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