1 Host-Microbe Relationships and Disease Classical ecological definitions –Mutualism: organisms live together, both benefit E. coli in GI tract; we get Vitamin K, protection from pathogens, it gets warm wet place to live, lots to eat. –Commensalism: one organism benefits, the other is not particularly benefited or harmed; to eat at the same table Most GI tract microbes; we provide a warm wet place to live with food, we don’t get all that much in return. –Parasitism: one organism benefits at the other’s expense Disease-causing bacteria; to them, we’re dinner. Classically, a “parasite” lives in or on host.

2 Terms, terms, and more terms Contamination: presence of microbes (where they don’t belong). Infection: multiplication of parasitic organisms in/on host. –Infestation: used to describe larger organisms, e.g. lice. Disease: malfunction in or damage to the host. –Many kinds of “disease”; here we discuss “infectious disease”. –Disease is a condition of the host, not an infectious microbe. Pathogen: a parasite capable of causing disease –Not all pathogens are equal as we will see.. Pathogenicity: ability of pathogen to cause disease

3 Terms, terms, and more terms-2 Virulence: relative ability to cause disease. –Especially variations in pathogenicity within a specific group –Virulence can be weakened (attenuation) or increased (animal passage); Growing pathogens on agar attenuates them. Normal microbiota: the microbes normally found on the body. Since people are not “normally” sick, pathogens are not normally consider “normal microbiota”. –The term “flora” is to be avoided as microbes are NOT plants! Resident microbiota: always found on human tissues. Transient microbiota: come and go, can include pathogens.

4 Where do they live? Microbes live where it is “topologically outside” –We are a tube within a tube. We have sacs open to the outside. Respiratory tract: –nasal passages, sinuses, trachea, lungs. Lungs well protected, other areas more populated. GI tract: Crowded! –Mouth is full, fewer in esophagus and stomach; toward end of small intestine, numbers increase greatly. –Feces consist largely of bacteria.

5 Where do they live?-2 Skin: largest organ in the body. –Colonized. Various factors keep the numbers down. Genito-urinary tract: –Female reproductive tract colonized, especially with Lactobacillus which is helpful and with yeast which is occasionally not helpful. –Lower portion of urethra contains some bacteria, but bladder, ureters, and kidneys normally sterile. All areas of the body have mechanisms for keeping us from being lunch; to be discussed soon.

6 What determines whether we get sick? Inf Dis: the likelihood of contracting an infectious disease. N: the numbers of infecting organisms. V: the virulence of the organism. HF: host factors, including overall health, nutritional status, genetic background, age, immune status.

7 How dangerous? Pathogen: causes disease. Opportunistic pathogen: can cause disease under the right circumstances –Dose in high numbers –Host is in a weakened state, e.g. HIV infection. –Organism gets where it doesn’t belong E. coli and urinary tract infections. –Lack of microbial antagonism, e.g. superinfection competition for space, nutrients; bacteriocins. Saprobe/saprophyte: decompose dead stuff.

8 Whether an organism will cause disease is not always a clear cut thing Not everything in biology can be neatly classified. There is a gradation from pathogen to opportunist to non-infectious, and what happens depends on the balance of these 3 factors.

9 Types of disease Inherited diseases: caused by a faulty gene –but susceptibility to microbes can be genetic too. Congenital: due to damage during development. –but microbes can be involved in this damage. Degenerative diseases, due to age or lifestyle –but microbes can be involved in some as well. Nutritional, endocrine, mental, immunological, neoplastic (cancer), idiopathic; same caveat. Iatrogenic: caused by doctor. –Nosocomial infections: occur in hospital.

10 Types of infectious diseases Communicable: can be spread from one person to another. –Example: tuberculosis, HIV Contagious: highly communicable, can easily be spread from one person to another. –Genital herpes, measles. Non-communicable: are not spread from one host to another. –Examples: your infected appendix bursts –You get tetanus from “rusty nail”