Bacteriology and Fungi

• Bacteria are unicellular and small: typically 0.5 – 5 µm. They usually have rigid cell walls containing peptidoglycan and are less complex than eukaryotes. Organelles not membrane bound.
• Bacteria have a diverse morphology. They can be spherical, rod-shaped, spiral-shaped, or coccus-shaped.
• Bacteria which cause disease are known as pathogenic bacteria.
• A typical bacterial cell has a capsule, cell wall, cell membrane, cytoplasm, and flagella/pili. Some also form endospores.
• Coccus bacteria can come singularly, in chains, pairs, or in clusters.
• Bacillus bacteria can come singularly or in chains.
• Spirochaete bacteria come in commas, coils, or corkscrews.
• Gram positive bacteria stain blue due to the simple, thick peptidoglycan in their cell walls.
• Gram negative bacteria stain red due to their outer membrane and periplasmic space with less peptidoglycan combined with outer lipopolysaccharides (LPS).
• The capsule of bacteria is a polymeric extracellular material adherent to cell well synthesized by some bacteria usually formed from a polysaccharide. It is associated with virulence.
• The function of the capsule is adherence to a host cell, to prevent phagocytosis, and aid in survival in the environment.
• Roughly half of all prokaryotes are capable of taxis, the directed movement towards or away from stimulus. Examples of this are positive (towards) or negative (away) chemotaxis (in response to chemicals).

  Some species move at high velocities. Flagella are the most common structure and are scattered over the surface or concentrated at one / both ends.
• Prokaryote genomes are structurally different from eukaryotes as there are many different types.
• Types of prokaryote genomes:

  • Single haploid circular chromosome containing dsDNA
  • No nuclear membrane
  • Plasmid DNA. Small circular DNA, separate from genome that replicates independently and can be transferred between bacteria.
• Bacteria go through replication by binary fission where single bacteria yield two daughter cells. The generation time ranges from 20 mins to 30 hours and there are limitations on growth such as nutrient supply and metabolic waste accumulation as well as competition against other microorganisms.
• The rapid reproduction rate of bacteria contributes to high levels of genetic diversity in bacteria. High level of mutation is possible in part due to rapidity of replication.
• Individuals that have acquired mutations that enable improved survival are effectively selected for. Genetic recombination via horizontal gene transfer also contributes to genetic diversity.
• Bacteriophage: viruses that infect bacteria.
• Transduction: viruses carry prokaryotic genes from one host to another. Often occurs during phage replicative cycle. If virus with prokaryotic DNA can’t replicate. Attaches to new recipient cell. Injects DNA from first donor cell. DNA crosses over into recipient cells chromosome.
• Conjugation: DNA transferred between two prokaryotic cells, temporarily joined together. Usually same species. DNA transfer one-way: one cell donates, the other receives it.
• Mesophile bacteria:

  • Aerobic
  • 37°C optimal – close to body temp
  • Can also grow between 20 – 45°C
  • Pathogenic bacteria
• Psychrophile bacteria:

  • 15°C optimal
  • Environmental bacteria
• Thermophile bacteria:

  • 60°C optimal
  • Environmental bacteria
• Obligate aerobe bacteria are aerobic and require oxygen for growth.
• Obligate anaerobic bacteria are those that cannot survive in the presence of oxygen and live via fermentation.
• Facultative bacteria can grow in both aerobic and anaerobic conditions depending on resources.
• Microaerophile bacteria grow best in the presence of less than 1% oxygen, known as sub-atmospheric conditions.
• Capnophile bacteria are aerobic and produce hydrogen sulfide gas, relying on the presence of carbon dioxide.
• Diagnosis of bacterial disease can be achieved by obtaining a sample for characterisation by microscopy, bacterial culture or other methods.
• Antimicrobial drugs can be used to kill bacteria or inhibit bacterial growth in the host by targeting structures of the bacterial cell.
• Fungi are eukaryotic and go through aerobic growth as non-photosynthetic heterotrophs. They derive nutrients from complex organic molecules and obtain nutrients by absorption.
• Heterotrophs use enzymes to digest living and dead substances and secrete hydrolytic enzymes which break down complex molecules and absorb nutrients. They use enzymes to penetrate the walls of other cells to absorb their nutrients.
• Fungi can be saprophytic or parasitic. Saprophytic bacteria are widespread in the environment and occasionally cause opportunistic infection. Parasitic fungi cause disease.
• Moulds have a hyphal cell wall composed of carbohydrate components including chitin and cellulose cross links.
• Yeasts have a cell wall that contains proteins complexed with polysaccharides.
• Sexual Reproduction in fungi uses hyphae from two mycelia which release pheromones. They extend towards the source and fuse together, and the union of the two parent mycelia = plasmogamy. Haploid nuclei pair off, two to a cell and divide without fusing = dikaryotic. Haploid nuclei contributed by two parents fuse to produce a zygote with diploid cells = karyogamy. Normally only diploid stage. Divide by meiosis (restores them to haploid). Generates genetically diverse spores.
• Exotoxins are made by both gram positive and gram negative bacteria. They cause cell membrane damage and interfere with protein synthesis and are not encoded on the chromosome.
• Endotoxins are a component of the cell wall of gram negative bacteria. The LPS containing Lipid A is responsible for the toxicity. The toxin can interact with immune cells, induce IL1 (fever) and activate complement. These toxins are encoded on the chromosome.
• Control methods for bacterial pathogens involve methods to disrupt or reduce transmission.
• Antimicrobial drugs are commonly used to treat bacterial infection. Their use should be based on bacterial isolation and sensitivity testing where possible.