1LECTURE 22 Genetics (cont.) & EpidemiologyTable 8.3. Summary of last lectureMovement of DNA within cells e.g. transposonsTransposition involves small segments of DNA (transposons, "jumping genes") that can move around chromosomes or plasmids.e.g. F-plasmids have insertion sequences that allow the plasmid to integrate into the chromosome => Hfr cell. Insertion sequencesare very simple and typically contain only the information needed for insertion(see figure 8.24). Also used in experiments to disrupt specific genes.Transposons (Composite Transposon in Figure 8.24) are more complex and may contain a number of genes that confer e.g. antibiotic resistance and/or toxin production (see next slide). Fig. 8.24 a. Insertion sequence, b. Composite transposon containing an antibiotic resistance gene…. Fig. 8.23. Movement of a transposon through a bacterial community.Epidemiology - the study of the factors influencing the frequency and distribution of diseases.Combines:microbiologyecologysociologystatisticspsychologyMajor goal of epidemiology:When a disease breaks out, epidemiologists try to determine root cause (or etiology), and determine the most effective measures for preventing or controlling the disease. Transmission routes and corresponding safety measures:1. animal bites - control animal populations or avoid contact2. personal contact - isolation or treatment of infected persons, immunization3. Ingestion - water quality and food regulations4. sexual contact - public education regarding "safe sex" and individual treatmentMostly about terminology…2Communicable diseases - can be transmitted from one person to another (as opposed to genetically inherited diseases or diseases such as those caused by toxins in food)Prevalence rate = total number of infected individuals in a populationDefinitions, “rates”Incidence = # of new cases in a population per time periodMorbidity ratenumber of cases per unit timenumber of individuals in the population at risk=Mortality ratenumber of deaths due to the diseasenumber of individuals with the disease=Terms to describe prevalence:Endemic - disease is constantly presente.g. common cold or flu see fig. 20.2Epidemic - unusually large number of cases in a given areaPandemic - a wordwide epidemic e.g. AIDSFig. 20.2. Weekly influenza and pneumonia mortality as a % of all deaths for 121 cities in the USA. Note the normal seasonal cycles and an occasional outbreak or epidemic.Reservoir - source of disease-producing organismAffects extent & distribution of diseaseCan be humans, animals, or environmental.3Infected humans are most sign. reservoirs of communicable diseases.• symptomatic -show obvious signs of the disease• asymptomatic carrier - an infected individual who is a potential source of infection.asymptomatic carriers, e.g.50% of woman infected with Neisseria gonorrhoeae have no symptoms.60% of the population carry Staphylococcus aureus during some part of their lives. 20% are chronic carriers.The Most (in)famous Carrier:Mary Mallon - “Typhoid Mary”She was a chronic carrier of Salmonella typhiin the early 1900s and worked as a cook all over NYC. She was arrested and imprisoned until she promised not to ever handle the food of others again. She broke that promise and escaped authorities for 5 more years, causing numerous deaths. She was re-captured and imprisioned for life!2-5% of typhoid victims are chronic, asymptomatic carriers….Animal reservoirsPoultry - Salmonella, CampylobacterRodents - plague, hantavirusRaccoons, bats, skunks - rabiesZoonoses -diseases that primarily exist in other animals, but can be transmitted to humans.Environmental ReservoirSome pathogens, e.g. Costridium tetani and C.botulinum are widespread in soils.Vector - an organism that spreads a disease from one host to another.e.g.Mosquitos, Ticks, Fleas, LiceAn example using some of these terms: (read pages 723-726) Plague (black death): Yersinia pestis (Gamma Proteobacteria, Enterobacteriaceae) infection is endemic in prairie dogs and other wild rodents (reservoirs). Fleas (vectors) on the wild rodents can transfer plague to the occasional human. This results in sporadic occurrence of "wild" plague or "sylvatic" plague (“cat case” in Colorado). Of greater threat is when fleas pass Y. pestis on to domestic rats, that can then pass (via fleas) Y. pestis on to large numbers of humans. This can result in an epidemic (India, 1994) or even a pandemic (middle ages). Y. pestis infects lymph nodes to make bubos, hence "bubonic" plague. Subcutaneous hemorrhaging makes dark areas, hence "Black Death". Because bubonic plague originates in a rodent, bubonic plague is considered a zoonosis.Yersinia pestis in lungs results in pneumonic plague (pneumon=Gk. for "lung"). Pneumonic plague can spread through the air or via direct contact.Mortality rate = 50 - 80%, almost always fatal in a few days if left untreated.4Fig. 28.6. Cells of Yersinia pestis have safety pin appearance when isolated from lymph nodes. Fig. 28.7. Fleas after a blood meala. Healthy Flea b. Flea infected with Y. pestis. Note intestinal blockage……. Results in very hungry flea (therefore bites over and over) and flea that vomits Y. pestiscells. Table 28.4. Plasmid and chromosomally borne virulence factors of Y. pestis. Note that F1 is only expressed in mammals (37°C) and not in fleas (26°C).Other modes of disease transmission:ContactDirect contact - one person touches another• handshake• sexual intercourse• fecal-oral transmissionMost pathogens transmitted this way cannot survive for long in the environment.Indirect Contact - transfer of a pathogen via an object or fomite.Droplet transmission - respiratory droplets with microbes can spread from person to person in close proximityAIR> Mucus droplets are emitted when people talk, laugh, or sneezeFood & Water• Many pathogens that infect the gastrointestinal tract use this mode.• Foods can become contaminated:> Directly in the food> During food preparationCross-contamination - one food to another> Improper storagePopulation Characteristics that affect epidemiology1. Immunity - a disease needs a populaiton of susceptible peopleHerd immunity - a critical concentration of immune hosts prevents the spread of a disease2. General HealthMalnutrition and crowding can make people more susceptible to disease.3. Age, gender, genetic background etc.5Epidemiological StudiesDescriptive studies - characteristics of