BIO 411 – Medical MicrobiologyChapter 9
Commensal and Pathogenic Microbial Flora
Introduction What is normal flora? “In the absence of these organisms, life as we
know it would be impossible.” (page 83) Human fetus is sterile
• After birth, many microbes take up residence
Exposure to a microbe leads to one of the following outcomes:• Transient colonization• Permanent colonization• Disease
Introduction Colonization vs. Disease
• Organisms that colonize a human do not interfere with normal body functions
• Those that cause disease lead to tissue damage
• Proliferation of the microbe
• Production of toxins/enzymes
• Host immune response
Introduction Strict pathogens always cause disease
• Mycobacterium tuberculosis• Neisseria gonorrhoeae• Rabies virus
Opportunistic Pathogens – many normal flora organisms
Normal Flora and Pathogens Mouth, oropharynx, nasopharynx:
• Normal Flora:• Many anaerobes (Peptostreptococcus spp.)• Aerobes: Streptococcus (viridans),
Haemophilus, Neisseria spp.
• Pathogens:• Streptococcus pyogenes (pharyngitis) • Sinusitis:• S. pneumoniae• Haemophilus influenzae• Neisseria meningitidis• Moraxella catarrhalis
Normal Flora and Pathogens How do we decide what is causing the
disease?• You must exclude other known
pathogens!
External Ear:• Normal Flora – Coagulase negative Staph• Pathogens – Pseudomonas aeruginosa,
etc.
Normal Flora and Pathogens Lower Respiratory Tract:
• Normal Flora – usually none• Pathogens:
• S. pneumoniae
• S. aureus
• Klebsiella pneumoniae
• Peptostreptococcus spp.
Normal Flora and Pathogens Esophagus:
• Normal Flora – usually no permanent flora
• Pathogens – rare for bacteria
Stomach:• Normal Flora – few acid tolerant species
(Lactobacillus and Streptococcus)• Pathogens – Helicobacter pylori
Normal Flora and Pathogens Small Intestine:
• Normal Flora:• Mostly anaerobes (Peptostreptococcus spp.,
etc.)
• Pathogens:• Salmonella and Campylobacter spp.
Normal Flora and Pathogens Large Intestine:
• Normal Flora (mostly anaerobes):• Bifidobacterium,
• Eubacterium,
• Bacteroides,
• Enterococcus, ect.
• Pathogens:• Bifidobacterium fragilis (most common anaerobe
causing intraabdominal disease)
• E. coli (most common aerobe causing intraabdominal disease)
• Enterococcus, Shigella spp.
• Clostridium difficile (antibiotic treatment)
Normal Flora and Pathogens Genitourinary System – normally
sterile with 2 exceptions:• Anterior urethra:
• Normal flora – lactobacillus, streptococcus, coagulase (-) staphylococcus
• Pathogens:– Enterococcus, E. coli, Candida spp.
– Neisseria gonorrhoeae and Chlamydia trachomatis
Normal Flora and Pathogens• Vagina:
• Normal flora – lactobacillus, streptococcus, staphylococcus, others
• Pathogens:– Candida albicans and C. glabrata
– Neisseria gonorrhoeae and Chlamydia trachomatis
Normal Flora and Pathogens Skin:
• Normal Flora• Almost all Gram +
• Coagulase (-) Staph
• Corynebacteria
• Propionibacteria
• Clostridium perfringens (20% of healthy individuals)
• Pathogens – Coagulase (+) Staph, Streptococcus pyogenes
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