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Introduction to Medical Microbiology, historical landmarks, Classification of microorganisms


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Title: Introduction to Medical Microbiology, historical landmarks, Classification of microorganisms

Introduction to Medical Microbiology, historical
landmarks, Classification of microorganisms
  • Dr. Anthony Onipede

  • To introduce students to the field of medical

  • Medical microbiology is the study of
  • parasites,
  • fungi,
  • bacteria, and
  • viruses that are the agents of infectious disease
    in humans.

  • Pathogen disease causing microbe
  • Pathogenesis how a disease develops
  • Infection growth of microbes in or on the body
  • Virulence degree of pathogenesis, how
    aggressive a microbe is

  • Disease can occur when a microbe is found in a
    part of the body where it is not normally found.
    e.g. E. coli is a bacterium normally found in the
    colon but will cause urinary tract infection if
    it gets into the urinary tract, where it is not
    normally found

Historical facts
  • The discipline of bacteriology evolved from the
    need of physicians to test and apply the germ
    theory of disease and from economic concerns
    relating to the spoilage of foods and wine.
  • What is the germ theory?

Robert Kochs Postulates - germ theory for
  • The same microbe should be isolated from ALL
    individuals with a given disease.
  • The microbe must be grown as a pure culture.
  • Inoculation of the microbe into a healthy
    individual should cause the SAME disease.
  • The SAME microbe must be now isolated from the
    newly inoculated individual.
  • Koch used this to prove that Mycobacterium
    tuberculosis caused tuberculosis and that
    Bacillus anthracis causes anthrax.

Major advances in bacteriology over the last
  • resulted in the development of many effective
    vaccines (e.g., pneumococcal polysaccharide
    vaccine, diphtheria toxoid, and tetanus toxoid)
    as well as of
  • other vaccines (e.g., cholera, typhoid, and
    plague vaccines) that are less effective or have
    side effects.
  • Another major advance was the discovery of

Major advances in bacteriology over the last
  • These antimicrobial substances have not
    eradicated bacterial diseases, but they are
    powerful therapeutic tools.
  • Their efficacy is reduced by the emergence of
    antibiotic resistant bacteria (now an important
    medical management problem)
  • In reality, improvements in sanitation and water
    purification have a greater effect on the
    incidence of bacterial infections in a community
    than does the availability of antibiotics or
    bacterial vaccines.

Historical facts
  • Most diseases now known to have a bacteriologic
    etiology have been recognized for hundreds of
  • Some were described as contagious in the writings
    of the ancient Chinese, centuries prior to the
    first descriptions of bacteria by Anton van
    Leeuwenhoek in 1677.
  • There remain a few diseases (such as chronic
    ulcerative colitis) that are thought by some
    investigators to be caused by bacteria but for
    which no pathogen has been identified.

Historical facts
  • Occasionally, a previously unrecognized diseases
    is associated with a new group of bacteria. An
    example is Legionnaire's disease, an acute
    respiratory infection caused by the previously
    unrecognized genus, Legionella.
  • Also, a newly recognized pathogen, Helicobacter,
    plays an important role in peptic disease.
  • Another important example, in understanding the
    etiologies of venereal diseases, was the
    association of at least 50 percent of the cases
    of urethritis in male patients with Ureaplasma
    urealyticum or Chlamydia trachomatis.

New trend
  • Recombinant bacteria produced by genetic
    engineering are enormously useful in
    bacteriologic research and are being employed to
    manufacture scarce biomolecules (e.g.
    interferons) needed for research and patient
  • The antibiotic resistance genes, while a problem
    to the physician, paradoxically are indispensable
    markers in performing genetic engineering.
  • Genetic probes and the polymerase chain reaction
    (PCR) are useful in the rapid identification of
    microbial pathogens in patient specimens.

New trend
  • Genetic manipulation of pathogenic bacteria
    continues to be indispensable in defining
    virulence mechanisms.
  • As more protective protein antigens are
    identified, cloned, and sequenced, recombinant
    bacterial vaccines will be constructed that
    should be much better than the ones presently
  • In this regard, a recombinant-based and safer
    pertussis vaccine is already available in some
    European countries. Also, direct DNA vaccines
    hold considerable promise.

Why is Medical Microbiology important?
  • Modern medicine relies on the control of
  • microorganisms to maintain human health and
    quality of life.

Why is Medical Microbiology important?
  • In developed countries, 90 percent of documented
    infections in hospitalized patients are caused by
  • These cases probably reflect only a small
    percentage of the actual number of bacterial
    infections occurring in the general population,
    and usually represent the most severe cases.
  • In developing countries, a variety of bacterial
    infections often exert a devastating effect on
    the health of the inhabitants.

Why is Medical Microbiology important?
  • Malnutrition, parasitic infections, and poor
    sanitation are a few of the factors contributing
    to the increased susceptibility of these
    individuals to bacterial pathogens.
  • The World Health Organization has estimated that
    each year,
  • 3 million people die of tuberculosis,
  • 0.5 million die of pertussis, and
  • 25,000 die of typhoid.
  • Diarrheal diseases, many of which are bacterial,
    are the second leading cause of death in the
    world (after cardiovascular diseases), killing 5
    million people annually.

Microbes and Humanity
  • Microbes may be the most significant life form
    sharing this planet with humans because of their
    pervasive presence and their utilization of any
    available food source, including humans whose
    defenses may be breached.

Microbes and Humanity
  • Depending on the food source, microbes may have
    either beneficial roles in maintaining life or
    undesirable roles in causing human, animals and
    plant disease.

Microbes and Humanity
  • Beneficial roles of microbes include recycling of
    organic matter through microbe-induced decay and
    through digestion and nutrition in animals and
  • In addition, the natural microbial flora provides
    protection against more virulent microbes.

Microbes and Humanity
  • Microbes that cause infectious diseases are
  • opportunistic diseases may also be caused by
    normally benign microbes.
  • Opportunistic infections occur when the host
    defense mechanisms are impaired, microbes are
    present in large numbers, or when microbes reach
    vulnerable body sites.
  • HIV for example impairs the host's defenses to
    multiple microbes.

Microbes and Humanity
  • death or severe impairment of an infected host
    compromises the survival of the infecting
  • natural selection favors a predominance of less
    virulent microorganisms, except when microbial
    transmission depends on disease manifestations
    (e.g., coughing and sneezing).

Microbes and Humanity
  • Understanding and employing the principles of
    microbiology and the molecular mechanisms of
    pathogenesis enable the physician and medical
    scientist to control an increasing number of
    infectious diseases

Divisions of medicalmicrobiology include
  • bacteriology, the study of bacteria that inhabit
    and/or colonize the human body and cause disease
  • mycology, the study of fungi as causative agents
    of human disease

Divisions of medicalmicrobiology include
  • parasitology, the formal study of the human
    parasitic organisms
  • protozoans,
  • Helminths nematodes, trematodes and
  • arthropods) and
  • virology, the study of viruses that cause
    infectious syndromes in humans.

Sizes for the pathogens considered include
  • viruses (50-100nm), (the smallest)
  • bacteria that range from 0.1 µm (Chlamydiae) to
    10µm (Bacillus rods),
  • fungi ranging from 8µm (yeasts) up to 10mm in
    size (filamentous fungi) and
  • metazoan parasites that are visible to the naked

Inter-relatedness with other discipline
  • Medical microbiology as a discipline requires a
    working knowledge of human anatomy and histology,
    and a comprehension of the pathologies associated
    with the infectious disease process.
  • The human immune response to pathogens is key to
    the consideration of infectious disease.
  • Understanding the relationship between pathogens
    and antimicrobial pharmacology is essential as

Inter-relatedness with other discipline
  • Microbiology places information about pathogenic
    organisms and their specific characteristics
    within the context of host disease.
  • Developing connections between microbiology and
    immunology will make learning more effective in
    both disciplines.

Special patient populations
  • are important to consider and should be
    appreciated for the distinctive infectious
    disease presentations they reflect.
  • Age-related immuno-compromised status makes the
    elderly and the very young, especially newborn
    infants, particularly at-risk for respiratory
  • Health-related immunosuppression can predispose
    organ transplant recipients, and patients with
    immunodeficiency disorders, cancer, and diabetes
    to difficult infections.
  • unique exposures due to occupation or travel can
    be a problem for some patient populations.

Special patient populations
  • An immunocompromised person is lacking in some
    aspect of innate or adaptive immunity due to
    either a primary or secondary immune deficiency.
  • the net result is that the immunocompromised
    individual becomes susceptible to infection with
    a range of opportunistic pathogens from the
    commensal microflora and conventional infectious
    agents that cause a more severe form of disease
    than in a 'normal' host.
  • Immunity to infection and sensitivity to normal
    commensal microbes as pathogens varies throughout
    life and not just in 'disease' states.

The Universal Cell
  • All living organisms, large and small, have one
    thing in common the cell.
  • This is a tiny living factory capable of
    converting simple food substances into energy and
    new cell material and of reproducing itself.

The Universal Cell
  • Large organisms, including people, are composed
    of billions of cells with many different roles.
  • Microorganisms, on the other hand, are made up of
    a very few or even a single cell capable of
    carrying on all of life.s processes.
  • So a basic understanding of cell structure and
    function is essential to understanding the
    actions of bacteria, yeasts and molds.

The Kingdoms of Microorganisms
  • Characteristics of the organism that classifies
    its kingdom cell type, body form, cell wall
    composition, mode of nutrition, nervous system,
    and locomotion

The Kingdoms of Microorganisms
  • Monera bacteria, most do not photosynthesize
    (parasitic) except for the blue-green bacteria.
  • Protista protozoa, some are motile and feed upon
    bacteria and other organic matter. Algae are part
    of this kingdom. Algae photosynthesizes and live
    in water.
  • Fungi molds, yeasts, mushrooms, rusts, and
    smuts. Parasites or decomposers. Fungi contain no
    chlorophyll and cannot synthesize food. Instead
    they secrete enzymes that digest food material
    outside the organisms.

The Kingdoms of Microorganisms
  • Plantae non-microscopic
  • Animalia non-microscopic

(No Transcript)
What are bacteria?
  • Bacteria are single-celled microorganisms that
    lack a nuclear membrane, are metabolically active
    and divide by binary fission.
  • They have circular double-stranded DNA and
    (except for Mycoplasma sp) cell walls.
  • Medically they are a major cause of disease.

What are bacteria?
  • Only a small number are human pathogens.
  • they are ubiquitous and have a remarkable
    capacity to adapt to changing environments by
    selection of spontaneous mutants,
  • the importance of bacteria in every field of
    medicine cannot be overstated.


Bacteria are classified by several criteria,
  • Morphology
  • cylindric (bacilli),
  • spherical (cocci), or
  • spiral (spirochetes).
  • Motility
  • A few coccal, many bacillary, and most
    spirochetal species are motile.


Bacteria are classified by several criteria,
  • Gram Reaction
  • Gram-positive bacteria retain crystal violet dye
    after iodine fixation and alcohol decolorization,
    whereas gram-negative bacteria do not.
  • Gram-negative bacteria have an additional outer
    membrane containing lipopolysaccharide
  • Presence of Capsule
  • Bacteria may be additionally enclosed in
    capsules, which may (eg, with Streptococcus
    pneumoniae and Haemophilus influenzae) impair
    their ingestion by phagocytes. Other factors may
    enhance bacterial pathogenicity.

Other properties
  • Aerobic bacteria grow in culture in the presence
    of air.
  • Anaerobic bacteria do not facultative bacteria
    can grow either aerobically or anaerobically.
  • Some bacteria (eg, Salmonella typhi , Legionella
    sp, Mycobacteria sp, and Chlamydia and
    Chlamydophila spp) preferentially reside and
    replicate intracellularly.
  • Most others do so extracellularly.

Other properties
  • Antibacterial drugs are derived from bacteria or
    molds or from de novo synthesis.
  • Antibiotic, which is often used synonymously
    with antibacterial drug, technically refers
    only to antimicrobials derived from bacteria or

Other properties
  • Antibacterials have many mechanisms of action,
  • inhibiting cell wall synthesis,
  • activating enzymes that destroy the cell wall,
  • increasing cell membrane permeability, and
  • interfering with protein synthesis and nucleic
    acid metabolism.

Other properties
  • Antibacterials sometimes interact with other
    drugs, raising or lowering serum levels of other
    drugs by increasing or decreasing their
    metabolism or various other mechanisms.
  • The most clinically important interactions
    involve drugs with a low therapeutic ratio (ie,
    toxic levels are close to therapeutic levels)

Classification of Common Pathogenic Bacteria Classification of Common Pathogenic Bacteria Classification of Common Pathogenic Bacteria
Aerobic vs Anaerobic Type Organism
Aerobic Gram-positive cocci, catalase-positive Staphylococcus aureus (coagulase-positive), S. epidermidis (coagulase-negative), other coagulase-negative staphylococci
Aerobic Gram-positive cocci, catalase-negative Enterococcus faecalis , E. faecium , Streptococcus agalactiae (Group B streptococcus), S. bovis , S. pneumoniae , S. pyogenes (Group A streptococcus), Viridans group streptococci, S. anginosus , S. mutans
Aerobic Gram-negative cocci Moraxella catarrhalis , Neisseria gonorrhoeae , N. meningitidis
Aerobic Gram-positive bacilli Bacillus anthracis , Corynebacterium diphtheriae , C. jeikeium , Erysipelothrix rhusiopathiae , Gardnerella vaginalis (gram-variable)
Aerobic Acid-fast bacilli Mycobacterium avium complex, Mycobacterium kansasii, M. leprae , M. tuberculosis , Nocardia sp
Aerobic Gram-negative bacilli Enterobacteriaceae (Citrobacter sp, Enterobacter aerogenes , Escherichia coli , Klebsiella sp, Morganella morganii , Proteus sp, Providencia rettgeri , Salmonella typhi , other Salmonella sp, Serratia marcescens , Shigella sp, Yersinia enterocolitica , Y. pestis)
Aerobic Fermentative, non-Enterobacteriaceae Aeromonas hydrophila , Chromobacterium violaceum , Plesiomonas shigelloides , Pasturella multocida , Vibrio cholerae , V. vulnificus
Aerobic Non-fermentative, non-Enterobacteriaceae Acinetobacter calcoaceticus , Flavobacterium meningosepticum, Pseudomonas aeruginosa , Pseudomonas alcaligenes, other Pseudomonas sp, Stenotrophomonas maltophilia
Aerobic Fastidious gram- negative coccobacilli and bacilli Actinobacillus actinomycetemcomitans , Bartonella bacilliformis , B. henselae , B. quintana , Brucella sp, Bordetella sp, Eikenella corrodens , Haemophilus influenzae, other Haemophilus sp, Legionella sp
Aerobic Curved bacilli Campylobacter jejuni , Helicobacter pylori
Aerobic Chlamydiaceae Chlamydia trachomatis , Chlamydophila pneumoniae , C. psittaci
Aerobic Rickettsiae Rickettsia prowazekii , R. rickettsii
Aerobic Mycoplasma Mycoplasma pneumoniae
Aerobic Treponemataceae (spiral organisms) Borrelia burgdorferi , Leptospira sp, Treponema pallidum
Anaerobic Gram-negative bacilli Bacteroides fragilis, other Bacteroides sp, Fusobacterium sp, Prevotella sp
Anaerobic Gram-negative cocci Veillonella sp
Anaerobic Nonspore-forming gram-positive bacilli Actinomyces sp, Bifidobacterium sp, Eubacterium sp, Propionibacterium sp
Anaerobic Endospore-forming gram-positive bacilli Clostridium botulinum , C. perfringens , C. tetani, other Clostridium sp
Anaerobic Gram-positive cocci Gemella morbillorum , Peptococcus niger , Peptostreptococcus sp
Parade of pathogens