HC17: Host versus invader

Commensals, colonization and normal host defense

Sterile versus non-sterile areas:

A host has both sterile and non-sterile areas:

• Nonsterile areas
  • Skin
  • Elementary tract
  • Alimentary tract
• Sterile areas → no bacteria are present
  • The "inside"
  • Stomach
  • Deep respiratory tract
  • Urinary tract

The human body has 10^14-15 bacteria and 10¹³ body cells. Most bacteria reside in the intestines.

Commensals:

Commensals are normally present in every person. The skin only contains gram-positive bacteria because they have a thick peptidoglycan layer, which makes them resistant against a dry environment. This doesn't necessarily cause symptoms → commensals are harmless unless the immune system is impaired.

Colonization:

Humans have colonization resistance → bacteria are present on skin and mucosae to protect against adherence by other microorganisms.

Normal host defense:

The human body has different measures to achieve sterility:

• Physical
  • Barriers: skin and mucosae
  • Airways: ciliar activity, mucus, coughing
  • Urinary tract: voiding, antimicrobial factors
• Chemical
  • Gastric acid → kills bacteria
  • Enzymes
    • Lysozyme in tears
• Immunological

Exposure versus infection

Infection:

An infection is a structural or functional change caused by a micro-organism or its components or products. This includes the host response such as inflammation.

The microorganism staphylococcus aureus is carried by 1/3 of all humans in the nose. This, however, isn't an infection → nothing has changed. It only is a carrier. Something only becomes an infection once something has changed, for example:

• Pain
• Redness
• Puss
• Fever

Fever can have many causes, but more often than not it is caused by infection. The body temperature is regulated in the hypothalamus:

1. Signals come from bacteria
2. Lipopolic saccharides interact with Toll-like receptors on the macrophages → triggers the production of cytokines
3. The interleukins attach to the epithelial cell and give signals → prostaglandins go to specialized cells which sets certain mechanisms in action:
   • Vasoconstriction → containing body warmth
   • Shivering → making body warmth
   • Brown adipose tissue → ATP → making body warmth

Exposure:

There are many forms of exposure:

• Oral
• Trauma/operation
• Needle injury, blood, catheters
• Artrhopod
• Penetration of skin by the micro-organism itself
• Ascending infection
• Contact
• Inhalation, aspiration

Exposure to micro-organisms leads to:

• Colonization
  • No symptoms
  • Commensal or primary pathogens on skin or mucosae
  • Transiently or permanently
  • Competition for food and adhesion sites
• Infection
  • Often, but not always
  • Symptoms and/or signs

Natural course of infection

There are 2 groups of pathogens which can cause infections:

• Primary pathogens: no host defense disorder needed to become ill
• Opportunistic pathogens: host defense disorder needed to become ill
  • The weaker microorganisms

Infection can be acquired endogenously or exogenously:

• Endogenous
  • Commensals: "friendly" microorganisms every human has
    • Skin
      • Staphylococcus epidermidis
        
        23. Gram-positive
    • Throat
      • Streptococci
        
        23. Gram-positive
      • Neisseria species
        
        23. Gram-negative
      • Candida albicans
        
        23. A yeast
    • Bowel
      • Escherichia coli
        
        23. Gram-negative
      • Bacteroides fragilis
        
        23. Gram-negative
      • Clostridium species
        
        23. Gram-positive
  • Colonization: virulent microorganisms
    • Staphylococcus aureus
    • Streptococcus pyogenes
    • Streptococcus pneumoniae
      • Gram-positive
      • Decapsulated
    • Neisseria meningitidis
      • Decapsulated
    • Haemophilus influenzae
      • Gram-negative
      • Decapsulated
• Exogenous
  • Other person
  • Zoonosis
  • Vectors
  • Environment

Staphylococci

Catheter infection:

A patient with acute myeloid leukemia starts chemotherapy. Chemotherapy wipes away all the bone marrow cells → there's a period of 3-4 weeks where there aren't any granulocytes → granulocytopenia. The following happens:

• Day 20: still granulocytopenia + central venous line to give medication
• Day 21: fever
  • Blood cultures are obtained
  • Prescription of antibiotics → vancomycin and gentamicin
• Day 22: positive blood cultures
  • Gram-positive cocci in groups → staphylococci

The diagnosis is an intravenous catheter infection.

Different staphylococci:

The name of a staphylococcus is very important:

• Staphylococci epidermidis
  • Commensal → present in everyone
  • Opportunist
  • Risk factors are required to cause problems:
    • Granulocytopenia
    • Foreign body
      • For example adhesion to catheters/prosthesis
• Staphylococci aureus
  • 30% are carrier
    • Mainly in the nose
  • Primary pathogen
  • No risk factors required, but there is an increased risk in case of:
    • Granulocytopenia
    • Foreign body (biofilm)
    • Intravenous drug use

Diagnostics:

The difference between cocci is firstly made with the catalase test, and with more tests subsequently:

• Positive catalase test → staphylococcus
  • Positive coagulase test → s. aureus
  • Negative coagulase test → s. epidermidis
• Negative catalase test → streptococcus
  • Positive hemolysin test → a-hemolytical or b-hemolytical
    • S. pneumoniae is an example of an a-hemolytical bacteria
    • S. pyogenes is an example of a b-hemolytical bacteria
  • Negative hemolysin test → indifferent streptococci

Re-exposure:

Re-exposure from a bacteria can lead to 2 things:

• Protective immunity → no disease
• Infectious disease
  • The protective immunity isn't strong enough