Bacteria and Antibiotics PDF

Bacteria and Antibiotics Advanced Study Skills in Biological Sciences Dr. Karim Mustafa Acknowledgement: Masoud Akhtar In this lecture we will cover: Bacterial morphology Structure of prokaryotes vs Eukaryotes Visualizing bacteria Bacterial growth Pathogenic...

Bacteria and Antibiotics Advanced Study Skills in Biological Sciences Dr. Karim Mustafa Acknowledgement: Masoud Akhtar In this lecture we will cover: Bacterial morphology Structure of prokaryotes vs Eukaryotes Visualizing bacteria Bacterial growth Pathogenic bacteria Antibiotics and antibiotic resistance  Prokaryotes are a broad and diverse kingdom of living organisms. Evidence of their existence for 3.5 billion years. All unicellular although some will form chain or clustered colonies. Classified by their shape: rod, spherical, comma & spiral are the Bacteria most common. All have a cell wall made of peptidoglycan (murein) & some have a slime capsule surrounding wall (gram negative bacteria). Reproduce asexually by binary fission (splitting). Prokaryotic vs Eukaryotic Cell Structure Structure of a prokaryote They have no nucleus, or any internal membranes, so they have no membrane-bound organelles. Certain prokaryotes increase the surface area of their cell membranes to support vital functions using structures like mesosomes and photosynthetic lamellae. These structures allow more space for enzymes and molecules involved in key processes (like energy generation or photosynthesis They are unicellular.  Structure of a prokaryote Contain a number of organelles eg ribosomes, pili, flagella, mesosomes & plasmids but none are membrane bound. The shape of bacteria is due to their rigid cell wall which has a unique structure: it contains a 3D mesh of peptidoglycan (murein).  Gram Stain The most widely used staining procedure in microbiology is the Gram stain. Discovered by the Danish scientist and physician Hans Christian Joachim Gram in 1884. Gram staining is a staining technique that differentiates bacteria into two groups: gram- positives and gram-negatives.  Gram positive bacteria have cell walls with a thicker layer of peptidoglycan/murein, which retains the crystal violet/iodine complex within their cells when washed with alcohol - staining purple. Whereas, when washed with alcohol the lipopolysaccharide capsule is removed from the gram-negative cells. They can then be counterstained with safranin and appear pink. Bacterial Cell Walls Bacteria are found everywhere 1 million / ml water 40 million / gram soil  Eukaryotic & Prokaryotic cells nucleolus Rough ER mitochondria Nuclear envelope nucleus ribosomes Smooth ER centriole golgi membrane vesicle  mitochondria Nuclear membrane Nucleus Cell membrane Vacuoles The eukaryotic cell Differences between eukaryotes / prokaryotes Cell membrane - both contain phospholipids, no sterols in prokaryotes Bacteria have a cell wall containing peptidoglycan Ribosomes (Protein synthesis) Eukaryotic: 80s (larger), Prokaryotic: 70s (smaller). Cellular ultrastructure is different Prokaryotes don’t have mitochondria, endoplasmic reticulum, chloroplasts, Golgi apparatus or vacuoles (membrane bound organelles). Prokaryotes have plasmids (small circular sections of DNA), pili (protein fibres extending from cell wall) & mesosomes (infoldings of cell membrane). Bacterial Growth Bacteria can divide (‘grow’) rapidly - Divide by binary fission (no mitosis or meiosis) - Prokaryotes (n=1, haploid). Eukaryotes (n=2, diploid) Phases of the Bacterial Growth Curve Lag Phase: Initial period of slow population growth, as numbers of individuals are small, so the increase in population size is also relatively small at the beginning of the population growth. Phases of the Bacterial Growth Curve Exponential Phase: Providing there is no limiting factors, so nutrients and other chemical requirements are plentiful, the number of individuals increase exponentially over time. Phases of the Bacterial Growth Curve Stationary Phase: The number of viable individuals remains constant as the production of new cells is roughly equal to the death of others. There is sufficient resources within the environment to maintain the population. Phases of the Bacterial Growth Curve Death Phase: When nutrients and or oxygen run out (if the bacteria are aerobes). Or, more likely, toxic waste products of the cells’ metabolism build up, then viable cell numbers will drop. The death rate is greater than the production of new cells in the population. Factors influencing bacterial growth Temperature (Most bacteria are ‘mesophiles’ = 25-45o C) Oxygen requiring (aerobes) Oxygen Do not require oxygen (anaerobes) Carbon & nitrogen source (organic material or CO2) Energy source (light or chemical) Water Minerals (suphur, iron etc) & trace elements (Zinc)  Pathogens (disease causing agents) Bacteria can gain entry to the body and then reproduce. As they grow, they produce ‘exotoxins’, usually as a by-product of their metabolism. Exotoxins are chemicals that can: - Damage host cell membranes - Interfere with host cell metabolism - Cause inflammation / septic shock Some examples: Tissue damage (tuberculosis, TB), Dehydration/ diarrhoea (cholera), Tuberculosis Mycobacterium tuberculosis Tuberculosis is a bacterial disease that is again on the increase, partly due to the link with the HIV epidemic. It can be spread rapidly in overcrowded conditions and is transmitted in airborne droplets when infected people cough and sneeze. The most common form of TB attacks the lungs and neck lymph nodes. Symptoms include coughing, chest pain and coughing up blood. Tuberculosis Mycobacterium tuberculosis Named for the tubercles or nodules of dead and damaged cells in the lungs of infected people. The tubercles may contain gas filled cavities that can be seen in X-rays. Tuberculosis is prevented by a Bacillus Calmette- Guerin (BCG) vaccination programme for children (live-attenuated). If infected, treatment involves a long course of antibiotics ( 4 types over 6 months in most cases). Tuberculosis (Mycobacterium tuberculosum) cough (+ blood) fever weight loss cavities lung damage Cholera: Vibrio cholerae Cholera is caused by a Gram- negative bacterium which is endemic in some areas of the world. Its toxins affect the gut lining causing watery diarrhoea leading to severe dehydration and frequently death. Humans act as reservoirs or carriers and contaminate water supplies in which the organism is transmitted. Cholera prevention is by the treatment of water and the provision of clean drinking water. Dehydration Diarrhoea Cholera: Vibrio cholerae A toxin is produced by V. chloreae in the small intestine that affects chloride channel proteins on the surface of gut cells. As a result, water and many ions including Cl¯ Na+ are pumped out of the cells into the gut rather than being absorbed into blood. This results in the patient has severe watery diarrhoea. Blood pressure falls and the patient may die.  How to develop antibacterial agents TB Find a molecule that will: Prevent growth [division] or Kill the pathogen without having an impact on human cells Exploit differences between eukaryotes & prokaryotes.  What types of antibiotic are there? Broad spectrum – kill a range of bacteria including those which are beneficial. Narrow spectrum – only kill a specific type of bacteria. Bactericidal – kill the bacteria. Bacteriostatic – prevent cell division. Penicillin A. Fleming (1881-1955) Bacteria Agar plate Fungus  Penicillin blocks cell wall synthesis Penicillin works by targeting bacterial cell wall synthesis. It does this by inhibiting (stopping) the transpeptidase enzyme that makes the polypeptide cross-links in the bacterial cell wall, which strengthen it. Penicillin contains a β lactam ring structure which acts as an irreversible competitive inhibitor within the active site of the transpeptidase Penicillin leads to bacterial lysis - Inhibits cross-linking of peptidoglycan by inhibiting transpeptidase enzyme - Weakens cell wall - Sensitive to water movement (osmotic shock)  Other Antibiotics Penicillin is ineffective against gram negative bacteria (eg salmonella sp. & E. coli) as they have a thick lipopolysaccharide outer layer to their cell wall. Therefore, the β lactam cannot block the enzyme. Antibiotics have been developed that have other modes of action The future: Antibiotic resistance  How does antibiotic resistance develop? Simple mechanism of evolution: A very small number of bacteria will have an allele of a gene which gives them resistance to the antibiotic & they can survive the toxic environment. The susceptible bacteria, without the allele are killed when antibiotics are used. The resistant ones survive and pass the resistance allele to the next generation. Antibiotic resistant genes can be found in plasmids, which can be exchanged between bacteria. There are now resistant strains of TB, Vibrio cholerae and Staphylococcus aureus, and many others.