Chapter 13

Microbe – Human Interactions

Three types of microorganisms are associated with human body

•         Normal microbiota – microorganisms colonizing the human body without harming it

•         Transient microbes – rapidly lost

•         Pathogenic microbes – invade and damage the cells and tissue

Acquiring Resident Biota

•         The uterus is germ-free

•         First exposure to microbes occurs during the passage through the birth canal

•         Feeding

•         Contact with the environment and family members

The Progress of Infection

•         Infection starts with an invasion of a pathogen

•         How successful this invasion will be, depends on:

–        Pathogenicity – the ability of a microbe to cause disease in  another organism

–        Virulence – the degree of pathogenicity; relative ability of a microbe to cause a disease

•         Virulence factors (enzymes, toxins, capsule…)

Establishing the Infection

•         The microbes enter the body through -Portal of entry

–        Attaching to the host

–        Surviving host defenses

–        Causing disease

How Microorganisms Enter a Host?

Portals of entry:

•         Mucous membrane (Lines the body cavities that are open to the environment)

•         Skin (openings or cuts)

Infectious agents that enter the skin

•         Skin is a tough barrier

•         Broken skin enables penetration of microbes

•         The common infectious agents:

–        Staphyloccus aureus (boils)

–        Streptococcus pyogenes (impetigo)

–        Clostridium (gangrene, tetanus)

•         Other ways of penetrating the skin

–        Digestive enzyme of microbes

–        Insect bites

–        Hypodermic needles

•         Mucous membrane

–        Respiratory tract is the most common portal of entry - through inhalation (common cold, influenza, pneumonia, tuberculosis …)

–        Gastrointestinal tract - through food, water and contaminated hands (hepatatis A, typhoid fever, amoebic disentery …)

–        Genitourinary tract - through sexual contact (STD’s)

Is the number of invading microbes important?

•         A certain (minimal) number of microbes have to enter the body in order to cause an infection – infectious dose

•         Infectious Dose for:

–        Q fever (Rickettsia) – 1 cell

–        Tuberculosis – 10 cells

–        Gonorrhea – 1000 cells

–        Cholera – 1,000,000,000

Preferred portal of entry

•         Pathogens have preferred portal of entry

Ex: Streptococcus that is inhaled can cause pneumonia, when swallowed  it does not show symptoms

Attaching to the Host

•         Microorganisms attach themselves to the host cell through:

•         Fimbriae (bacteria)

•         Capsules (bacteria)

•         Spikes (viruses)

Adherence

•         Attachment is based on binding of specific molecules on both host and pathogen

•         Surface molecules of the pathogens (ligands) bind to the specific molecules of host tissue cells (receptors)

•         Ligands are located on glycocalyx or on fimbriae

Surviving Host Defense

•         Pathogenic microbes are recognized by white blood cells – phagocytes

•         Phagocytes engulf and destroy the pathogen

•         Some microbes know how to evade phagocytosis

–        Produce toxins that kill phagocytes (leukocidins)

–        Produce capsule

–        Can survive inside the phygocyte

Causing Disease

How Virulence Factors Contribute to Tissue Damage?

•         Virulence factors:

1. Exoenzymes

2. Toxins   

3. Blocked phagocytic response 

Exoenzymes

•         Exoenzymes can dissolve structural chemicals in the body

–        Hyaluronidase - hydrolyses hyaluronic acid  (a substance that cements animal cells together)

–        Coagulases – coagulate (clot) the fibrinogen (protection against phagocytes)

–        Kinases – digest the blood clots - formed to isolate the infection

Bacterial Toxins

•         Exotoxin

–        Actively excreted by a living bacterial cell

•         Endotoxins

–        Part of the cell wall of Gr negative bacteria

Exotoxins

•         Released from bacterial cells (Gr+ or Gr-)

•         They are proteins, some are enzymes

Naming exotoxins:

–        Hepatotoxins

–        Cytotoxins

–        Neurotoxins

–        Enterotoxins

Representative Exotoxins

•         Botulinum toxin – Clostridium botulinum

–        Neurotoxin that prevents the transmission  of impulses from the nerve cell to the muscle

–        Flacid paralysis

•         Tetanus toxin – Clostridium tetani

–        Blocks the relaxation pathway of muscles; uncontrollable muscle contraction.

Endotoxins

•         Is part of the outer layer of the cell wall of Gram negative bacteria

•         Endotoxins are lipopolysaccharides

•         Endotoxin has to be released from the cell wall in order to cause the symptoms

•         The symptoms are: chills, fever, weakness aches, shock and even death

•         Microbial toxins can cause:

•         Fever, diarrhea, cardiovascular disturbance, shock, inhibition of protein synthesis, disrupt the nervous system

The Patterns of Infection

•         Localized infection -infection that is limited to a small area of body (abscesses)

•         Systemic infection -microbes are spread throughout the body (measles)

•         Focal infection – Infectious agent migrates from a local infection to other tissues

•         Mixed (polymicrobial) infection – more than one infectious agent is involved

•         Primary infection - infection that causes initial illness

•         Secondary infection - infection by opportunistic pathogen

•         Acute infection - develops rapidly but lasts a short time (influenza)

•         Chronic infection - develops slowly but lasts for long period o time (tuberculosis, hepatitis B

Stages of Clinical Infections

•         Incubation period - time period between initial infection and first symptoms of disease (several hours in pneumonic plague; several years leprosy)

•         Prodromal stage – feeling of discomfort (1-2 days)

•         Period of invasion -  expression of all symptoms

•         Convalescent period – decline of symptoms - recovery

Signs and Symptoms

•         Inflammation – swelling of the tissue

–        Edema – accumulation of fluid

–        Granulomas and abscesses – walled-off cells of damaged cells and microbes

–        Rashes, skin eruption

•         Signs of infection in the blood

–        Leukocytosis – increased level of blood cells

–        Bacteremia – bacteria present in the blood; do not multiply

–        Septicemia – microorganisms multiply in the blood

•         Unnoticed infections

–        Asymptomatic, subclinical

The Portal of Exit

The discharged microorganisms are the source of a new infection

•         Respiratory or salivary portals

•         Moist secretion (mucus, nasal discharge) – coughing, sneezing, laughing, talking

•         Fecal

•         Intestinal infection (damage of intestinal mucus) causes rapid peristalsis (movement of intestines) – diarrhea

•         Urogenital tract

•         vaginal discharge, semen, urine (STDs, tuberculosis of kidneys)

•         Blood

•         When removed for testing purposes or when released during tissue injury

•         AIDS, hepatitis

Persistence of Microbes

•         Microbes start an infection from the reservoirs

•         Reservoir:

–        Living reservoir

•         Human body

•         Animal body

–        Non-living reservoir

•         Soil

•         Water

•         Plants

Living reservoir

Source of infection:

•         Diseased person or animal

•         Carrier - people that harbor pathogens without any signs of illness (AIDS, hepatitis, gonorrhea, streptococcal infection)

–        Incubation carriers – spreads the disease during the incubation period

–        Convalescent carriers – recuperating patients

–        Chronic carrier – carry the agent for long period of time

Passive carriers – medical personnel

Animals as Reservoirs

•         Animals are Vectors – they transmit the infectious agent from one organism to another

•         Biological vectors

–        The agent multiplies within the vector (Plasmodium - Mosquito - Malaria)

•         Mechanical vectors

–        Mechanically transmits the agent to food or directly to humans

Transmission of Infectious Agents

•         Communicable diseases – infected host transmits the agent to another host

•         Noncommunicable diseases – infection acquired through

–        contact with an agent from its own body (opportunistic pathogens)

–        from nonliving reservoir (Clostridium tetanus)

Nosocomial Infections  -Hospital acquired

•         5-15% of hospital patient acquire nosocomial infection

•         Factors responsible for nosocomial infections:

–        The characteristics of microbes in the Hospital (opportunistic, antibiotic resistant)

•         Patients are compromised by:

–        Broken skin or mucous membranes

–        Suppressed immune system

•         Chain of transmission

–        Transmission from hospital staff to patient

–        From patient to patient

–        From fomites (non-living objects) to patients

–        Through ventilation system

Epidemiology

Study of frequency and distribution of disease in human population

•         Frequency of cases:

–        Incidence of a disease –Number of new cases of a disease during a given time interval, usually one year

•         Used to determine probability of developing a specific disease 

–        Prevalence of a disease – Total number of current cases in the entire population regardless of when it first appeared

Frequency of Occurrence

•         Endemic disease – constantly present in a population (common cold)

•         Sporadic disease – occurs occasionally (typhoid fever in the U.S.)

•         Epidemic disease – many people in a given area acquire a disease in short period of time (influenza)

•         Pandemic disease – epidemic disease that occurs in a large geographic region

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