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Normal Microbiota: Locations and Predominant Microbes
Skin
Eye (conjunctiva)
Oral cavity
Teeth
Subgingiva
Respiratory Tract
Nostrils
Sinuses
Upper respiratory tract
Lower respiratory tract
Gastrointestinal Tract
Upper GI Tract
Small Intestine
Large Intestine
Urogenital Tract
Predominant Microbes
Staphylococcus epidermidis, Micrococcus, Corynebacterium
Staphylococcus epidermidis, Propionibacterium acnes
Oral cavity
Streptococcus mutans, Streptococcus sanguis
Anaerobic bacteria (e.g., Actinomyces)
Respiratory Tract
S. epidermidis, Corynebacterium, Staphylococcus aureus
Usually sterile
Many (non-Beta hemolytic streptococci, Neisseria)
Sparse
Gastrointestinal Tract
Helicobacter pylori
Sparse (Lactobacillus, Micrococcus)
G- coliforms, G+ enterococci, Clostridium, methanogens
Lactobacillus acidophilus, S. epidermidis, Enterococcus
Normal Microbiota: Benefits and Detriments
Benefits
• Compete with pathogens for attachment sites and resources.
• Stimulate cross-reactive antibodies.
• Antagonize potential pathogens through the production of antimicrobial
chemicals.
• Synthesize vitamins (K, B12, other B vitamins) and energy.
Detriments
• Dental caries and gingivitis.
• Opportunistic pathogens
o Normal microbes in the wrong place
o Normal microbes with additional genes (plasmids or prophage)
o Immunocompromised individuals
Steps to Pathogenicity
Transmission
Direct contact Indirect contact (spread by fomites)
Droplet contact (airborne within 1 meter)
1. Transmission
2. Colonization
3. Evasion from host defenses
4. Toxicity
Vehicle Transmission
(food, water, airborne > 1M)
Mechanical vector
Biological vector
Colonization
Evasion of Host Defenses
Bacterial Cell
1a. Nonspecific adhesion due to
hydrophilic interactions,
Brownian motion, van der
Waals forces.
Evading phagocytosis
Invading tissues where phagocytes do not patrol
“Hiding” in host antigens (staphylococcal coagulase, Treponema fibronectin)
Evasion of phagolysosome digestion
1b. Specific adhesion due to binding of
bacterial adhesins (e.g., surface proteins,
lipoteichoic acid, lipopolysaccharide) to
host cell receptor sites.
2.
Secretion of Invasins which are proteins that allow
bacteria to invade host tissues and/or cells.
Hyaluronidase and Collagenase which break down
hyaluronic acid and collagen (which connect host
cells together)
Inhibition of lysosome action (Salmonella, Mycobacterium, Chlamydia)
Escape from phagosome (Ricketssia)
Killing phagocytes (hemolysins)
Induction of ineffective antibodies
Antigenic variation
Bacterial Kinases prevent clotting of blood.
Host Tissue
Neuraminidase destabilizes host cell membrane and
enhances intracellular invasion.
Bacterial Toxins
Exotoxins
Exotoxin
Lysogenic
conversion
A-B toxin. Inhibits protein
synthesis. Cytotoxin.
+
• Streptococcus pyogenes
Membrane-disrupting.
Erythrogenic.
+
• Clostridium botulinum
A-B toxin. Neurotoxin
+
• Corynebacterium diphtheriae
Source
Gram– Source
Mostly Gram+ (but also G-)
Metabolic product
Metabolic product
LPS (Lipid A) of cell wall
By-products of growing cell
Chemistry
Lipid
Symptoms
Fever?Inflammation
Fever, Shock,
Various but specific
• C. tetani
A-B toxin. Neurotoxin
Toxoid Formation
No
Neutralized by
antitoxin
LDlarge
50
Relatively
Yes
• Vibrio cholerae
A-B toxin. Enterotoxin
Small
• Staphylococcus aureus
LD50
Chemistry
Protein
Superantigen. Enterotoxin.
+
Extent of Host Involvement
Classifying Infectious Diseases
•
Local infection
Pathogens limited to a small area of the body
•
Systemic infection
An infection throughout the body
•
Bacteremia
Bacteria in the blood
•
Septicemia
Growth of bacteria in the blood
•
Toxemia
Toxins in the blood
•
Viremia
Viruses in the blood
•
Primary infection
Acute infection that causes the initial illness
•
Secondary infection
Opportunistic infection after a primary infection
•
Subclinical disease
No noticeable signs or symptoms (inapparent infection)
Communicable disease
A disease that is spread from one host to another.
Contagious disease
A disease that is easily spread from one host to another.
Noncommunicable disease A disease that is not transmitted from one host to another.
Endemic disease
Disease constantly present in a population.
Epidemic disease
Disease acquired by many hosts in a given area in a
short time.
Pandemic disease
Worldwide epidemic.
Herd immunity
Immunity in most of a population.
Nosocomial (Hospital-Acquired) Infections
The Stages of a Disease
Are acquired as a result of a hospital stay
5-15% of all hospital patients acquire nosocomial infections
Why are nosocomial infections so prevalent?
Pathogenic microbes present in health care settings
Immunocompromised patients present in health care settings
Multiple modes of transmission (air, puncture wounds, direct contact)
Why are nosocomial infections so dangerous?
Multiply antibiotic resistant strains
Sequela
Figure 14.5