Microbiology Chapter 27: Microbial Interactions with Humans

Most microorganisms are benign

Humans colonized by microorganisms at birth

Animals provide favorable environment for growth of many microorganisms

Infections frequently begin at sites of animal's mucous membranes

Microorganisms usually found associated with human body tissue

Microbial parasites

The ability of a parasite to inflict damage on the host

Measure of pathogenicity

Relative ability of a pathogen to cause disease

Causes disease only in the absence of normal host resistance

Situation in which a microorganism is established and growing in a host, whether or not the host is harmed

Availability of nutrients MOST IMPORTANT in affecting pathogen growth

Damage or injury to the host that impairs host function

Skin is a dry, acid environment that does not support growth of most microorganisms
-compared to sweat glands that readily colonize gram-positive bacteria

Skin microflora composition is influenced by environmental factors and host factors

Oral cavity is a complex, heterogeneous microbial habitat

Saliva contains antimicrobial enzymes
-But high concentrations of nutrients near surfaces in the mouth promote localized microbial growth
-includes lysozymes (enzyme that cleaves glycosidic linkages in peptidoglycan of bacterial cell walls)

The tooth consists of a mineral matrix (enamel) and surrounding living tissue (dentin and pulp)
-bacteria colonize teeth by attaching to acidic glycoproteins deposited on it by saliva

Extensive growth of oral microorganisms (especially streptococci) results in thick bacterial layer (dental plaque)
-as plaque develops, anaerobic bacterial species grow & produce high concentrations of acid = decalcification of the tooth enamel (dental caries)

Lactic acid bacteria Streptococcus sobrinus and Streptococcus mutans are common agents of dental caries

Human Gastrointestinal (GI) Tract
-consists of stomach, small intestines, large intestines
-responsible for digestion of food, absorption of nutrients, and production of nutrients by indigenous microbial flora
-contains 10^13 - 10^14 microbial cells

Acidity of the stomach and the duodenum of the small intestine (pH 2) prevent many organisms from colonizing the GI tract

Carry out a variety of metabolic reactions that produce various compounds

The type and amount produced is influenced by the composition of the intestinal flora and diet

Compounds produced by intestinal flora include:
-vitamins
-gas, organic acids, odor
-enzymes

Restricted group of organisms colonizes the upper respiratory tract
-include staphylococci, streptococci, diphtheroid bacilli, gram-negative cocci

Lower respiratory tract lacks microflora in healthy individuals
-as air passes into the lower respiratory tract, flow rate decreases and organisms settle onto walls of respiratory passages

Bladder is sterile in males and females

Altered conditions (change in pH) can cause potential pathogens in the urethra (E.coli and P.mirabilis) to multiply and become pathogenic
--usually cause UTIs in women

Vagina is weakly acidic and contains glycogen
-Lactobacillus acidophilus, organism in the vagina, ferments glycogen and produces lactic acid
-Lactic acid maintains a local acidic environment

Exposure -> adherence -> invasion -> colonization & growth -> toxicity/invasiveness -> tissue damage/disease

Virulence estimated by LD50 (lethal dose 50)
-amount of an agent that kills 50% of animals in a test group

Highly virulent pathogens show little difference in the number of cells required to kill 100% or 50%

Decrease or loss of virulence

Organism causes disease by means of toxin that inhibits host cell function or kills host cells

Toxins can travel to sites within host not inhabited by pathogen

Ability of a pathogen to grow in host tissue at densities that inhibit host function
-can cause damage without producing a toxin

Starts at the site of adherence

May spread throughout the host via the circulatory or lymphatic systems

Pathogen must gain access to host tissues and multiply before damage can be done

Bacteria and viruses that initiate infection often adhere to epithelial cells through macromolecular interactions on the surfaces of the pathogen and the host cell

Facilitated by:

-Extracellular macromolecules that are not covalently attached to the bacterial cell surface
---ex. slime layers and capsules

-fimbriae and pili

Pathogens produce enzymes that:

-enhance virulence by breaking down or altering host tissue to provide access to nutrients
---ex. hyaluronidase (enzyme that promotes spreading of organisms in tissue by breaking down polysaccharide hyaluronic acid-intercellular cement in animals)

-protect the pathogen by interfering with normal host defense mechanisms
---ex. coagulase (breaks down tissue supporting collagen network, spreading organism)

Proteins released from the pathogen cell as it grows

Three categories:
-cytolytic toxins
-AB toxins
-superantigen toxins

Work by degrading cytoplasmic membrane integrity, causing cell lysis and death

Toxins that lyse red blood cells are called hemolysins

Staphylococcal alpha-toxin kills nucleated cells and lyses erythrocytes (releases cytoplasmic components out of cell)

Consist of two subunits, A and B

Inhibits protein synthesis

Works by binding to host cell receptor (B subunit) and transferring damaging agent (A subunit) across the cell membrane

Examples: diphtheria toxin, tetanus toxin, botulinum toxin

Produce potent AB exotoxins that affect nervous system

C.tetani grows in the body in deep wounds that become anoxic

C.botulinum grows directly in the body, causing infant wound or botulism and grows and produces toxins in preserved foods

Botulinum toxin consists of related AB toxins that are potent biological toxins

Tetanus toxins are AB protein neurotoxins

Exotoxins whose activity affects the small intestines

Generally cause massive secretion of fluid into the intestinal lumen, resulting in vomiting and diarrhea
-ex. chloera toxin

Lipopolysaccharide portion of the cell envelope of certain gram-negative Bacteria, which is a toxin when solubilized

Cell bound and released in large amounts only when the cells lyse

Generally less toxic than exotoxins

Presence of endotoxin can be detected by the Limulus amoebocyte lysate (LAL) assay

One or more resistance mechanisms inactive

Probability of infection increases

Age is important factor (old more susceptible)

Stress, diet, genetic conditions can also compromise a host

Hosts have innate resistance to most pathogens
-natural host resistance, tissue specificity, physical/chemical barriers

Removal of particles including microorganisms by rapid passage of air over cilia in nasopharynx

Physical barrier

Produces antimicrobial fatty acids and its normal flora inhibits pathogen colonization

Stomach acidity (pH 2) inhibits microbial growth

Flushing of urinary tract prevents colonization

In tears and other secretions dissolves cell walls

Mucus and cilia lining in trachea suspend and move microorganisms out of the body

Mucus and phagocytes in lungs prevent colonization

Blood and lymph proteins inhibit microbial growth