Microbiology: Normal Flora and Pathogens

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