PPY 400: Unit 3 Infection - Past Slides PDF

PPY 400: Unit 3 *Infection Chapter 6 Infection Infections are caused by microorganisms, called pathogens that gain entry into the body Major types of pathogens: bacteria, viruses, fungi and parasites Transmission of infection involves a chain of events that must occur...

PPY 400: Unit 3 *Infection Chapter 6 Infection Infections are caused by microorganisms, called pathogens that gain entry into the body Major types of pathogens: bacteria, viruses, fungi and parasites Transmission of infection involves a chain of events that must occur unbroken to allow one human to infect another See Fig. 8-1 Transmission of Infection pg. 161 Pathogen must live and reproduce in a reservoir (human for influenza virus, animal for rabies etc.) Must have a portal of exit and mode of transmission (ex. semen in sexual contract) from the reservoir to a susceptible victim Direct vs. Indirect Transmission Transmission can be direct (contact is made between the reservoir and the new host) or indirect (the reservoir deposits the organism on an inanimate object (a fomite), which then transmits the pathogen to a new host Direct contact can be physical, fecal contamination, or airborne droplets Transmission of Infection The pathogen gains entry to the victim through a “portal” (nasal cavity, skin puncture) If the host is susceptible to the pathogen, it will grow in numbers to overwhelm the host’s immune system and infection occurs Then the cycle repeats itself (the susceptible victim/host becoming the reservoir Factors that make individuals susceptible: age, gender, genetics, immune system state, stress level etc.  “colonization” occurs if a balance between the host and microorganism is reached to control the location and total number of the pathogen These microorganisms will not cause infectious disease, but can be passed on to others Staphylococcus aureus is found in the nasal passages of 25-30% of the population  can cause disease if transmitted to immunocompromised individuals. Control of Disease Breaking the chain of transmission can occur epidemiology studies aim to identify vulnerabilities in the links of the transmission chain For example, rabies vaccinations help to eliminate one reservoir of potential rabies transmissions Blocking the portal of exit can stop transmission  having patients with TB wear masks The Host The human body has a number of mechanical and biochemical barriers Epithelial barriers physically block foreign material from entering the body Epithelial cells of the skin provide multiple layers of protection especially if dry (microorganisms prefer moist) Constant shedding of the epidermis and mucous membranes aid in removal High fat content of the skin inhibits the growth of bacteria and fungi  Mucous membrane linings provide barriers separating the sterile internal body from the external environment (ex. Lungs are lined with a layer of mucous that traps and removes microorganisms) Chemical barriers enhance the effectiveness of mechanical barriers  acidic nature of skin, urine and the vagina inhibit bacterial growth Hydrochloric acid makes the pH of the stomach between 1 and 2  inhospitable to many microorganisms Saliva, mucous, tears and sweat contain antimicrobial chemicals such as lysozyme (destroys bacterial cell walls)  Lactoferrin is a protein present in body fluids like saliva, tears and mucous  reduces the availability of free iron needed for bacterial replication Sebaceous gland secretions act as antifungals Immunoglobins (IgA and IgG) prevent entry of bacteria and viruses through mucous membranes Barriers (Fig. 8-3) Infection If these barriers are removed or degraded, infection becomes very likely Burn victims that are missing skin are at high risk of infection If a urinary catheter is in place, urine no longer flushes bacteria from the urinary tract opening Immunization Immunization of a population is the most cost- effective method of reducing susceptibility to a pathogen Goals of immunization: 1. Give immunity to a host by direct exposure to the altered (dead or weakened) pathogen 2. Decrease the number of susceptible hosts in the population; thus limiting disease transmission *The concept of “herd immunity”  disease can be controlled or eliminated without immunizing everyone, as long as a high enough percentage of the population is immunized Immunization Immunization exposes the potential host to altered bacteria, viruses, or their derivatives, which stimulates the body to produce antibodies against these pathogens without causing the disease Once these antibodies exist, the body can destroy the pathogen before it causes disease  Environmental Effects Environmental factors (weather, population etc) affect the prevalence and transmission of infections Parasitic infections occur more often in hot and humid climates and in areas of over-population Infections may be transmitted by inhalation of polluted dust or air Fecal contamination of meat and vegetables by improper cleaning or slaughtering techniques lead to infections Salmonella contamination of poultry during slaughter and dressing of meat, coupled with improper preparation and incomplete cooking  very common! Environmental Effects Some infections have seasonal patterns  getting a cold in the winter, septic meningitis tends to occur in the summer or fall Some infections occur only in certain regions of the world Sanitation, air quality, living conditions are also important factors Host-Parasite Relationship (pg. 166) relationship – the interactions Host-parasite between a host and the microorganisms that reside on or in it Normally, large numbers of microorganisms reside on the skin, in the GI tract and in the vagina of the human host  cause no harm These are considered “normal parasitic flora” The host and flora co-exist and benefit one another  symbiotic relationship  Normal flora Certain bacteria have a role in protecting the host from other pathogens and other metabolic processes like synthesizing vitamin K in the colon Remember: harmless inhabitation of the skin or mucous membranes by these microorganisms is called “colonization” Resident floras – specific types of microorganisms that occupy certain environments on or in their host Ex. Corynebacterium, Staphylococcus epidermidis, Streptococcus viridans, fungi, yeasts, and mycobacteria are normal resident floras of the skin Opportunistic Infection Opportunistic infections occur when normal floras and/or microorganisms unlikely to cause infection become pathogenic  occurs when the immune system is compromised The use of steroids depresses the immune system and increases the chance of opportunistic infections  Microorganism Certain factors influence the microorganism’s Characteristics ability to infect a host: virulence, toxin production, microbial adherence and invasiveness Microbial adherence – the ability of the microorganism to latch onto and gain entrance into the host Pathogens need to directly penetrate or stick to a host’s tissue surface Adhesion molecules on the microorganism cell surface help them adhere to the host epithelial cells  Microbial development of a slime layer facilitates adherence  typically made of mucopolysaccharide by enzymes on the bacteria surface “Glycocalyx” (slime layer) acts as glue or adhesive that sticks the bacteria to the host surfaces Streptococcus mutans produces a slime layer that allows it to stick to tooth enamel and form plaque.  Virulence – the ability of a microorganism to evade host defences and cause disease The more virulent the microorgansim = the more likely to cause disease = the more harm caused to the host Virulence and invasiveness factors contribute to the pathogenicity of the microorganism by enabling it to penetrate natural barriers, resist death by phagocytosis, or survive antimicrobial therapy  Virulence and Invasiveness factors Bacterial enzymes (fibrinolysin, coagulase, hyaluronidase) aid in the microorganism’s ability to spread, or invade tissues These enzymes dissolve or hydrolyse blood, collagen, or connective tissue components Fibrinolysin – produced by streptococci and dissolves coagulated plasma and allows microbial spread into host tissues Hyaluronidase – secreted by staphylococci, clostridia, streptococci and other organisms and helps microbes to spread by dissolving a connective tissue component called hyaluronic acid.  Encapsulation – prevents recognition and binding of antibodies  prevents phagocytosis The bacterial slime layer also prevents phagocytosis Endospore Formation – some microorganisms can survive harsh environments by producing endospores An endospore is a temporary resting/dormant cell that can be regenerated when favourable conditions return Endospore state is triggered by bacterial genes when adequate nutrition is not possible Endospores are highly resistant to heat, lack of moisture and chemicals  Microorganisms that are able to mutate in response to changes in the host’s environment in order to successfully infect the host have “antimicrobial resistance”. Staphylococcus aureus has become resistant to antibiotics as a result of successful mutation Note: Not taking your full course of antibiotics leads to antibiotic resistance. Microorganisms that can survive a low-dose antibiotic become the dominant species  this resistant form of bacteria is spread and becomes common in the general population  Viruses have also developed resistances to antiviral medications  requirement for multi-drug “cocktails” Bacteria with “flagella” allows them to move or swim towards nutrients , usually sugar or amino acid concentrations Toxins Some microorganisms produce toxins that affect the host’s cells Exotoxins – polypeptide toxins that are produced and released by the organism; cause immediate immune reaction, are highly toxic and unstable when exposed to heat -exotoxins cause illness and death by binding to specific receptors in target organs and by interfering with vital metabolic pathways Exotoxins For example, C.Tetani produces an exotoxin called tetanospasmin, a powerful neurotoxin that interferes with synaptic transmission of inhibitory interneurons in the spinal cord  leads to tetany (involuntary contraction of muscles), muscle spasms, “lock jaw” and seizures  Endotoxins – associated with Gram- negative bacteria and are the common causative agents of septic shock An endotoxin is a part of a bacterial cell wall that triggers a massive immune response when the bacterium lyses/breaks down. The immune response leads to shock and multiple organ failure  Types of Pathogens (pgs 168- 179) Bacteria – single-celled organisms that have no internal organelles Surprisingly, only a small percentage of bacteria are known to be harmful to humans Bacteria are classified into 4 groups (based on cell wall shape and movement mechanism): 1. Gliding bacteria 2. Spirochetes 3. Mycoplasmas 4. Rigid bacteria  Free-living bacteria (pg 168) Rigid bacteria include free-living bacteria Streptococcus  strep throat Three basic shapes of free-living bacteria: 1. Cocci – round and nonmotile, may stick together in clumps (staphylococci), in pairs (diplococci), or in long strands (streptococci) 2. Bacilli – rod shaped; approx. half are motile 3. Spiral – cork-screw shaped *See figure 8-6 on page 170 Gram Stain A gram stain reaction is based on whether or not a bacteria can retain a basic dye after iodine fixation Gram-positive organisms look dark purple under a microscope Gram-negative organisms look pink, resist staining  How bacteria infects a host Once bacteria have penetrated the initial defense mechanisms, they will multiply and create a colony The body tries to fight the infection with an acute inflammatory response  neutrophils and macrophages ingest and destroy the microorganisms If this doesn’t work, bacteria move throughout the body via blood, interstitial fluids and lymph Bacteria eventually move to the lymph nodes where they stimulate an immune response If they overwhelm the lymph nodes, clumps of bacteria can cause bacteremia and worse, sepsis, hypotension, organ failure and death More types of Pathogens Viruses – smallest known infective agent (20-300nm), composed of DNA or RNA and a capsule surrounding the genetic material Only contain enough genetic info to code for 2-60 proteins and thus are dependent on the host cell for energy and the ability to replicate How viruses infect All viruses must enter a host cell to replicate A virus can adhere to the cell membrane and cause phagocytosis  once inside the virus capsule is removed to expose the viral genetic material to the cel l’s interior Some viruses will stick to the surface of the cell and inject genetic info into the host cell Some viruses will manufacture an envelope made of host cell membrane and viral proteins that surround the viral capsid The virus kills the host cell by continually breaking down the cell’s maintenance mechanisms with build- up of waste and more virus! Two categories of viruses Viruses can contain RNA (retroviruses and straight replicating viruses) or DNA DNA viruses (ex. herpes simplex virus) – first produce messenger RNA within this host cell’s nucleus, viral proteins are made and the DNA of the virus is replicated by the host cell enzymes  new DNA viruses are made within the host cell 2. RNA viruses (ex. Influenza) – have the ability to create both messenger RNA and DNA from their own genome  viral DNA is incorporated into the host cell DNA. When the host cells DNA replicates, so does the viral DNA! *most viral infections are self-limited and few antiviral agents exist to treat them More types of pathogens Fungi – eukaryotic protists that can reproduce by simply dividing or by combining their genetic information before dividing Infections caused by fungus are called mycotic infections or mycoses. Fungi first colonize in an area by adhering to and proliferating on the site of infection Fungi such as candida live naturally in the body as normal flora  when host’s natural immune system is compromised, overgrowth can occur causing infection  Patients on antibiotics lose some of their normal flora along with their targeted pathogen  more susceptible to overgrowth of fungi  fungal infection Very common in the urinary tract of women and in diabetics due to the high amount of nutrients Topical antifungal drugs can be used to treat superficial infection Types of pathogens Parasites – protozoa, roundworms, flatworms and arthropoda, live on or in the human body, depend on host for shelter and sustenance Symptoms of a parasitic infection depend on the location: protozoan infection of the GI tract produces cramping, abdominal pain and diarrhea Identification of the infection agent is usually by visualizing the adult parasite or its ova or by microscope examination of blood, feces or tissue samples