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Bacteria

  • Two major criteria used to classify bacteria:
    • Gram positive/gram negative
      • Difference is due to cell wall – one has a thicker peptidoglycan layer than the other
      • Positive = purple
      • Negative = pink
    • Bacterial shape: cocci/rod

Capsules

  • Particularly prevalent in gram negative bacteria (although some gram positive have them as well)
  • Capsules are composed of complex polysaccharides
  • Colonies of bacteria with capsules tend to have a mucoid appearance
  • Appears as a white area around the bacteria
  • The capsule has a negative charge
  • Functions of the capsule:
    • Allows it to attach to the host cell
    • Prevents the bacteria from being phagocytosed by PMNs (neutrophiles)
    • This gives the bacteria a chance to multiply and survive as our bodies need to develop a specific response
  • Streptococcus pneumoniae
    • Has 75 different types of capsule, based on the sugar content. If you recover from one infection with the bacteria, then you are not protected from others that have a different capsule type
    • Major cause of community-acquired pneumoniae (as opposed to hospital-acquired pneumonia)
  • Other encapsulated bacteria include the following two organisms that cause meningitis:
    • Haemophilus influenzae
    • Neisseria meningitides

Flagella

Examples of Bacterial Flagella
  • Responsible for motility
  • Different bacteria have different numbers and locations
  • Motility is often important for colonisation and nutrient acquisition. They need to get to the correct site/s to attack or feed.
  • Appear as fine filaments under the microscope
  • Flagella are complex, like a boat motor
  • Works on proton motor force:
    • Movement of ions across the membrane powers the motor (flow of acid powers it)
    • Has a stator, rotor, U-joint etc. They really are real motors

Pili/fimbrae

  • Pili are surface filaments that are much smaller than flagella
  • Function: adhesion. Very important for sticking to host
  • Facilitate attachment of bacteria such as Eschericia coli in infection of the urinary tract and intestines
    • Continuous movement via peristalsis (fast in small intestine, slow in large intestine)
    • Bacteria need to lodge themselves in place

Bacterial spores (endospores)

  • Spores don’t only form in fungi
  • Allow bacteria to survive for long periods under adverse conditions
  • Some bacteria form spores when environmental conditions become adverse. Spores can survive for 100s of years
    • Dipicolinic acid and calcium dehydrate the bacteria and put it in a dormant form where it is not multiplying
    • This is called the dormant state
  • When conditions improve, they germinate and start to reproduce again
    • This state is called the vegetative state
  • Spores are associated with gram positive rods
  • Important spore forming bacteria
    • Bacillus species
      • Bacillus anthracis – Anthrax
        • Very dangerous if it gets in the lung; bioterrorism
      • Bacillus cereus – Food poisoning
    • Clostridial species
      • Clostridium tetani – Tetanus
      • Clostridium botulinum – Botulism (food poisoning; lethal)
      • Clostridium perfringens – Gas gangrene; Food poisoning. Example scenario:
        • serious accident
        • spores get in
        • start to reproduce (can germinate in better conditions)
        • anaerobic respiration produces gas and toxins

Bacterial growth

  • Bacteria multiply by simple division, called binary fission
  • Cannot be divided into prophase, metaphase, anaphase and telophase because prokaryotes have no nucleus and no cetromeres

Process of binary fission

Binary Fission
Bacterial Growth Numbers

Animation: http://en.wikipedia.org/wiki/File:Binary_fission_anim.gif

  1. Cell elongates and DNA is replicated
    • Starts at only a single origin of replication, unlike eukaryotic cells that have multiple origins of replication.
    • The replication bubble grows and copies the template DNA
    • Each circular DNA strand attaches to the cell membrane, causing the DNA to separate
  2. Cell wall and plasma membrane begin to divide. Invagination of cell wall and membrane
  3. Cross-wall forms completely around the divided DNA
  4. Cells separate
  • Hence bacterial growth is as 2^t. It is exponential growth (though for some reason they call it logarithmic growth)
  • This is important in collecting urine samples:
    • If you leave the urine sample at room temperature then the bacteria will rapidly replicate
    • Many urinary tract infections are caused by proliferation of normal flora deep within the urinary tract
    • Diagnosis of these infections is by measuring a large number of normal flora in the urine
    • Hence incorrect diagnosis is possible
  • Youtube = “Logarithmic growth of bacteria”
  • Growth is divided into phases:
    • Lag phase
      • Individual bacteria are maturing and are unable to divide
      • First few hours or days (depending on how long it takes for the organism to multiply)
      • Organism adapts to its new environment (change in media e.g. from outside to inside the body)
      • Has to synthesise proteins to live in the new environment
    • Exponential growth (log) phase
      • Replication starts and rapidly continues
      • Bacterial cells undergo binary fission at a constant rate. The number of bacterial increases exponentially
    • Stationary phase
      • As the bacteria continue to replicate and grow, the nutrients are used up and waste products from the bacteria build up in the environment
      • The loss of nutrients or build up of waste in the environment can cause the pH or temperature to shift (or there is a build up of toxic metabolic wastes). This makes the environment a less optimal growth area for the bacteria
      • The reproduction rate slows and the total number of bacteria present stabilises as cells begin to die off or stop reproducing
      • Death rate = growth rate
      • (Inside the body, phagocytosis contributes to this increased death rate)
      • Because antibiotics usually act in latter stages of the cell cycle (such as replication of DNA, synthesis of cellular components, or mitosis), bacteria in the stationary phase are less susceptible to antibiotics than bacteria in the lag phase
    • Death/Decline phase
      • All nutrients in the environment are getting used up (or, inside the body, the number of phagocytes etc. reaches a critical level)
      • Individual bacteria begin to die off from lack of nutrients (or phagocytosis)
      • Eventually the number of dead cells exceeds the number of live ones
      • At this stage, the number of live bacteria is either radically reduced or completely destroyed

Growth Factors

  • Optimum bacterial growth temperatures
    • Psychrophiles: 0-20 C E.g. Antarctica
    • Mesophiles: 20-40 C Associated with human disease (37 C)
    • Thermophiles: 40-90 C E.g. Thermal springs
  • Optimum bacterial growth oxygen requirements
    • Aerobic: Require oxygen for growth
    • Anaerobic: Cannot grow in the presence of oxygen
    • Facultative: Can grow with or without oxygen
    • Microaerophilic: Require a low concentration of oxygen
  • Optimum bacterial growth pH
    • Most bacteria grow best at pH 7
    • Human blood and tissues have pH 7.2-7.4 (they are adapted to us!)
    • Majority of bacteria do not grow well in acidic conditions
    • The stomach is highly acidic (pH 2), inhibiting bacterial growth
    • Exceptions:
      • Helicobacter pylori – enzyme called “urease” breaks down urea and produces an ammonia cloud that allows it to survive. pH is higher the closer you get to epithelium (mucous lining less acidic)
      • Lactobacilli are normal flora of the human vagina (pH 4-5). The lactic acid they produce (in combination with fluids secreted during arousal) are responsible for the characteristic scent of the vagina.
      • Acidophiles are acid tolerant bacteria
  • Bacterial growth media
    • Liquid media
      • Useful for increasing the numbers of bacteria (e.g. for ELISAs you need a very large population, which you are very unlikely to get in a blood test… hence need to multiply them)
    • Solid media
      • Solid surface allows bacteria to grow into single colonies and facilitates identification of pathogens. You can see how the colonies look, which is a big part of identification.
      • E.g. agar plates
        • Agar is added to a nutrient broth to provide a solid surface
        • Agar melts at 100C and sets at 44C. This is useful in sterilisation.
        • The medium is sterile, even if we use blood (note: blood is sterile unless you’re sick)
        • A bacterial colony results from the multiplication of a single bacterium
        • One colony = one bacterium originally
        • Can use this to distinguish between normal flora and pathogens etc.
        • Some cause changes in the agar, which can also be seen

Bacterial Infections

  • Bacteria are associated with infections at many sites of the body
  • Skin infections
    • Impetigo e.g. Streptococcus pyogenes (school sores – what Mum had at school)
  • Lung infection
    • Pneumonia e.g. Streptococcus pneumoniae
  • Nervous system
    • Meningitis e.g. Haemophilus influenzae
  • Urinary tract infection
    • Cystits, pyelonephritis e.g. E. Coli (which causes 80% of diseases of the urinary tract in the community)
  • Genital tract infection
    • Gonorrhoea e.g. Neisseria gonorhoeae
  • Stomach
    • Peptic ulcer disease e.g. helicobacter pylori. [Inflammation, ulcers; predispose to gastric cancer]
  • Intestine
    • Typhoid fever e.g. Salmonella typhi
      • This is a gram-negative rod. Feature: gets inside of a macrophage, survives, divides and spreads to other organs and eventually returns to the gall bladder, where the immune system is ready to deal with it

Archaea

  • Prokaryotes
  • Resemble bacteria microscopically
  • Distinguishing features:
    • Cell walls don’t contain peptidoglycans
    • Cell membranes contain different lipid composition
    • Unique nucleotide sequences in RNA of their ribosomes
  • Have been isolated from the human colon, vagina and oral cavity, but have not (yet) been established as causes of human disease
  • Very little evidence of their role in disease

Eukaryotes

Fungi

  • Mushrooms (some are toxic)
  • Moulds
    • Are fungi that grow in the form of multicellular filaments called hyphae
    • A connected network of these tubular branching hyphae has multiple, genetically identical nuclei, and is considered a single organism (called a colony)
    • Cause spoilage of food
  • Yeasts
    • Are microscopic fungi that grow as single cells
    • Most reproduce asexually by budding, although a few do so by mitosis
    • Small bud (daughter cell) is formed on the parent cell. The nucleus of the parent cell splits into a daughter nucleus and migrates into the daughter cell. The bud continues to grow until it separates from the parent cell
    • E.g. Candida albicans, an opportunistic (reproduce when conditions are favourable and are dormant otherwise) yeast that causes thrush (oral and vaginal). Most harmful in the immunocompromised.
  • Fungi (such as Penicillium and Aspergillus) are identifiable by their spores or hyphae. The fungal spore is a reproductive body. The spore type is useful in identification.
  • Hyphae search out and assimilate nutrients. They also produce organs for reproduction.
    • Common fungal infections include
      • Tinea capitis
      • Tinea versicolor
      • Onychomycosis
      • Tinea corporis
      • Tinea pedis (tinea on your feet)

Protozoa

  • Largest of all microorganisms
  • No chlorophyll
  • Hence are heterotrophic
  • Some are unicellular, others are multi-cellular
  • Obtain nutrients by engulfing food particles
  • Asexual reproduction
  • Common protozoal diseases
    • Amoebiases – Dysentery (Entamoeba histolytica)
    • Giardasis – Diarrhoea (Giardia llamblia)
    • Trichomoniasis – STDs: discharge (Trichomonas vaginalis)
    • Toxoplasmosis
      • Infectious mononucleosis type symptoms (similar to Glandular fever)
      • Causative agent: Toxoplasma gondii
    • Malaria
      • Affects more than 250 million people worldwide
      • Causative agents: Plasmodium species
      • Transmitted by Anopheles mosquito

Worms

  • Parasitology includes worms
  • Example: Necator americanus (Hookworm)
    • Small intestinal roundworms
    • Enters body via skin
    • Blood sucking
    • Can lead to anaemia and protein loss
  • Example: Ascaris lumbricoides
    • Large intestinal round worm (~30 cm)
    • Ingested
    • Can obstruct intestine
    • Affects digestion and food absorption
  • Example: Enterobius vermicularis
    • Pinworm that is ingested
    • Common in children, causing Perianal pruritis (itching around the anus)
    • Can be detected using Scotch tape
      • Shine a torch on the bottom, use Scotch tape to get the worm out, then look at it under the microscope
  • A lot of these worms need to pass from someone’s faeces to the mouth of someone else. Occurs when someone doesn’t wash his or her hands properly after going to the toilet.