Chapter 14: Infection, Infectious Disease, and EpidemiologyWhy is this important? • Beginning of discussion of how microbes affect our bodies• Microbes are simply living their lives – undergoing metabolism, binary fission, etc. – and are not on a mission to harm humansSymbiotic Relationships Between Microbes and Their Hosts• Symbiosis means “to live together”– We have symbiotic relationships with countless microorganisms that live in and on our bodiesTypes of Symbiosis- Mutualism: bacteria in human colon: E. coli: GI tract is a great habitat for the organisms and the E. Coli produces our vitamin K for us (we cant make vitamin K by ourselves)- Commensalism: ongoing gray area; the 2nd organism may receive a benefit notdiscovered yet - Parasitism: one organism gets a place to live at the expense of the host; ebola,common coldNormal Microbiota in Hosts• Although many parts of your body are axenic other parts shelter millions of mutualistic and commensalistic symbionts• Normal microbiota – the microbes that colonize the surfaces (external and internal) of the body without normally causing disease– Also termed normal flora and indigenous microbiota• Two types– Resident flora– Transient floraResident Flora• Born with this flora • Remain a part of the normal microbiota of a person throughout life• Most are commensal– Feed on excreted cellular wastes and dead cells without causing harm• Exact combination of species and size may vary somewhat, but resident flora are characteristic and permanent• Know locations and read notes: don’t need to know the species • Organisms are usually well adapted for the region of your body that they reside inTransient Microbiota• Remain in the body for only a few hours, days, or months before disappearing• We are always colonized by them at some time in our life • Found in the same regions as resident members• Cannot persist in the body– Competition from other microorganisms– Elimination by the body’s defense cells– Chemical or physical changes in the body that dislodge themNormal  Opportunists  PathogensAcquisition of Normal Micrbiota• Start getting normal flora when you are born • Usually placenta is sterile • The womb is generally an axenic environment• Microbiota begins to develop during the birthing process• Coming out of the birth canal and getting handled by the nurses, etc. • Much of one’s resident microbiota is established during the first months of lifeHow Normal Micrbiota Become Oppurtunistic Pathogens• Pathogens: organism that cause disease • Opportunistic pathogens are normal microbiota or other normally harmless microbes that can cause disease under certain circumstances• Can cause disease if given the opportunity• 40% of poop is bacteria • If e. coli gets into the urethra it will cause infection, but it is okay in the intestines • Conditions that provide opportunities for pathogens – Introduction of normal microbiota into unusual site in the body– Immune suppression – disease, malnutrition, emotional or physical stress, extremes of age, radiation or chemotherapy, or immunosuppressive drugs– Changes in the normal microbiota– Microbial antagonism – normal flora use nutrients, take up space, and release toxic waste products; make it less likely that other microbes can compete well enough to become established– Changes in relative abundance of normal microbiota may allowone member of the normal microbiota to become an opportunistic pathogen (antimicrobial treatment-more of onenormal flora than the other, hormonal changes, stress, changes in diet, exposure to large numbers of pathogens)Capabilities of a Pathogen• Maintain a reservoir• Leave the reservoir (transmission) • Enter a host through a portal of entry (infection) and adhere to the surface of the host• Evade defenses, multiply, and affect body function• Leave the body and return to its reservoir or enter a new hostReservoirs of Infectious Diseases of Humans• Reservoir: where the pathogen stays most of the time- come out to infect • Most pathogens cannot survive for long outside of their host• Sites where pathogens are maintained as a source of infection are termed reservoirs of infection • Three types of reservoirs– Animal reservoir– Human reservoirs– Humans with active disease– Carriers– Nonliving reservoirAnimal Reservoirs• Many pathogens that infect domesticated or wild animals also infect humans– The more similar the animal’s physiology to that of humans, the more likely the pathogen is to infect humans• Gorillas foe example • Zoonoses – diseases that are naturally spread from their usual animal host to humans• Acquire zoonoses through various routes– Direct contact with animals or their wastes– Eating animals: especially if the animals are uncooked – Bloodsucking arthropods: like mosquitos• Human infections with zoonoses are difficult to eradicate because of the sheer number and types of reservoirs • Humans generally do not serve as reservoirs for reinfection of animals– Generally we do not transfer the disease back to a gorilla Human Carriers• Humans with active disease• Carriers – people with no obvious symptoms before or after an obvious disease may be infective and some may be asymptomatic (never experience signs and symptoms) and infective for years– Some individuals will eventually develop illness – Others remain a continued source of infection without ever becoming sick– Healthy carriers may have defensive systems that protect them from illnessNonliving Reservoirs• Soil, water, and food can be reservoirs of infection– Presence of microorganisms is often due to contamination by feces or urine– Food: water that lettuce was washed in and was contaminated Modes of Infectious Disease Transmission• Transmission is either from a reservoir or a portal of exit to another host’s portal of entry• Three basic modes of transmissionContact Transmission• Direct contact transmission – typically involves body contact between hosts– Physically touching – Includes person-to-person spread– Touching, kissing, sexual intercourse, biting, scratching– Transfer from mother to fetus– Self-inoculation• Indirect contact transmission – pathogens spread from one host to another by fomites (inanimate objects that are inadvertently used to transfer pathogens to new hosts; not a reservoir-they don’t get any