Chapter 20 Viruses and Bacteria Section 1: Viruses Section 2: Bacteria.
How do we compete with pathogens? Bacteria and viruses reproduce much more rapidly Bacteria and...
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Transcript of How do we compete with pathogens? Bacteria and viruses reproduce much more rapidly Bacteria and...
How do we compete with pathogens?
Bacteria and viruses reproduce much more rapidly Bacteria and viruses are very numerous Bacteria and viruses tolerate vast fluctuations in population size
Recover so quickly 1% of bacterial population is insignificant but 1% of human population is a
major proportion Enormous genetic diversity with rapid evolution
Summary: Pathogens have large populations that undergo rapid fluctuation, evolving at a much higher rate than their hosts.
Pathogens have a shared interest in our survival; with few exceptions, a dead
host inhibits the spread of the pathogen.
Evolutionary mechanisms include intraclonal (within the cell) and interclonal
(between different cells) processes.
Intraclonal Processes
DNA Replication: microbes are constantly exposed to chemical and physical mutagens, resulting in high variation of genomes
RNA Replication: no repair mechanisms means a mutation rate so high that for many RNA viruses virtually every particle is genetically different
Haploid Organisms: single copy genomes means no delay in expressing new genetic factors
Phase Variation: many pathogens seem to have an archive of genetic material that may be silent under normal conditions but can become expressed and give the appearance of an adaptive change
Interclonal Processes
Genetic Exchange: conjugation, transduction, and transformation increase the genetic, and often the phenotypic, diversity
Host-Parasite Coevolution: hosts and parasites coadapt with parasites generally prolonging their survival by overcoming host defenses and mitigating their virulence
Symptoms: symptoms may be a host defense or a virulence mechanism
Foundations of Microbiology Antoni van Leeuwenhoek first observed
microorganisms Louis Pasteur laid the foundation for modern
microbiology Robert Koch developed methods and concepts
Koch’s Postulates The disease-causing organism must be present in all
diseased hosts and absent in healthy hosts. The disease-causing organism must be isolated and grown
in pure culture. Inoculation of a healthy host with the disease-causing
organism must result in the original disease. The disease-causing organism must be re-isolated and
shown to be the same as was inoculated.
Ignaz Semmelweis
Observed that mortality rate of new mothers was higher with physicians than with midwives
Suggested better hygiene to reduce mortality Friend died of puerperal sepsis contracted during
autopsy Hypothesized “invisible agents” responsible for
the spread of disease
Darwinian Medicine:The influence of evolution on disease.
How does a disease affect reproductive fitness? An inherited disease with a severe fitness cost will
be selected against.
“When diseases have been present in human populations for many generations and still have a substantial negative impact on people’s fitness they are likely to have an infectious cause.”
---- Paul Ewald
Darwinian Medicine and Microbiology
Evolutionary change occurs very rapidly in the microbial world
Evolutionary drive is to reproduce Do reproductive and transmission strategies influence
disease symptoms? Are disease symptoms caused by the pathogen or the host?
Evolution of Infectious Disease
Too many copies of an infectious agent can kill or immobilize a host before it can spread
Pathogens transferred by a vector tend to be more virulent
Chronic Diseases
Diseases that could not fulfill Koch’s Postulates were attributed to heredity,
environmental, or “multifactorial” causes
Darwinian Medicine and Microbiology II
Some of the diseases attributed to genetic or environmental factors, including some forms of heart disease, cancer, and mental illness, are in many cases actually due to microbial
infections.
Heart Disease
Clogged coronary arteries have been found full of Chlamydia pneumoniae
Some estimates that 80% of coronary heart disease is caused by C. pneumoniae
C. pneumoniae also found in autopsied brains of Alzheimer’s patients (not in controls)
HIV
High mutation rate Preventative measures curb the spread of infection Would transmission-prevention programs be more
effective than the search for a vaccine?
Cancer
HTLV-1, a retrovirus endemic in parts of Asia, Africa, and the Caribbean, has been linked to some leukemias and lymphomas
Epstein-Barr Virus, the cause of mononucleosis, has been linked to some lymphomas and nasopharyngeal cancers
HPV has been shown to cause cervical cancer Hepatitis B and C have been linked to liver cancer
Ulcers
Barry Marshall demonstrated that up to 75% of ulcers may be due to Helicobacter pylori
Over 90% of cases are cured with antibiotics Infection by H. pylori causes a 6-fold greater risk
of stomach cancer