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 (STDs)
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:
Exoenzymes
Toxins
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