Human Microbe Interactions (see text pages 701-725) Microorganisms are ubiquitous

• Skin no exception • Nether regions no exception

Normal Flora

• Bacteria that ‘normally’ inhabit the body

• Commonly found on >90% of the population

• Beneficial • Breaches in the normal flora present

opportunities for infection • Infection is defined as an invasion of

the body that disrupts normal function

o No all organisms are capable of causing infection

• Pathogens are those that have been identified as potentially dangerous

o Review the concept of Virulence and the Opportunistic Pathogen

Ps. aeruginosa • Adults.. low

likelihood of infections

• Children..much greater likelihood of infection • Burn victims..the leading cause of bacterial infection

Review establishment of the Normal Flora

• Birth (in utero life is sterile) • Food • Exposure to people • Exposure to environment

o Fomites o Insects o Animals/pets o Air o water

• Influences upon o Temperature o pH

Candida albicans (pathogenic yeast) normal flora of the vagina. pH normally acidic (4.0-4.5) maintained by Lactobacilli (also normal flora). Keep yeast in check. Metabolism altered by drugs (like birth control pills) cause changes n flora resulting in yeast infections

o metabolism of host o Age o Diet o Stress

So how can we depict the relationships among bacteria and host?

Bacteria

So what happens during the infection process?

• Hosts have means of defense o Non-specific

Passive Active

o Specific • Bacteria have means of

o Invasiveness Ability to grow

o Toxicity Ability to cause disease

by production of toxins

Non-infectious Non-pathogenic

Atypical normal flora

Typical normal flora

Transient bacteria Opportunity

INFECTION

Non-specific Passive defenses • Skin

o Sebaceous glands secrete fatty acids and lactic acid which lower pH

• Mucus membranes o Traps with ciliated cells

• Stomach o Acidity

• Organs o Eyes and kidneys bathed in

lysozyme • Blood

o contains beta lysins that cytoplasmic membranes

• Tissue specificity o Presence of specific

receptors that permit adherence

What to the bacteria have that permit invasiveness or toxicity?

• Some initial points o Size of initial innoculum is important in establishing infection o Initial infection usually localized by may spread by the lymphatic system o Generalized infection characterized by bacteria in the blood stream is

termed Bacteremia or even more general, Septicemia • Virulence factors

o Hyaluronidase • Known as spreading factor • Digests hyaluronic acid, a polysaccharide that holds cells in place • Produced by some Streptococci, Staphylococci and some

Clostridia o Collagenase

• Digests collagen, a fibrous material supporting tissues • Produced by some Clostridia

o Streptokinase • Dissolves fibrin • Produced by some Streptococci

o Coagulase • Clots fibrin, localizes infection • Produced by some Staphylococci

o Exotoxins • A host of proteins released by organisms

o Endotoxins • Component of the cell envelope of Gram negative cells

Toxins • Fall into two broad categories

• A look at some toxins • Cytolytic Toxins

o Produced by various pathogens (Streptococci, Clostridia) o Act on the cell membranes and lyse cells o May attack the phospholipids (i.e. phospholipases)

Hemolysins • Act on red blood cells

Streptolysin O • Acts on sterols

Leucocidins • Act upon white blood cells

• Diptheria Toxin o Produced by Corynebacterium diptheriae o Effectively prevents protein synthesis o Toxin actually coded for by cells lysogenized with phage beta

• Tetanus Toxin o Produced by Clostridium

tetani o Protein stops the release of a

neurotransmitter that blocks the release of acetylcholine. Acetylcholine is the neurotransmitter that signals the muscles to contract. Consequently, the muscles are in a constant state of contraction. Very nasty.

• Botulinum Toxin o Blocks the release of

acetylcholine so here the muscles cannot contract. Results in flaccid paralysis

o Very potent…1 mg can kill up to one million guinea pigs

o Some of the forms of botulinum are coded for by viruses.

• Enterotoxins o All act on the small lintestine causing secretion of fluid into the lumen o Cholera Toxin

Produced by Vibrio cholerae o Others

Produced by Escherichia and Salmonella Most have similar effects but may differ in mode of action

• Endotoxins o Lipopolysaccharide-lipoprotein complexes of the outer envelope of Gram

negative cells o Cause fever, diarrhea, generalized inflammation o Not as toxic as exotoxins

A summary of some of the known toxins

All figures in this section are taken from Madigan et al. 2002

References:

Madigan, M. T., J.M. Martinko, and J. Parker. 2002. Brock Biology of Microorganisms 10th ed. Prentice Hall.