Bacteria (single celled, prokaryotic) Only small number are pathogens Asexually reproduction by binary fission / 2 identical daughter cells Grow best at optimum conditions (human body) Constant temperature Neutral pH Constant supply of food, H2O, O2 Mechanism removing waste Most bacteria are aerobic / obligated aerobes Aerobic bacteria growing with absent oxygen / facultative anaerobes Bacteria which find oxygen toxic / obligate anaerobes Sigmoid growth curve shows the number of bacteria plotted against time Bacteria MUST grow in closed system and nutrient medium → BACTERIAL CULTURE Population grows slowly - LAG PHASE Rapid increase of population growth - LOG PHASE Reaches equilibrium when number remains constant - STATIONARY PHASE Lag Phase → initial phase Low number of reproducing organisms Bacteria increase in size before division This requires nutrients which need to be digested Digestion requires enzymes, proteins → activation of genes → time consuming process Log Phase → exponential phase; max growth rate; steep curve Optimum conditions: no limiting factors, waste does not accumulate to a toxic level Bacteria most susceptible due to production of new cells Antibiotics inhibit cell wall formation Antibiotics inhibit DNA replication Antibiotics inhibit protein synthesis Stationary Phase → reduced growth rate New cell production balanced by death of cells Limiting factors, declining nutrients, accumulating waste influence population size Decline Phase → bacteria stop dividing Death rate increases; numbers may fall to zero Lack of nutrients, build up of toxic waste products Aseptic conditions Sterilise equipment, instruments, thus, to prevent contamination with the culture. Use high temp and disinfectants Total cell count Number of cells whether living or dead Count cells with haemocytometer Stop bacteria entering the flask with a stopper. Important as bacteria would have been caused reduced growth rate of yeast/killed yeast and competed for space/nutrients Culture is shaken to achieve a uniform distribution of yeast cells/spread out yeast Larger number is taken to avoid anomalies/produce an average Measurement of growth Generation time: time taken for a bacterial population to double Rate of population = increase in number of yeast cells/time = number larger - number less/(time larger - time less) Suppose the number of cells in one square are 6 8 9 5 7 The sum of the cells in 5 squares is 35 The mean for one type B square is 7 Therefore 25 squares have 25 x 7 = 175 in 0.1mm3 In 1mm3 there will be 1750 cells or 1,8 x 103 Viable cell count Only living cells since these are the only ones capable of dividing 1cm3 original sample is diluted in 9cm3 distilled H2O Mix 1cm3 from last dilution with 9cm3 distilled H2O - serial dilution 1cm3 of each dilution is put on an agar plate and counted. Number is multiplied by the dilution factor Measurement of growth Number of colonies on the 10-3 dilution plate = 35 Number of viable colonies in 1cm3 of 10-4 dilution of milk 35 x 1/0.1 = 350 Sample was diluted by 10-3 Number of bacteria in 1cm3 of the original sample = 350 x 103 Biological factors Bacteria are effected in growing by nutrients: C, H, N, P, S Temp: low → low speed of enzyme reactions; high → denaturisation of enzymes pH → tolerate a wider range of pH than plant and animal cells O2 → some grow better in presence, but some grow in absence Koch's Postulates1 A list of postulates (criteria) must be fulfilled to proof an infective cause for a disease "Organism must be sufficiently abundant in every case to account for the disease Organism associated with the disease can be cultivated artificially in pure culture Cultivated organism produces the disease upon inoculation into another member of the same species Antibodies to the organism appear during the course of the disease"1 Exceptions are possible Number of organism causing disease might be very low (eg tuberculosis) Cultivation might be difficult Animals must be used as it is unethical to infect a human with a causative organism Antibodies may not appear if the immune system is inhibited Entry of Microorganisms (Pathogens) into the Body Damaged skin Skin acts as a barrier to infections Tetanus occurs when the bacterium Clostridium tetani enters a wound Mucus membrane of respiratory tract Air containing droplet of infectious material are breathed in Mycobacterium tuberculosis causes tuberculosis Digestive track Vibrio cholerae causes cholera when drinking water infected with faeces Salmonella enteritis causes food poisoning when eating undercooked food These organisms are resistant to acidic conditions in the stomach Acid protects against microorganisms by providing a hostile environment Others Transmission by vectors (e.g. malaria via Plasmodium parasite when mosquito vector takes blood) Direct entry through the intact skin (e.g. Schistosomiasis where the larval stage schistosome burrows through the skin of the feet) Pathogenesis: How Microorganisms Cause Disease Damage or destroy host cells - e.g. HIV, Salmonella Organism is taken up by epithelial cells in the intestine HOST SPECIFIC: ligand on pathogen must fit onto receptor proteins on host Some hosts are more susceptible than others because proteins depend on gene coding Destroy brush border of microvilli Host creates a ruffled surface / Invaded cells detach from intestinal wall, creating inflamed lesions / Secretion of large amounts of watery fluid into the lumen of the gut → diarrhoea Produce toxic waste - e.g. Vibrio cholerae Are harmless but produce harmful "exotoxins" - toxins released from the cell Causes loss of chloride and hydrogencarbonate ions from the intestinal cells Osmotic loss of up to 10 litres of water per day Impaired absorption of water and salt from the gut This explains severe watery diarrhoea and death from dehydration Body's own immune response to the presence of microorganisms which produce the symptom e.g. Mycobacterium tuberculosis Body tries do destroy the invading bacteria This causes inflammation and damage to the surrounding cells occur Lesions may become hard or spongy, leaving "holes" in the lungs, sometimes damaging blood vessels Some bacteria will cause all of the 3 ways above; Some require a large number of bacteria for a disease; Some will only a few number of bacteria Microorganisms may enter the lymphatic system via tissue fluid and are carried around the body in this way Ability of bacteria to cause disease relies on Location - what tissue is colonised Infectivity - how easily a bacterium can enter the host cell Invasiveness - how easily a bacterium or its toxin spreads within the body Pathogenicity - how a bacterium cause disease Tuberculosis (Myobacterium tuberculosis) Lung most common infected organ Inhaling droplets exhaled from a carrier during coughing causes the infection Latent period: bacteria may lie inactive for up to 30 years and become active as primary tuberculosis (TB) Symptoms are fever, loss of weight and persistent coughing Bacteria destroy lung tissue and cause accumulation of fluid in the pleural cavity Coughing up blood is common because bacteria destroy lung tissue Treatment Antibiotics for a six-month period to ensure eradication Combinations of drugs prevent development of resistant strains Vaccine is a live attenuated strain of TB Salmonella Symptoms: typhoid fever, intestinal infections Food poisoning caused from uncooked poultry, beef, and eggs "Salmonella enters the body in contaminated food/drink The bacterium passes from the esophagus, through the stomach, into the intestine It enters cells lining the small intestine to multiply population increases Some bacteria die and release an endotoxin This causes (symptoms) diarrhoea, vomiting, nausea, abdominal pain (food poisoning)"1