Microbiology: Normal Flora and Pathogens

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5 Questions

What is the term used for microbes that can cause disease in normal hosts?

Aggressive pathogens or primary pathogens

Define opportunistic pathogens.

Microbes that do not cause disease in normal hosts but do so in those with impaired defenses

Normal flora can cause disease in immunocompromised individuals. (True/False)


Which type of pathogens lie dormant in a normal host but cause clinical disease when host defenses are compromised?

Latent pathogens

Endotoxins are the cell wall lipopolysaccharides of _______ bacteria.

Gram negative

Study Notes

Normal Flora and Pathogens

  • Normal flora: microbes that inhabit the surfaces of the body, including skin, hair, nails, and mucous membranes of digestive, respiratory, and urogenital tracts
  • Classified into four groups:
    • Normal flora: inhabit body surfaces
    • Aggressive pathogens: cause disease in normal hosts
    • Opportunistic pathogens: cause disease in individuals with impaired defenses
    • Latent pathogens: lie dormant in normal hosts, causing disease when host defenses are compromised
  • Virulence: the relative ability of a microorganism to cause disease or the degree of pathogenicity

Contamination, Colonization, and Infection

  • Contamination: transient presence of microbes on the skin or body surfaces without injury or invasion
  • Colonization: continuing presence of microbes on the skin or body surfaces for weeks, months, or years without injury or invasion
  • Infection: injury to or invasion and damage of tissues by microbes
  • Tissue invasion is usual in microbial attack, but some diseases, like cholera, can occur without significant tissue invasion

Normal Flora and Disease

  • Normal flora can cause disease in immunocompromised and debilitated individuals
  • Normal flora provides colonization resistance, occupying attachment sites on skin and mucous membranes, preventing pathogenic bacteria from colonizing
  • Normal flora may serve a nutritional function, producing vitamins like B and K
  • Reduction of normal flora through antibiotics can lead to vitamin deficiencies

Oral Flora

  • Oral flora varies greatly among individuals
  • Viridans streptococci are primary microbes in the oral cavity
  • Streptococcus salivarius is associated with the tongue, while Streptococcus sanguis and Streptococcus mutans are associated with tooth enamel
  • Candida species are indigenous flora in 10-15% of people
  • Anaerobic bacteria inhabit plaque and gingival crevices with low oxygen levels

Ecology of Oral Flora

  • Oral ecosystem consists of oral flora, different oral habitats, and associated surroundings
  • Major oral habitats include buccal mucosa, teeth, crevicular epithelium, and prosthodontic and orthodontic appliances

Acquisition of Normal Oral Flora

  • Infant mouth is sterile at birth, but bacteria start to colonize within hours
  • Bacteria from the birth canal, mother's breast, and environment colonize the infant's oral cavity
  • During and after tooth eruption, aerobic bacteria occupy hard tissue surfaces, while anaerobic bacteria occupy gingival crevices

Factors Affecting Oral Microflora

  • Anatomical factors: tooth shape
  • Saliva: immunoglobulins (IgA) and non-specific immunologic action (lysozyme, lactoferrin, and myeloperoxidase)
  • Gingival crevicular fluid: flushing out microbes and containing defense factors like IgA and phagocytes
  • Microbial factors: normal flora producing inhibitory substances
  • Miscellaneous factors: local environment pH, oxidation-reduction potential, antibiotic therapy, diet, and iatrogenic factors

Pathogenesis of Bacterial Disease

  • Four important portals of entry for pathogens: skin, respiratory tract, gastrointestinal tract, and genitourinary tract
  • Pathogenicity: structural and biochemical mechanisms by which microorganisms cause disease
  • Invasiveness and toxigenesis are two mechanisms by which pathogenic bacteria cause disease
  • Virulence factors help bacteria invade the host, cause disease, and evade host defense mechanisms
  • Specific virulence factors include surface proteins, carbohydrates, enzymes, and toxins
  • Toxins can destroy or inactivate vital components of the host, allowing the pathogen to survive and flourish

Learn about the different types of microbes that inhabit the human body, including normal flora and aggressive pathogens, and how they interact with the host's defense mechanisms.

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