Symbiosis, Infectious Diseases and Antibiotics
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- Created by: amazingemilyjones
- Created on: 14-04-19 22:05
Symbiosis, Infectious Diseases and Antibiotics
Symbiosis, Infectious Diseases and Antibiotics
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Symbiosis
- Any microorganism that spends a portion of part of its life or all of its life with another organism of a different species is called a symbiont
- Relationship is symbiosis
- Normal flora
- Mixture of organisms regularly present at any site in an animal
- Bacteria
- Fungi
- Virus (including bacterial viruses; bacteriophages)
- Protozoa
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Symbiosis/Parasitism
- If the symbiont either harms or lives at the expense of another organism it is a parasitic organism
- Relationship is parasitism
- Ectoparasite: a parasite that lives on the surface of a host organism
- Endoparasite: a parasite that lives inside the body of an organism
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Benefits of the Normal Flora
- Synthesise and excrete vitamins
- Enteric bacteria secrete vitamin K and vitamin B12
- Prevent colonisation by pathogens
- Competing for attachment sites or for essential nutrients
- Oral cavity, intestine, skin and the vaginal epithelium
- Antagonise other bacteria
- Production of substances which inhibit or kill non-indigenous species
- Intestinal bacteria
- Fatty acids and peroxides to highly specific bacteriocins, inhibit or kill other bacteria
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Benefits of the Normal Flora
- Stimulate the development of certain tissues
- Caecum
- Caecum of germ-free animals is enlarged, thin-walled and fluid filled
- The normal flora stimulate the production of cross-reactive antibodies
- Imbalance of the normal flora leads to diseases
- Obesity
- Infections
- Probiotics are microorganisms that are believed to provide health benefits when consumed
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Normal Flora
Infectious Diseases
- Infectious diseases, also known as transmissable diseases or communicable diseases, comprise clinically evident illness (i.e. characteristic medical signs and/or symptoms of disease) resulting from the infection, presence and growth of pathogenic biological agents in an individual host organism.
- In certain cases, infectious diseases may be asymptomatic for much or even all of their course in a given host.
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Determinant Factors
- Disease is intially transported into the host
- Direct (human to human): coughing, sneezing, contact
- Indirect: food, water, soil, surfaces
- Adhere to colonise or invade the host
- Multiply or complete its life cycle on or in the host or the host's cells
- Initially evade the host's defence mechanism
- Possess the mechanical, chemical or molecular ability to damage the host
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Normal Flora, Opportunistic Pathogens and Disease
- Some normal flora are able to cause disease at a time of immune compromise - opportunistic pathogens
- S. aureus, E. coli and Proteus vulgaris, C. albicans
- Bacteria not normal flora but represents pathogenic organisms that may be present in a large percentage of the population without causing disease
- MRSA, Clostridium difficile
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Long Term Use of Antibiotics and Opportunists
- Extended use of broad spectrum antibiotics (e.g. lindamycin, lincomycin, third generation cephalosporins, fluoroquinolones) may give rise to a severe gastroenteritis resulting from a toxin produced by C. difficile
- The inflammation of the colon is pseudomembranous colitis and is generally caused by antibiotic assocated diarrhoea (AAD)
- C.difficile normally are indigenous bacteria, able to proliferate when most of the normal intestinal flora are killed by the antibiotic
- Non-pathogenic strain may acquire virulence gene, e.g. toxin which cause disease
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Virulence Factors
- Virulence: the degree or intensity of pathogenicity of an organism
- Virulence factor: a microbial product that contributes to virulence
- Examples of virulence factors in E.coli:
- Adhesin - colonisation factor (CFA1, CFA2)
- Endotoxin - lipopolysaccharise (pyrogen)
- Exotoxin - heat labile (LT) enteroxin, vero cytotoxin-producing E.coli (VTEC) e.g. E.coli O157
- Invasin (uropathogenic E.coli)
- Capsule
- Antibiotic resistance is not a virulent factor
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Toxins
- Toxin: substance produced by organism that damages the host
- Exotoxin
- Usually proteins released by bacteria as it grows and divides
- neurotoxins
- enterotoxins
- cytotoxins
- Usually proteins released by bacteria as it grows and divides
- Endotoxin
- Bound to bacterium and is released when the bacterium lyses
- Gram-negative bacteria; lipopolysaccharide
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Exotoxins
- Required in very small amounts to cause disease
- Usually strong antigenic: produce antitoxin in host
- Inherently unstable: artificial immunisation
- Often composed of protein subunits (e.g. Shigella toxins)
- Mediate their effects outside and inside cells
- membrane-damaging
- membrane acting (disrupting signal following attachment to receptors)
- intracellular
- Bacterial exotoxins are some of the most toxic substances known
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Mechanism of Action of Bacterial Exotoxins
Endotoxins
- Toxic at high doses compared with exotoxins
- Stable
- Weakly immunogenic
- Capable of producing systemic effects
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Pathogenicity Islands
- Virulence genes are contained in specific regions of DNA
- Inserted DNA sequences within chromosome (e.g. Shigella, Salmonella and Yersinia, E. coli)
- Specialised for export of virulence factors directly into cells
- Confer a variety of virulence traits (e.g. cell adherence, cell entry, toxins)
- Many seem to have been acquired in a single step from foreign source
- Provides sudden radical changes in bacteria-host interactions
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Microbial Diseases
- Infection - symptoms
- Outside body
- Inside: microflora
- Acute
- Silent
- Infection - severity
- Infection - causative agents
- Similar symptoms (viruses, bacteria)
- Identification
- Isolation, identification (media, antibody/antigen, DNA)
- Treatment
- Identification is key, severity, choice of treatment
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Antibiotic of Choice
- Determinants
- Selective toxicity, therapeutic dose, toxic dose
- Spectrum of activity
- Broad, narrow, bacterial type (genus and species), antibiotic susceptibility profile of isolate
- Resistance
- Propensity for bacterial isolate to develop resistance
- Activity
- Bacteriostatic vs. bactericidal
- Route of administration
- Oral, intravenous
- Side effects
- Cost of antibiotic and duration of treatment
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Therapeutic Index
- Therapeutic index/therapeutic dosage level is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes death in animal studies or toxitity in human studies
- Therapeutic index = toxic dose/therapeutic dose
- Therapeutic index = TD50/ED50
- TD50: dose that provides a toxicity in 50% of the population
- ED50: minimum effective dose for 50% of the population
- Higher the index the more effective and less toxic than antibiotic
- Especially careful when treating diseases caused by eukaryotes
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Classes of Antibiotics
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Bacterial Resistance to Antibiotics
- Methicillin-resistant Staphylococcus aureus (MRSA)
- Vancomycin-resistant enterococci (VRE)
- PEnicillin-resistant enterococci
- Vancomycin insensitive Staphylococcus aureus (VISA)
- Multidrug resistant (MDR) Mycobacterium tuberculosis (XDR, TDR)
- NDM1 (New Delhi metallo-beta-lactamase 1) enterobacteria
- ESBL Escherichia coli (extended spectrum beta-lactamase)
- Multidrug resistant Acinetonbacteria spp.
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Mechanisms of Bacterial Resistance to Antibiotics
- Intrinsic resistance: natural property of microorganism
- Impermeability barrier (change in permeability)
- Efflux
- Enzymatic inactivation
- Acquired resistance: can be transferred between bacteria
- Decreased uptake/permeability
- Decreased accumulation (efflux)
- Enzymatic inactivation
- Duplication and overproduction of targets
- Modification of target site
- Bypass of a metabolic pathway/enzyme
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