Infections Lecture Notes PDF

Infection, Infectious Diseases & Epidemiology  Types of Symbiotic Relationships Symbiosis = “to live together”. 3 Types include: Commensalism: Neutral situation; host & bacteria live together having no effect on each other's life cycle Mutualism: Species live together with...

Infection, Infectious Diseases & Epidemiology  Types of Symbiotic Relationships Symbiosis = “to live together”. 3 Types include: Commensalism: Neutral situation; host & bacteria live together having no effect on each other's life cycle Mutualism: Species live together with mutual benefit; each receives a valuable contribution from the other Parasitism: Unequal relationship where one organism clearly benefits from an association to the detriment of another.  Normal Microbiota aka normal flora, indigenous microbiota – colonize the body from birth – non-pathogenic Resident microbiota: remain throughout life Transient microbiota: remain for a period of time, then disappear Human body: 1x1013 human cells 1x1014 bacterial cells!! 1cm2 skin = 3x106 bacteria Resident Microbiota Resident Microbiota  Axenic sites Axenic environment (Ax = no, xenis = foreigner) – Sterile sites, devoid of “foreigners” – Examples: In utero Kidney Prostate Middle and inner ear Other internal sites  Normal Flora Can Become Opportunistic Pathogens 1. Immune suppression: – Weakened immune defences allow infection by organisms that wouldn’t generally be able to cause infection in healthy host – AIDS patients more susceptible to Pneumocystis jirovecii, systemic C. albicans 2. Location change for microbiota: – Introduction of member of normal flora to an unusual site in body – eg. Gut E. coli entering urethra causing UTI 3. Changes in the relative number of normal flora: – microbial antagonism or competition usually prevents this overgrowth of one type of normal flora (bacteriocins, pH balance, O2 availability) – altered number may allow one to thrive & cause disease – eg. E. coli produce bacteriocins to inhibit growth of others, C. difficile overgrow after aggressive antibiotics, C. albicans overgrow after antibacterial treatment  How Microorganisms Enter a Host Portal of entry: route by which the pathogen gains access to the body some microorganisms have a “preferred” portal of entry and can only cause disease via specific route – Neisseria gonorrhoeae can adhere to cervical epithelial cells, but not vaginal epithelial cells  Mucous Membranes Mucous Membrane: membranes that line body openings providing moist, warm environment hospitable to pathogens – respiratory tract is the most common site of entry through the nose or mouth – GI tract is another route- some pathogens are able to survive the acidic pH of the stomach – genitourinary tract (UTI, STI) – Conjunctiva (many viruses enter the the respiratory via the eyes)  Skin One of largest organs in body Outer layer of packed, dead, skin cells usually acts as barrier – Unbroken skin very good defense against invading pathogens – some fungi grow on skin Some pathogens enter through openings (sweat glands, hair follicles) Others enter by burrowing into or digesting the outer layers of skin (hookworm larvae, Schistosomes)  Placenta Nutrients and wastes diffuse between the vessels of the developing child and the mother, the two bloods do not contact each other. The placenta forms an effective barrier to most pathogens Some pathogens which cross the placenta and infect the fetus can cause: – Spontaneous abortion – Birth defects – Premature birth Pathogens that cross the placenta HIGH POSSIBILITY Bacteria - T pallidum - Listeria Virus - CMV (DNA) - HIV (RNA) - Rubella (RNA) - B19 (DNA) -Hepatitis B Protozoa - Toxoplasma gondii  Parenteral Route organisms gain access via broken skin – injections, punctures – bites – cuts, wounds – surgery – cracking/splitting of (dry) skin  Measures of Pathogenicity if too many microorganisms enter body at the same time, host defense may become overwhelmed LD50: lethal dose – # microbes that kill 50% of test animals ID50: infectious dose – # microbes required to produce demonstrable infection in 50% of test animals These values are NOT absolute, they are relative comparisons of experimental results Examples of Infectivity: expressed as ID50 – Legionella ID50 = 1 cell most virulent – Salmonella ID50 = 105 cells – Shigella ID50 = 100,000 deaths per year in USA – may be opportunistic infections affecting seriously ill patients – infections w/ Methicillin-Resistant Staphylococcus aureus (MRSA), may occur because of special nature of hospital environment  Common Nosocomial Infections Urinary tract infection (UTI): most common nosocomial infection – urethral catheters, bypass normal bladder “flushing” Lower respiratory tract infection (LRTI) – bronchitis, tracheitis, pneumonia, from endotracheal tubes, sedation Surgical wound infection – foreign material (joint replacement), prolonged procedures, tissue damage Bloodstream infection – considerable morbidity, mortality, develop from iv therapy (skin flora)  Relative Frequency & Common Microorganisms of Nosocomial Infections  Epidemiology Epidemiology: study of the occurrence, distribution, & spread of disease – Etiology of disease: causative agent? – mode of transmission? – geographical distribution? Important factors to consider and patterns to identify: – age, sex, occupation – personal habits; socioeconomic status – history of immunization – other diseases – common history of affected individuals – initial contact point – timing (seasonal?)  Occurrence of Disease: definitions incidence: number of new cases over a given time – indicator of disease spread prevalence: number of cases at a single defined point in time – indicator of how seriously & how long a disease affects a population sporadic disease: only present occasionally – typhoid fever in USA endemic disease: always present – common cold epidemic disease: many cases in a short time – influenza pandemic disease: worldwide epidemic – >35 million people living with HIV