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Microscope

Microbiology:
Week Four

Chapter Five: Microbial Diversity - Eucaryotic Microbes

Characteristics of Fungi

  • The study of fungi is called mycology; scientists who study fungi are called mycologists.

  • Fungi are found virtually everywhere.

  • Some fungi are harmful, some are beneficial.

  • Fungi represent a diverse group of eucaryotic organisms that include yeasts, moulds, and fleshy fungi (e.g., mushrooms).

  • Fungi are the “garbage disposers” of nature.

  • Fungi are not plants – they are not photosynthetic.

  • Fungal cell walls contain a polysaccharide called chitin.

  • Some fungi are unicellular, while others grow as filaments called hyphae.

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Microscopic Appearance of Various Fungi

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Fungi : Yeast

Yeasts are found in soil and water and on the skins of many fruits and vegetables.

  • Yeasts have been used for centuries to make wine and beer.

  • Saccharomyces cerevisiae is a yeast used in baking.

  • Candida albicans is the yeast most frequently isolated from human clinical specimens and is also the fungus most frequently isolated from human clinical specimens.

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Yeast colonies may be difficult to distinguish from bacterial colonies.

  • A simple wet mount can be used to differentiate yeast colonies from bacterial colonies.

  • Yeasts are larger than bacteria and are usually oval-shaped.

  • Yeasts are often observed in the process of budding.

  • Bacteria do not bud.

Colonies of C. albicans on blood agar

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Fungi : Moulds

  • Often spelled “molds.”

  • Moulds are often seen in water and soil and growing on food.

  • Moulds produce cytoplasmic filaments called hyphae.

        –Aerial hyphae extend above the surface of whatever the mould is growing on.

        –Vegetative hyphae grow beneath the surface.

  • Reproduction is by spore formation, either sexually or asexually, on the aerial hyphae (also known as reproductive hyphae).

  • Moulds have great commercial importance.

        –Some produce antibiotics. Examples: Penicillium and Cephalosporium

        –Some moulds are used to produce large quantities of enzymes that are used commercially.

        –The flavor of cheeses like bleu cheese, Roquefort, camembert, and limburger are due to                 moulds that grow in them.

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Fungi: Medical Significance

  • A variety of fungi including yeasts, moulds, and some fleshy fungi, are of medical, veterinary and agricultural importance because of the diseases they cause in humans, animals, and plants.

  • The infectious diseases of humans and animals that are caused by moulds are called mycoses.

  • Fungal infections of humans are categorized as superficial, cutaneous, subcutaneous, and systemic mycoses.

Fungi: Fleshy Fungi

  • Include mushrooms, toadstools, puffballs and bracket fungi

  • Consist of a network of filaments or strands (the mycelium) that grows in soil or on rotting logs

  • The fruiting body that grows above the ground forms and releases spores

  • Some mushrooms are edible; some are extremely toxic

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Superficial & Cutaneous Mycoses

  • Superficial mycoses are fungal infections of the outermost areas of the human body – hair, nails and epidermis.

  • Cutaneous mycoses are fungal infections of the living layer of the skin, the dermis.

        –A group of moulds collectively referred to as dermatophytes cause tinea (“ringworm”)                  infections.

        –Note that “ringworm” infections have nothing to do with worms.

        –The yeast, Candida albicans, can also cause cutaneous, oral, and vaginal infections.

Chapter Nine: Controlling Microbial Growth in Vivo Using Antimicrobial Agents

Introduction

  • Chemotherapy is the use of any chemical (drug) to treat any disease or condition.

  • A chemotherapeutic agent is any drug used to treat any condition or disease.

  • An antimicrobial agent is any chemical (drug) used to treat an infectious disease, either by inhibiting or killing pathogens in vivo. Some antimicrobial agents are antibiotics.

  • Drugs used to treat bacterial diseases are called antibacterial agents; those used to treat fungal diseases, antifungal agents; those used to treat protozoal diseases, antiprotozoal agents; those used to treat viral diseases, antiviral agents.

  • •An antibiotic is a substance produced by a microorganism that kills or inhibits growth of other microorganisms.

  • •Antibiotics that have been chemically modified to kill a wider variety of pathogens or reduce side effects are called semisynthetic antibiotics; examples include semisynthetic penicillins such as ampicillin and carbenicillin.

The discovery of penicillin by Alexander Fleming. (A) Colonies of Staphylococcus aureus are growing well in this area of the plate. (B) Colonies are poorly developed in this area of the plate because of an antibiotic (penicillin) being produced by a colony of Penicillium notatum (a mould), shown at C.

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Characteristics of an Ideal Antimicrobial Agent

  • The ideal antimicrobial agent should:

       –Kill or inhibit the growth of pathogens

       –Cause no damage to the host

       –Cause no allergic reaction in the host

       –Be stable when stored in solid or liquid form

       –Remain in specific tissues in the body long enough to be effective

       –Kill the pathogens before they mutate and become resistant to it

Characteristics of an Ideal Antimicrobial Agent

  • The ideal antimicrobial agent should:

       –Kill or inhibit the growth of pathogens

       –Cause no damage to the host

       –Cause no allergic reaction in the host

       –Be stable when stored in solid or liquid form

       –Remain in specific tissues in the body long enough to be effective

       –Kill the pathogens before they mutate and become resistant to it

      Click to study               Week Four Vocabulary words

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How Antimicrobial Agents Work

  • The 5 most common mechanisms of action of antimicrobial agents are:

       –Inhibition of cell wall synthesis

       –Damage to cell membranes

       –Inhibition of nucleic acid synthesis (either DNA or RNA synthesis)

       –Inhibition of protein synthesis

       –Inhibition of enzyme activity

Antibacterial Agents

  • Colistin and nalidixic acid destroy only Gram-negative bacteria; they are referred to as narrow-spectrum antibiotics.

  • Antibiotics that are destructive to both Gram-positive and Gram-negative bacteria are called broad-spectrum antibiotics (examples: ampicillin, chloramphenicol and tetracycline).

  • Multidrug therapy

       –Sometimes one drug is not sufficient; 2 or more             drugs may be used simultaneously, as in the                   treatment of tuberculosis.

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  • Synergism Versus Antagonism

       –Synergism is when 2 antimicrobial agents are used together to produce a degree of                       pathogen killing that is greater than that achieved by either drug alone. Synergism is a                 good thing!

       –Antagonism is when 2 drugs actually work against each other. The extent of pathogen                 killing is less than that achieved by either drug alone.  Antagonism is a bad thing!

Antifungal Agents

  • Most antifungal agents work in one of 3 ways:

       –By binding with cell membrane sterols (e.g.,                       nystatin and amphotericin B)

       –By interfering with sterol synthesis (e.g.,                             clotrimazole and miconazole)

       –By blocking mitosis or nucleic acid synthesis (e.g.,             griseofulvin and 5-flucytosine)

  • Antifungal agents and antiprotozoal agents tend to be more toxic to the patient because, like the infected human, they are eucaryotic organisms.

Antiviral Agents

  • Antiviral agents are the newest weapons in antimicrobial methodology.

  • Difficult to develop these agents because viruses are produced within host cells.

  • Some drugs have been developed that are effective in certain viral infections, but not others; they work by inhibiting viral replication within cells.

  • Antiviral agent “cocktails” (several drugs that are administered simultaneously) are being used to treat HIV infection.

Drug Resistance: "Superbugs"

  • Superbugs are microbes (mainly bacteria) that have become resistant to one or more antimicrobial agent. Infections caused by superbugs are difficult to treat!

  • Bacterial superbugs include:

          –methicillin-resistant Staphylococcus aureus (MRSA); vancomycin-resistant                  Enterococcus spp. (VRE); multidrug-resistant Mycobacterium tuberculosis                  (MDRTB); multidrug-resistant strains of Acinetobacter, Burkholderia, E. coli,                Klebsiella, Pseudomonas, Stenotrophomonas, Salmonella, Shigella. and N.                  gonorrhoeae; β–lactamase-producing strains of Streptococcus pneumoniae                and Haemophilus influenzae; carbapenemase-producing Klebsiella                                  pneumoniae.

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Drug Resistance: How Bacteria Become Resistant

  • Some bacteria are naturally resistant because they lack the specific target site for the drug or the drug is unable to cross the organism’s cell wall or cell membrane and thus, cannot reach its site of action. Resistance of this type is known as intrinsic resistance.

  • If bacteria that were once susceptible to a particular drug become resistant, this is called acquired resistance.

  • Before a drug enters a bacterial cell, it must first bind to proteins on the surface of the cell; these proteins are called drug-binding sites. A chromosomal mutation that affects the structure of a drug-binding site can prevent the drug from binding, resulting in drug resistance.

  • To enter a bacterial cell, a drug must be able to pass through the cell wall and cell membrane; chromosomal mutations may alter the structure of the cell membrane, thus preventing the drug from entering the cell; this results in drug resistance.

  • Bacteria can develop the ability to produce an enzyme that destroys or inactivates a drug.

          –Many bacteria have become resistant to penicillin because they have                               acquired the gene for penicillinase production during conjugation.

  • A plasmid that contains multiple genes for drug resistance is known as a resistance factor (R-factor).

  • Bacteria can also become resistant to drugs by developing the ability to produce multidrug-resistance (MDR) pumps (also known as MDR transporters or efflux pumps).

          –An MDR pump enables the cell to pump out drugs before they can damage                 or kill the cell.

  • Summary: Bacteria can acquire resistance to antimicrobial agents by chromosomal mutation or by the acquisition of new genes by transduction, transformation and, most commonly, by conjugation.

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Strategies in the war Against Drug Resistance

  • Education of healthcare professionals and patients

  • Patients should stop demanding antibiotics every time they are, or their child is, sick

  • Physicians should not be pressured by patients and should prescribe drugs only when warranted

  • Clinicians should prescribe a narrow-spectrum drug if lab results indicate that it kills the pathogen

  • Patients should destroy any excess or out-dated medications

  • Antibiotics should not be used in a prophylactic manner

  • Healthcare professionals should practice good infection control

  • Patients should take drugs in manner prescribed

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