• General Characteristics: Gram-negative, anaerobic cocci.

• Normal Habitat: Oral cavity, particularly dental plaque.

• Metabolic Function: They convert lactic acid (produced by cariogenic bacteria) into weaker acids. These weaker acids have less capability to degrade tooth enamel, potentially providing a protective effect.

• Significant Orally-Associated Species:

  • V. dispar

  • V. atypica

  • V. parvula (considered the prototype species)

Parvobacteria Group

• General Characteristics: This group consists of Gram-negative bacilli including Haemophilus, Brucella, Bordetella, Pasteurella, Francisella, Aggregatibacter (formerly Actinobacillus), Gardnerella, and Eikenella.

• Haemophilus influenzae:

  • Morphology: Gram-negative, aerobic coccobacilli. Non-sporing and non-motile. Some strains possess a capsule.

  • Normal Habitat: Upper respiratory tract.

  • Growth Requirements: They require X factor (hematin) and V factor ($NAD$ or $NADP$).

  • Associated Infections:         A. Meningitis         B. Osteomyelitis         C. Epiglottitis         D. Arthritis

  • Virulence Factors:

    1. IgA protease

    2. Capsule

    3. Endotoxin

  • Treatment and Prevention:

    • Ampicillin is a good candidate for treatment.

    • The Hib vaccine is used for prevention against type b strains.

    • Rifampin is used as a prophylactic antibiotic for close contacts of meningitis patients.

• Bordetella pertussis:

  • Morphology: Gram-negative, capsulated bacilli.

  • Normal Habitat: Respiratory tract.

  • Transmission: Transmissible via respiratory droplets.

  • Clinical Significance: Causative agent of pertussis (whooping cough), characterized by a paroxysmal cough with a "whoop" sound.

  • Virulence Factors:

    1. Fimbrial antigen

    2. Endotoxin

    3. Tracheal cytotoxin

  • Treatment and Prevention: Erythromycin is the Drug of Choice (DOC). The DPT vaccine is necessary for prevention.

• Aggregatibacter (Actinobacillus) actinomycetemcomitans:

  • Morphology: Gram-negative, aerobic, star-shaped bacilli.

  • Normal Habitat: Oral cavity (specifically supragingival sites).

  • Co-isolation: It is usually isolated alongside Actinomyces species.

  • Clinical Significance: Causative agent of localized aggressive periodontitis (specifically affecting tooth 6) in adolescents and rapidly destructive generalized periodontitis in adults.

  • Virulence Factors:

    1. Endotoxin

    2. Leukotoxin

    3. Collagenase

    4. Cytolethal distending toxin (CDT)

    5. Epitheliotoxin-bone resorption-inducing factor

    6. Protease-cleaving IgG

  • Treatment: Tetracycline is the Drug of Choice (DOC).

• Eikenella corrodens:

  • Morphology: Gram-negative, capnophilic coccobacilli.

  • Normal Habitats: Intestine and oral cavity.

  • Clinical Significance: Causative agent of abdominal and brain abscesses, peritonitis, meningitis, endocarditis, and osteomyelitis.

• Capnocytophaga:

  • Morphology: Gram-negative, capnophilic fusiforms.

  • Normal Habitat: Subgingival area.

  • Significant Species: C. ochracea (the prototype).

  • Virulence Factors: Includes IgA protease.

  • Clinical Significance: Causative agent of gingivitis and systemic infections, particularly in immunocompromised patients.

Family Enterobacteriaceae

• General Characteristics:

  • Gram-negative, facultative anaerobe coccobacilli.

  • Many possess flagella, pili, and capsules.

  • Normal Habitat: Gastrointestinal tract.

  • Transmission: Via tainted food and the fecal-oral route.

• Virulence and LPS Structure:

  • The most significant virulence factor is endotoxin (LPS/lipopolysaccharide).

  • LPS consists of Lipid A and Polysaccharide (O antigen).

  • Lipid A is specifically responsible for endotoxic shock, which includes fever, hypotension, disseminated intravascular coagulation (DIC), and death.

• General Treatment: A broad spectrum of antibiotics are utilized, including ampicillin, amoxicillin, cephalosporins, trimethoprim, chloramphenicol, aminoglycosides, and ciprofloxacin.

• Escherichia coli:

  • Normal Habitat: Human intestinal tract.

  • Associated Infections:         A. Urinary tract infection (UTI): Ranges from urethritis to pyelonephritis, especially common in elderly adults and young women.         B. Neonatal meningitis and septicemia: Often follows wound infection.         C. Diarrhea: Produced by four distinct types:             i. EHEC (Enterohemorrhagic)             ii. EIEC (Enteroinvasive)             iii. ETEC (Enterotoxigenic)             iv. EPEC (Enteropathogenic - the prototype)

  • Mechanisms of EPEC Pathogenesis:         a) Enterotoxins (including LT and ST).         b) Adhesive factors (pili).         c) Vero cytotoxicity (verotoxin), which induces Cytopathic Effect (CPE) on Vero cells.         d) Enteroinvasiveness.

• Salmonella spp.:

  • Normal Habitats: Intestinal tract and poultry products (occupational salmonellosis noted in veterinary and slaughterhouse workers).

  • Infections:         A. Enteric fever (typhoid fever): Caused by S. typhi and S. paratyphi.         B. Gastroenteritis: Includes nausea, cramps, vomiting, and diarrhea, caused by S. enteritidis.         C. Septicemia: Caused by S. choleraesuis.

  • Prevention: A typhoid vaccine derived from S. typhi is available.

• Shigella spp.:

  • Normal Habitat: Human intestine.

  • Important Species: S. dysenteriae, S. sonnei, S. flexneri, and S. boydii.

  • Pathogenesis: Invasive to the ileum and colon, causing bloody diarrhea (bacillary dysentery).

  • Virulence Factor: Neurotoxin.

  • Treatment: Electrolyte therapy is cited as the best treatment.

• Klebsiella pneumoniae:

  • Normal Habitats: Intestinal tract, respiratory tract, and oral cavity.

  • Clinical Significance: Causes pneumonia and UTI.

  • Primary Virulence Factor: Capsule.

• Other Enterobacteriaceae:

  • Enterobacter spp.: Found in plants, the oral cavity, and the intestinal tract; causes UTI.

  • Serratia marcescens: Causative agent of fatal diseases in neonates and immunocompromised individuals.

  • Proteus spp.: Causative agent of UTI.

  • Yersinia pestis: Causative agent of the plague (pest).

Pseudomonas aeruginosa

• Morphology: Gram-negative, aerobic, motile bacilli.

• Normal Habitats: Human intestine, moist surfaces, and dental unit water lines.

• Clinical Significance: The most important opportunistic agent of nosocomial (hospital-acquired) infections.

• Virulence Factors:

  1. Protease

  2. Elastase

  3. LPS (Lipopolysaccharide)

  4. Exotoxin

  5. Antiphagocytic extracellular slime layer

• Treatment: Best drugs include aminoglycosides, piperacillin, polymyxin, and cephalosporins.

Vibrios, Campylobacters, and Helicobacter

• Vibrios:

  • Characteristics: Gram-negative, curved (comma-shaped) bacilli exhibiting high motility.

  • Habitat: Contaminated water.

  • Species of Importance:

    1. V. cholerae: Causes cholera (watery diarrhea). Virulence is driven by an enterotoxin that binds to intestinal mucosal cells, activating adenylate cyclase, leading to $cAMP$ accumulation and subsequent excretion of water and electrolytes. Treatment requires ORS and intravenous electrolytes.

    2. V. parahaemolyticus: Causes mild diarrhea associated with seafood consumption.

• Campylobacters:

  • Characteristics: Gram-negative, curved, and motile bacilli.

  • Habitat: Animals (transmission via contaminated food and milk).

  • Species of Importance:

    1. C. jejuni: Causes self-limiting gastroenteritis, especially in children.

    2. C. fetus: Possesses a strong enterotoxin.

    3. C. rectus: Linked to periodontal diseases.

• Helicobacter pylori:

  • Clinical Significance: Causative agent of gastritis, duodenal ulcers, and gastric cancers.

  • Management: Can be effectively eradicated with antibiotics.

• Wolinella:

  • Characteristics: Gram-negative, motile bacilli.

  • Habitat: Oral cavity (specifically the gingival sulcus).

  • Pathogenic Species: W. succinogenes, which may cause periodontal diseases.

Gram-Negative Anaerobic Bacilli

• Bacteroides fragilis:

  • General Characteristics: Gram-negative, anaerobic, non-motile, non-sporing, but capsulated bacilli.

  • Normal Habitat: Intestine.

  • Virulence Factors: Capsule, endotoxin, and proteases.

  • Clinical Significance: Causes intra-abdominal infections, liver abscesses, and peritonitis.

  • Treatment: Metronidazole and clindamycin are the Drugs of Choice (DOC).

• Tannerella forsythia:

  • General Characteristics: Gram-negative, anaerobic, non-motile, non-sporing, and non-capsulated bacilli.

  • Normal Habitat: Subgingival site.

  • Virulence Factors: Fatty acid, endotoxin, and methylglyoxal secretion.

  • Clinical Significance: Causative agent of periodontal diseases.

• Porphyromonas gingivalis:

  • General Characteristics: Gram-negative, anaerobic coccobacilli. Non-motile and non-sporing, but capsulated.

  • Normal Habitat: Subgingival site.

  • Clinical Significance: Considered a consensus periodontal pathogen or periodontopathic organism; causes aggressive periodontal disease.

  • Virulence Factors:

    1. Capsule

    2. Fimbriae

    3. Endotoxin

    4. Collagenase

    5. Phospholipase

    6. Fibrinolysin

    7. Hemolysin

    8. Proteases

• Prevotellae:

  • General Characteristics: Gram-negative, anaerobic bacilli. Non-motile, non-sporing, and non-capsulated.

  • Normal Habitat: Oral cavity (subgingival sites).

  • Significant Species:

    • P. melaninogenica: The prototype species.

    • P. intermedia: Causes periodontal diseases.

    • Other species are isolated from healthy gingival sites but share virulence factors with P. gingivalis.

References

• Samaranayake L. (2024). Essential Microbiology for Dentistry. (6th edition). Churchill Livingstone Inc., London, United Kingdom. 424 p.

• evolve.elsevier.com