Bacteria And Disease Flashcards
What are pathogens?
Microorganisms that cause disease
What does it mean to culture microorganisms?
Microorganisms are provided with the nutrients, level of oxygen, pH and temperature that they require to grow in large numbers so they can be observed and measured
Why is it important to take care when culturing microorganisms?
- even if the microorganism you are culturing is completely harmless there is always the risk of a mutant strain arising that may be pathogenic
- there is a risk of contamination of the culture by pathogenic microorganisms from the environment
- when you grow a pure strain of a microorganism the entry of any other microorganisms from the air or your skin into the culture will contaminate it
What does sterile mean?
It is a term ised to describe something that is free from living microorganisms and their spores
What are the precautions you must take when culturing?
- All of the equipment must be sterile before tje culture is started
- once a culture has grown it must not leave the lab
- all cultures should be disposed of safely by sealing them in plastic bags and sterilising them at 121°c for 15 mins under high pressure before throwing them away
What is the nutrient medium?
A substance used for the culture of microorganisms, which can be in liquid form or in solid form
What is the nutrient broth?
A liquid nutrient for culturing microorganisms commonly used in flasks test tubes or bottles
What is nutrient agar?
A jelly extracted from seaweed and used as a solid nutrient for culturing microorganisms commonly used in petri dishes
Why is agar a useful jelly?
Because although it sets at 50°c it does melt until it is heated until 90°c
What is a selective medium?
A growth medium for microorganisms containing a very specific mixture of nutrients so only a particular type of microorganism will grow on it
What are selective medium good for?
Indentifying particular mutant strains of microorganisms, antibiotic resistant strains, and strains that are genetically modified
What is inoculation?
The process by which microorganisms are transferred into a culture medium under sterile conditions
How does inoculation take place?
- sterilise rhe inoculating loop by holding it in the Bunsen burner until it glows red hot and then leave to cool
- dip the sterilised loop in the suspension of the bacteria. Streak the loop across the surface of the agar, avoiding digging into the agar. Replacing the petri dish lif, tape closed and label. Turn dish upside down
How is the inoculating loop used?
By scraping off bacteria from onee solid media surface either into a liquid medium or streaking across another solid medium plate
How can an inoculating broth be used?
Make a suspension of the bacteria to be grown and mix a known volume with the sterile nutrient broth in the flask. The flask is then stoppered again as quickly as possible with cotton wool to prevent contamination from the air and clearly labelled. The flask is incubated at a suitable temperature and is often shaked to make sure the broth is aerated, allowing oxygen to the growing bacteria
What are ways of isolating the desired microorganism to get a pure culture?
- growing a culture under anaerobic conditions will ensure only anearobic bacteria will survive. Growing it under aerobic conditions will ensure only aerobic bacteria survive. This may not allow you to complete the seperation of microorganisms necesssry for a pure culture but it will reduce the variety
- the nutritional requirements of different organisms vary greatly. You can produce a medium that will favour the growth of the organism you wish to culture and inhibit the growth of others this allows you to identify the colony you want and then reinoculate it to produce a single pure culture.
- there are indicator media that cause certain types of bacteria to change colour
- we can only culture about 1% of the known species of bacteria
What are ways of producing a medium specific to a bacteria?
Controling the range of nutrients available or introducing selective growth inhibitors, antibiotics or antifungal chemicals
What is a haemocytometer?
A thick microscopic slide with a rectangular indentation and etched grid of lines that is used to count cells
How do you use a microscope and haemocytometer to count cells?
The sample of nutrient broth is diluted by half with an equal volume of trypan blue, a dye that stains dead cells blue so you can identify and count only the living cells. The cells are viewed using a microscope and counted.
Each corner of the haemocytometer has a square divided into 16 smaller squares. The number of cells in each of these four sets of 16 squares is usually counted and the mean calculated. The haemocytometer is calibrated so that the number of cells in one set of 16 squares equates to the number of cells x10’4 per cm3 of broth. By taking measurements at regular time intervals throughout the life of a bacterial colony, a pciture of changing cell numbers can be built up
What is turbidimetry?
A method of measuring the concentration of a substance by measuring the amount of light passing through it
What does turbid mean?
Turbid is a term used to describe something that is opaque or thick with suspended matter
How does turbidity take place?
As the numbers of bacterial cells in a culture increase it becomes more turbid. As a solution gets more turbid it absorbs more light and lets less pass through. A colourimeter measures how much light passes through the sample and therefore, indirectly how many microorganisms are present. A calibration curve is produced by growing a control culture and taking samples at regular time intervals. The turbity of rach sample is measured and a cell count using a haemocytometer is made for each sample. This gives us a relationshio between the turbidity of the culture and the number of bacterial cells present. Using this calibration curve we can measure the number of microorganisms simply using turbidimetry if we wanted to e.g. investiagte the effect of different conditions on the growth rate of the microorganism
What is dilation plating?
A method used to obtain a culture plate with a countable number of bacterial colonies
What is a total viable cell count?
A measure of the number of cells that are alige in a given volume of a culture
What is the technique of dilution plating based on?
The idea that each of the colonies on an agar plate has grown from a single viable organism on the plate
What is dilation plating used to find?
The total viable cell count
How does the technique of dilation plating work?
Often a solid mass of microbial growth is present after culturing and it is not possible to work out the individual colonies. The orignal culture is dilyted in stages until a point is reached where the colonies can be counted. If the number of colonues is multiplied by the dilution factor then a total viable cell count for the original sample can be determined. Because there are often two or more plates where counting individual cells is possible it is possible to reach a mean giving a reasonably accurate number of the cells in a sample
How can the accuracy of dilution plating be checked?
Using a haemocytometer to count the cells in the original culture
What is a sinole way to asses growth when culturing fungi and what can this be used to compare?
Measure the diameter of the patches of mycelium
This can be used to compare growth rates in different conditions
What is a good method to find the optimum temperature for the growth of a fungi?
Identical petri dishes containing indentical growth medium are inoculated with the same number of spores of a fungus. The petri dishes are cultured at different temperatures with several identical dishes grown at each temperature. After a set period of time the diameter of each fungal colony is calculated and the mean value for the diameter at each temperature is calculated. The temperature that has resulted in the largest mean diameter is the optimum temperature for growth.
Why is measuring the diameter of colonies of bacteria grown not a useful way to measure growth?
Because the microorganisms are so small the colony grows slower and so isn’t as easy to measure
What is a very effective way to discover the best array and concentration of nutrients ir the optimum pH at which to grow fungi?
Testing the dry mass of the organism
How is the technique of measuring the dry mass of an organism done?
Normally a liquid growth medium is used. Samples of broth can be removed at regular intervals and the fungi seperated from the liquid by centrifugation or filtering. The material is then dried thoroughly to the point that no more loss of mass is recorded e.g. in an oven overnight at 100°c. This gives a measure of the dry mass of biological material in a certain volume of the culture medium and an increase or decrease in the dry mass gives an indication of the increase of decrease in the mycelial mass. The conditions which produce the greatest dry mass of fungus are the optimum ones for growth
What is the time span between bacterial divisions known as?
Generation time
What are the two reasons exponential growth in a bacterial culture does not continue?
- a reduction in the amounts of nutrients available. At the start of the culture there are more than enough nutrients for all the microorganisms but as the numbers multiply exponentially in the log phase of growth the excess is used up. Unless fresh nutrients are added, the level of nutrients available will become insufficient to support further growth and reproduction and so will limit growth of the organism
- a build up of waste products. At the beginning of the growth cycle of a bacterial culture the waste products are mineral but as cell numbers rise the build up of toxic material is enough to inhibit further growth and even to poison and kill the culture. In particular as co2 produced bu the respiration of the bacterial cells builds up the pH of the colony falls to a point where the bacteria can no loner grow
Why are log numbers used to represent the bacterial population?
Because the difference in numbers from the initial organism to the billions of descendants is too great to represent using standard numbers
What are logarithms?
Powers of a base number
Why is a growth of bacteria against time graph semi logarythmic?
Because growth of the y axis is logarithmic and time is kept on a normal scale on the x axis
What is the equation to calculate the number of bacteria in a population and what does each symbol mean?
Nt = N0 x 2kt
Nt = number of organisms at time t N0 = number of organisms at time 0 k = the exponential growth rate constant t = the time the colony has been growing
How do you work out the exponential growth rate constant?
k = log10Nt - log10N0/ log102 x t
What are the four stages to the growth curve in a bacterial culture?
- the lag phase - when bacteria are adapting to their new environment and are not yet reproducing at their maximum rate
- the log phase or exponential phase - when the rate of bacterial reproduction is close to or is at its theoretical maximum, repeatedly doubling in a given time period
- the stationary phase - when the total growth rate is zero as the number of new cells formed by binart fission is equal to the number of cells dying
- the death phase or decline phase - when reproduction has almost ceased and the death rate of cells is increasing
How do most bacteria make people unwell?
Through the toxins they make as a by product of their metabolism
What are endotoxins?
Lipopolysaccharides that are an integral part of the outer layer of the cell wall of gram negative bacteria
What are exotoxins?
Soluble proteins that are produced and released into the body by bacteria as they metabolise and reproduce in the cells of their host
What are lipopolysaccharides?
Large molecules containing a lipid element and a polysaccharide element
In endotoxins what do the lipid part and the polysaccharide part do?
The lipid part of the lipopolysaccharides that acts as the toxin, while the polysaccharide stimulates an immune response
What are the pathogenic effects produced by bacterial endotoxins?
Symptoms shch as fever, vomiting and diarrhoea
Why can antobiotic treatments that destroy the bacteria cell by lysis of the cell wall also lead to further endotoxin release?
Due to the lipopolysaccharides component of the cell wall
What happens with salmonella infection?
The bacteria invade the lining of the intestine and the endotoxins cause inflammation. The cells no longer absorb eater so the faeces become liquid the gut then goes into spasms of peristalsis that result in diarrhoea
How is salmonella spread?
By ingestion of food and water contaminated with infected faeces. Salmonella bacteria live in the guts of many food animals akd can easily contaminate the meat. If food is not cooked properly the bacteria can survive and pass into your gut when you eat the food. If someone handles raw meat and doesn’t wash their hands before preparing other food they can transmit the bacteria. Salads eashed in contaminated water or contaminated drinking water can cause infection
Why are the salmonella bacteria able to get through the stomach to the small intestine?
Because they can survive the stomach acid
What does atibiotics for salmonella do for most people and who should it be prescribed to?
In most peiple antibiotics just reduce symptoms allowing the person to feel better but act as a carrier for longer
Patients who are very young, very old or has a comprised immune system
Where do endotoxins have an effect?
Around the site of infection by the bacteria
Are exotoxins produced by gram positive or negative bacteria?
Both
Why is the effect of exotoxins often more widespread than endotoxins?
Because they often act at sites at a distance from the infecting bacteria
What are some of the effects of exotoxins?
- some damage cell membranes causing cell breakdown or internal bleeding
- some act as competetive inhibitors to neurotransmitters
- directly poison cells
When do staphylococcus bacteria cause disease?
If they get inside the tissues of the body, if the normal skin flora is changed or if the person has a compromised immune system due to another disease or has been recieving treatments such as chemotherapy
What type of bacteria are staphylococcus?
Gram positive bacteria
What does s. Auereus cause?
- Skin infections such as styles, boils and impetigo
- infections of the joints in septic arthiritis
- infections of the membranes around the heart
- gastroenteritis
- diseases such as septicaemia, toxic shock syndrome and necrotising pneumonia, all of which can be fatal
What can both S. aureus and S. epidermis cause if they get access to the brain as a result of surgery or through the brain membranes in poorly controlled diabetes?
Bacterial meningitis
Can staphylococcus infections be treated with antibiotics?
Yes if they are diagnosed quickly but the exotoxins they produce are often very powerful and can cause death very rapidly
Apart from by the release of toxins how else can bacteria cause disease?
By invading host tissues and damaging cells. It is the response of the host organism to the cell damage that causes the symptoms of the disease
What is tuberculosis?
A lung disease caused by the mycobacterium tuberculosis and M. bovis
How is the mycobacterium tuberculosis bacteria spread?
By droplet infection
What are ways in which tuberculosis is spread and who is more vulnerable?
- crowded living or working conditions increase the likelihood of it spreading as people cough, breathe and sneeze near eachother
- People who are malnourished, ill or have problems with their immune systems are more vulnerable to the disease and much more likely to develip active TB than healthy well fed individuals
- peiple living with HIV/AIDS are vulnetable as a result of their reduced immune response
- you can also get infected by the bacterium mycobacterium bovis which affects cattle so people become infected by drinking infected milk or living and working in close contact with cattle
What does TB do?
Affect the respiratory system, damaging and destroying lung tissue. It also supresses the immune system, making the body less able to fight the disease. Symptoms such as coughing up blood and weakness come at the end of the disease process
What is the primary infection of tuberculosis?
The inital stage when the M. tuberculosis has been inhaled into the lungs and multiplied slowly often causing no obvious symptons
What happens if you have a healthy immune system and get a primary infection of tuberculosis?
There will be a localised inflammatory response forming a mass of tissue called a tubercle containing dead bacteria and macrophages. In about 8 weeks the immune system controls the bacteria, the inflammation dies down and the lung tissue heals
How do mycobacterium tuberculosis cause active tuberculosis and what does this mean in terms of natural selection?
Bacteria produce a thick waxy outer layer that protects them from enzymes of the macrophages. Bacteria with an effective coating will remain deep in the tubercles in the lungs, dormant or growing slowly for years until the person is malnourished, weakened or their immune system does not work well. These bacteria then cause active TB
In this way the most effective bacteria are selected and will be passed on
What is an antibiotic?
A drug that either destroys microorganisms or prevents them from growing and reproducing
What are antibiotics used to treat?
Bacterial diseases
What is the principle of selective toxicity?
It means that a substance is toxic against some types of cells or organisms but not others
Why do antibiotics run under the principle of selective toxicity?
They interfere with the metabolism or function of the pathogen with minimal damage to the cells of the human host
What is penicillin?
The first antibiotic discovered. It affects the formation of bacterial cell walls and is bactericidal
What do abtimetabolites do and give an example of them?
They interupt metabolic pathways such as blocking nucleic acid synthesis causing death
Eg. Sulfonamides
What do protein synthesis inhibtors do and give an example of them?
Interrupt or prevent transcription and/or translation of microbial genes so protein production is affected
E.g. tetracylines, chloramphenicol
What do cell wall agents do and give an example of them?
Prevent formation of cross linking in cell walls so bacteria are killed by lysis
Example: beta -lactams e.g. penicillin
What do cell membrane agents do and give an example of them?
Damage the cell membrane so metabolites leak out of water leaks in killing the bacteria
E.g. some penicillins, cephalosporins
What do DNA gyrase inhibitors do and give an example of them?
Stop bacterial DNA coiling up so it not longer fits within the bacterium
E.g. quinolone
What does it mean if an antibiotic is bacteriostatic?
It inhibits the growth of bacteria
What is tetracyline?
A bacteriostatic antibiotic that inhibits protein synthesis. It is used to tract acne, UTIs, respiratory tract infections and chlamydia
What is a bactericidal antibiotic?
An antibiotic that kills bacteria.
What is a bactericidal antibiotic?
An antibiotic that kills bacteria.
E.g. penicillins are bactericidal antibiotics used to treat skin infections, chest infections and UTIs
When are bactericidal antibiotics used?
- often in severe and dangerous infections
* treating infections where the immune system of the patient is weakened
What is a broad spectrum antibiotic?
An antibiotic that destroys a wide range of harmful bacyeria, pathogens and neutral and good bacteria
What are narrow spectrum antibiotics?
Antibiotics that targets one or two specific pathogens
What are the factors that the effectiveness of a antimicrobial drug depends on?
- the concentration of the drug in the area of the body infected, which will be affected by how easily the drug can reach the tissue and how quickly it is excreted
- the local pH
- whether either the pathogen or the host tissue destroy the antibiotic
- the susceptibility of the pathogen to the particular antibiotic used
What is an antibiotic resistant microorganism?
A microorganism that is not affected by an antibiotic, one that may have been effective in the past
How do you investigate the effect of different antibiotics on bacteria?
An agar plate is inocculated with a known bacterial culture. Filter paper discs containing different antibiotcs or different concnetrations of the same antibiotic and the plates covered, allowing oxygen in. A control culture microorganisms with known sensitivity to the antibiotic is grown at the same time under the same conditions. The level of inhibition of bacterial growth gives a measure of the effectiveness of the drugs
What is methicillin-resistant staphylococcus aureus (MRSA)?
A strain of S. Aureus that is resistant to several antibiotics including methicillin
Why does widespread use of antibiotics accelerate the process of natural selection with bacteria that have advantageous mutations,
Because it increases the selection pressure for the evolution of bacteria that are resistant to all of them
Where are superbugs commonly found?
In hospitals and care homes where people are ill or have had surgery and where antibiotic use is at it’s highest
What is clostridium difficile?
A type of bacteria that often exists in the gut and causes no problems unless it becomes dominant as a result of the normal gut flora being removed/ damaged by anitbiotic treatment
What are healthcare-associated infections (HCAIs)?
Infections that are acquired by patients while they are in hospital or care facilities. They may be the result of poor hygiene between patients or the result of antibiotic treatment and may be antibiotic resistant
What is MRSA?
A mutated strain of S. aureus that is methicillin resistant (and causes boils or infections throughout the body). The infections can be treated but only with high doeses of a very small number of antibiotics which are used very sparingly
What does C. difficile do?
Produces two different toxins that damage the lining of the small intestines causing severe diarrhoea that can lead to bleeding from the gut and even death
What are the codes if practice that have been drawn up for healthcare workers tontry and prevent the spread of (HCAIs)?
- controlling the use of antibiotics
- hygiene measures
- isolation of patients
- prevention of infection coming into the hospital
- monitoring levels of HCAIs
How should healthcare workers help control the use of antibiotics?
- only prescribing antibiotics when absolutely necessary
- making sure the pateint finishes their course of antibiotics as if the patient doesn’t finish it then there still may be bacteria left over which could reproduce
What are hygiene measures that should be taken in hospitals to reduce the spread of HCAIs?
- washing hands or using alcohol based gels between patients
- no wearing of long ties which may dangle and carry bacteria from one patient to another, no wristwatches or long sleeved shirts
- some hospitals use scrubs
- thorough cleaning of hospital wards, toilets and equipment such as bedpans
How can patients be isolated to reduce spread of diseaseM
Nursing infected individuals in seperate rooms with high level of hygiene and infection control
How is infection prevented from coming into the hospital?
- screening patients as they come into the hospital allows people who are carrying MRSA or other infections to be treated immedietly and isolated until the bacteria have been destroyed
- hospitals advice people not to visit if they are unwell
- hospitals provide alcohol gels for people to use as they come in and out of the hospital
How is monitoring levels of HCAIs done?
It has been made mandatory that hospitals measure and report levels of MSRA and C. difficile infections. These results are published and available to the general public as well as the government. This has focussed attention on the problem and led to increased efforts to reduce and overcome infections
